Kristian Spilling

• Reeder, Christian Furbo; Friesen, Lisa W. von; Lemke, Josephin; Spilling, Kristian & Farnelid, Hanna (2026). Composition of non-cyanobacterial diazotrophs in suspended and sinking particles is driven by productivity phase. Microbiology spectrum. 14(9). doi: 10.1128/spectrum.01386-26. Full text in Research Archive Show summary

  Biological nitrogen fixation is a key process impacting productivity in the Baltic Sea. Most studies of this process have focused on filamentous cyanobacterial diazotrophs, leaving the ecological roles of non-cyanobacterial diazotrophs (NCDs) poorly resolved. Here, we investigate the diazotrophic community across different primary productivity phases (productive and post-productive) and within distinct particle fractions (suspended and sinking) across the Northern Baltic Proper, Gulf of Finland, and Gulf of Bothnia. Using nifH gene amplicon sequencing, we show that NCDs dominate the diazotrophic community, accounting for 81%–100% of all sequences, whereas filamentous cyanobacteria contribute up to 19% and are largely restricted to the post-productive phase. Productivity phases emerged as a descriptor for structuring the diazotroph community composition, with Thermodesulfobacteriota dominating during the productive phase (up to 64.8%) and Gammaproteobacteria during the post-productive phase (up to 77.1%). Suspended and sinking particles were broadly similar, yet each exhibited distinct taxon-specific enrichments between phases, which might suggest different ecological strategies within the major NCD groups. This suggests potential roles in aggregate-associated nitrogen fixation; such mechanisms could include polysaccharide degradation and anaerobic metabolism. Phytoplankton composition (18S rRNA) varied between productivity phases and co-occurred with shifts in the diazotrophic community, suggesting potential cross-domain interactions. Comparing across basins, we did not observe significant differences in diazotroph community composition, except for a higher relative abundance of cyanobacterial diazotrophs in the Gulf of Finland. Taken together, our findings show diverse NCD lineages inhabiting particle-associated niches, highlighting the need to further investigate their ecological significance in the Baltic Sea. IMPORTANCE Nitrogen is a key nutrient that supports life in marine ecosystems. Traditionally, nitrogen fixation in the Baltic Sea has been attributed mainly to filamentous cyanobacteria. In this study, we show that non-cyanobacterial diazotrophs dominate the nitrogen-fixing community across productivity phases. We also demonstrate that suspended and sinking particles provide habitats for these microorganisms, allowing different groups to thrive under varying conditions. We further show that shifts in the phytoplankton community co-occur with changes in the diazotrophic composition. Together, our findings reveal that nitrogen fixation in the Baltic Sea extends beyond filamentous cyanobacteria and involves a diverse particle-associated diazotrophic community. Understanding the role of these overlooked microorganisms is important for predicting how nitrogen cycling will respond to ongoing changes, such as warming, eutrophication, and shifts in plankton communities.

• Lähteenmäki-Uutela, Anu; Judl, Jáchym; Silander, Jari; Lavikko, Sonja; Mäkelä, Tuukka & Löscher, Carolin Regina [Show all 9 contributors for this article] (2026). Tackling the technological, environmental, legal, and economic challenges of ocean alkalinity enhancement: A case study from the Baltic sea. iScience. 29(8), p. 1–16. doi: 10.1016/j.isci.2026.116928. Full text in Research Archive Show summary

  Ocean alkalinity enhancement (OAE) is a possible climate mitigation technology that can remove CO2 from the atmosphere. Here, we studied the feasibility of OAE in a case study using serpentinite gangue and examined the carbon dioxide removal (CDR) potential, the legal requirements, and the economic viability. The carbon emission factor was estimated to stay below 70 kg CO2e t−1 CO2 removed. Permitting would require the risks for marine ecosystems at the site to be carefully considered. Under the assumptions of this case study, OAE would require CDR credit prices higher than 100 € t−1 CO2 removed to be economically viable. Commitments of public and private actors to buy OAE credits would reduce market uncertainty and could be a way to promote OAE if the environmental impacts are acceptable.

• Borg-Stoveland, Stian; Draganovic, Vukasin; Spilling, Kristian & Gabrielsen, Tove M. (2026). Microbial succession and dominance patterns in modular RAS and their implications for effluent-based Phaeodactylum tricornutum cultivation. Aquaculture. ISSN 0044-8486. 621. doi: 10.1016/j.aquaculture.2026.743977. Full text in Research Archive Show summary

  Microbial communities are central to water quality, fish health, and operational stability in modular recirculating aquaculture systems (M-RAS). The composition and succession of microbial communities in the main production tank and the effluent were investigated during an Atlantic salmon grow-out period using 16S rRNA gene sequencing. Effluent from the aquaculture system was further reused for cultivation of Phaeodactylum tricornutum, enabling assessment of how effluent-derived microbial communities respond to algal growth conditions. In the main production tank, microbial communities showed an early shift from Rhodobacterales toward increasing dominance of Flavobacteriales, followed by a community assemblage with strong contributions by taxa of Thiotrichales and Chitinophagales. Operational disturbances, including reduced feeding and salinity adjustment, coincided with increased relative abundances of ASVs affiliated with Arcobacteraceae, Weeksellaceae, and Pseudoalteromonadaceae, taxonomic groups that include species described as opportunistic or pathogen-associated in marine fish systems. The effluent harboured a temporally variable microbial community throughout the grow-out period, characterized by fluctuations in the relative abundances of dominant orders such as Flavobacteriales, Enterobacterales, and Bacteroidales. Microbial communities in effluent studied during a two-week cultivation of P. tricornutum shifted toward taxa typical of algal phycospheres, including Caulobacterales and Rhodobacterales, which are often linked to mutualistic functions between algae and bacteria such as vitamin provision and nutrient remineralization. However, the effluent also harboured Bacteroidetes taxa (e.g., Maribacter, Zobellia, Aurantivirga), some of which have been reported in other systems to compete with diatoms for resources. Their presence during algal cultivation may therefore warrant consideration if the resulting biomass is intended for use in aquafeeds. These findings suggest that microbial community composition in M-RAS responds to operational adjustments, and that effluent reuse for algal cultivation not only enables resource recovery but also reshapes downstream microbial community assembly.

• Koch, Sophie J.I.; Filgueira, Ramón; Alberg, Jóhanna; Angel, Dror L.; Byron, Carrie J. & Cerca, Mariana [Show all 17 contributors for this article] (2026). Identifying a sustainable operating window for seaweed aquaculture in the Global North: balancing expansion barriers and carrying capacity. Aquaculture. ISSN 0044-8486. 611. doi: 10.1016/j.aquaculture.2025.742907. Full text in Research Archive Show summary

  Seaweed aquaculture is a growing blue sector that provides many benefits to society (e.g. biomass provision for food, feed and cosmetics) and the environment (e.g. eutrophication mitigation, carbon uptake and habitat provision). Successful and sustainable production expansion requires that these activities are operated within limits of acceptable change (LAC) i.e. align with ecological and social carrying capacity. Emerging from a three-round Delphi study, this work presents, from a Global North perspective, the most relevant 1) limiting variables from the socio-environmental spheres that influence the cultivation unit (inputs), such as high operating costs or underdeveloped markets or uncertain impacts from climate change, and 2) the negative impacts of aquaculture on environment and society (outputs), such as overhyped and unrealistic expectation for seaweed cultivation, conflicts with fisheries or pollution. Consolidated lists of these inputs and outputs are accompanied by specific thresholds beyond which unacceptable changes are likely to occur. These results are placed into a globally applicable holistic framework for a multidimensional assessment of seaweed aquaculture including barriers and carrying capacity, which has been outlined for the first time for seaweed cultivation. Our results emphasize the need to include socio-economic aspects into ecosystem approaches, like profitability or social license to operate – and the need for broad stakeholder participation. This study provides thus a directly useable lists of aspects to consider for cultivators and decision-makers. And as is as such a crucial contribution for the ongoing discourse on sustainable growth of this emerging blue sector.

• Santoro, Mariano; Vanharanta, Mari; Villena-Alemany, Cristian; Hassenrück, Christiane; Hagemann, Martin & Grossart, Hans Peter [Show all 9 contributors for this article] (2026). Impact of extreme weather events on pelagic cyanobacterial communities: an in situ mesocosm study in the Gulf of Finland. Journal of Marine Systems. ISSN 0924-7963. 253. doi: 10.1016/j.jmarsys.2025.104174. Full text in Research Archive Show summary

  Cyanobacteria play a critical role in regulating carbon, nitrogen, and phosphorus cycles in the Baltic Sea, seasonally forming extensive blooms under nitrogen-limited conditions in summer. Understanding their responses to environmental disturbances is crucial in the Baltic Sea where increasing and persistent surface water temperature anomalies were observed over the past two decades. During a 17-day mesocosm experiment in the southwestern Finnish archipelago (Gulf of Finland) designed to investigate microbial community dynamics under varying nutrient conditions, the unexpected occurrence of a natural disturbance event created a unique opportunity to assess the effects of pronounced physical changes on cyanobacterial dynamics. The natural disturbance overshadowed the expected effects of the nutrient treatments, which was especially evident for cyanobacteria. The picocyanobacterium Cyanobium spp. emerged as the dominant species throughout the study, particularly following the occurrence of the near gale wind-driven rainfall, likely due to its adaptability to rapid environmental changes. Conversely, the filamentous cyanobacteria Aphanizomenon spp. and Pseudoanabaena spp. thrived under stable conditions. These findings highlight the resilience of picocyanobacteria to environmental fluctuations and their primary role in driving cyanobacterial community dynamics during summer in the Baltic Sea, where natural perturbations are expected to occur with increasing frequency due to climate change.

• Thielecke, Antonia U.; Fernández-Méndez, Mar; Arístegui, Javier; Baumann, Moritz; Behncke, Jacqueline & Berger, Stella Angela [Show all 27 contributors for this article] (2025). Disentangling upwelling: how light and nutrient supply shape primary producers and stoichiometry in the Humboldt upwelling system. Deep-Sea Research Part II: Topical Studies in Oceanography. ISSN 0967-0645. 223. doi: 10.1016/j.dsr2.2025.105522. Full text in Research Archive Show summary

  The Humboldt upwelling system (HUS) is known for its extraordinary productivity due to wind-driven upwelling of nutrient-rich deep water, resulting in the highest fish catches per unit area worldwide. However, contrary to other Eastern boundary upwelling systems, upwelling intensity is highest in winter, while primary productivity reaches its peak during the summer months. Our current understanding of the counterintuitive relationship between upwelling intensity and productivity is insufficient to predict the consequences of climate change on this ecosystem. To elucidate the drivers of the upwelling-productivity relationship in the HUS, we tested the hypothesis that low light availability limits primary productivity in winter despite strong upwelling intensity supplying plenty of nutrients into the surface layer, while light availability in the shallower mixed layer in summer enables an effective use of the upwelled nutrients. To disentangle the interactive effects of light and nutrients on primary production and carbon cycling, we enclosed natural plankton communities off the coast of Callao (Peru) during a 35-day mesocosm experiment by recreating summer-time high light and winter-time low light conditions under different upwelling intensities (0 %, 15 %, 30 %, 45 % and 60 %). Primary productivity and phytoplankton biomass scaled with both nitrate and light availability. Comparing the same upwelling intensity at different light levels, our data confirmed the hypothesis that light limitation due to deepening of the mixed layer is a key driver for the out-of-phase observations in primary productivity in the Humboldt upwelling system. Under light limiting conditions phytoplankton had low POC:Chla ratios indicating photoacclimation and low POC:PON ratios indicating light limitation of nitrate uptake which leads to increased food quality for grazers in winter. Our study indicates that small seasonal changes in phytoplankton biomass (estimated using Chla) might hide larger changes in primary productivity (carbon uptake), and highlights the importance of combining satellite studies with in situ observations and experimental studies to predict the fate of upwelling systems in a changing ocean. Increased stratification caused by global warming in upwelling systems such as the HUS would lead to less phytoplankton biomass with higher POC:Chla and POC:PON ratios. This phytoplankton community would have lower food quality for grazers and might lead to a decline in the transfer to higher trophic levels, but at the same time might lead to increased CO2 drawdown in an otherwise CO2 emitting ecosystem. Understanding the unique relationship between upwelling intensity and productivity in the HUS contributes to predicting the reaction of this valuable ecosystem for fisheries to the impacts of climate change.

• Bansfield, D.; Spilling, Kristian; Mikola, A. & Piiparinen, J. (2025). Fungal Cultivation in a Dairy Side Stream: Valorisation Potential of Whey into Added-Value Biomass and By-Products. Waste and Biomass Valorization. ISSN 1877-2641. doi: 10.1007/s12649-025-03160-z. Full text in Research Archive Show summary

  Here we investigated the potential of fungi to valorise cheese whey into added-value biomass while removing nutrients. Batch cultures of four species (Geotrichum candidum, Penicillium corylophilum, Pleurotus ostreatus, and Penicillium restrictum) were set up in non-supplemented cheese whey in aerated bioreactors. Biomass yields and nutrient removal efficiencies were calculated, and potential biomass uses were assessed through fungal-assisted microalgal (Scenedesmus sp.) harvesting tests and biomass composition analysis. The results demonstrated species-specific differences, with P. corylophilum achieving the highest biomass yield (16.9 g DW L− 1) and the highest overall nutrient removal efficiencies (COD: 43%, TOC: 32%. TN: 54%, TP: 34%). G. candidum showed similarly high carbon removal efficiencies (COD: 43%, TOC: 34%). G. candidum and Scenedesmus sp. were rich in protein with balanced amino acid profiles, P. corylophilum’s fatty acid content and profile were suitable for biodiesel production, and P. ostreatus efficiently harvested Scenedesmus sp. (88% in 1 h) adding to the potential of the biomass. These results show that, with species-specific optimisation, these fungal species have significant potential to convert whey into value-added biomass and high-value by-products, thereby promoting sustainable bioeconomy practices for small-scale dairy producers.

• Ho, Lik Theng; Horn, Henriette Gesine; Weeraman, Pabasara; Spilling, Kristian & Gabrielsen, Tove M. (2025). Vertical and temporal variability of protist and prokaryotic summer plankton communities in a North Sea fjord. Aquatic Microbial Ecology. ISSN 0948-3055. 91, p. 75–92. doi: 10.3354/ame02020. Full text in Research Archive Show summary

  Microbial plankton communities play key roles in biogeochemical cycling and primary production in marine coastal ecosystems. Given the impacts of global change on Norwegian coastal waters, there is a need to understand the drivers of microbial plankton community structure and diversity. Environmental drivers such as temperature, salinity, and light influence the dynamics of microbial community structure and abundance in temperate oceans. Here we characterize the summer diversity of protist and prokaryotic plankton communities, using DNA metabarcoding and light microscopy, in the surface, upper mixed layer, and deep waters of Spindsfjorden, a South Norwegian fjord facing the North Sea. The sampling site was vertically stratified throughout the summer, with compositionally variable communities dominated by phototrophs and mixotrophs in the surface and upper mixed layers, and stable communities dominated by heterotrophs and parasites in the deep layer. Late summer blooms were dominated by the diatom Cerataulina pelagica in the surface and dinoflagellates of Tripos spp. in the upper mixed layer. Positive co-occurrences between certain diatom taxa and flavobacteria, and between diatoms and Alphaproteobacteria suggest potential symbiotic relationships or overlapping environmental preferences. Negative associations were observed between certain dinoflagellate groups and Syndiniales, possibly due to parasitic interactions. Temperature emerged as a key environmental driver of community change, underscoring its role in shaping the dynamics of microbial communities in temperate, coastal ecosystems.

• Macaulay, Samuel J.; Jeppesen, Erik; Riebesell, Ulf; Nejstgaard, Jens Christian; Berger, Stella Angela & Lewandowska, Aleksandra M. [Show all 33 contributors for this article] (2025). Addressing grand ecological challenges in aquatic ecosystems: how can mesocosms be used to advance solutions? Oikos. ISSN 0030-1299. 2025(5). doi: 10.1111/oik.11020. Full text in Research Archive Show summary

  Rapid and drastic anthropogenic impacts are affecting global biogeochemical processes and driving biodiversity loss across Earth's ecosystems. In aquatic ecosystems, species distributions are shifting, abundances of many species have declined dramatically, and many are threatened with extinction. In addition to loss of diversity, the ecosystem functions, processes and services on which humans depend are also being heavily impacted. Addressing these challenges not only requires direct action to mitigate environmental impacts but also innovative approaches to identify, quantify and treat their effects in the environment. Mesocosms are valuable tools for achieving these goals as they provide controlled environments for evaluating effects of stressors and testing novel mitigation measures at multiple levels of biological organisation. Here, we summarise discussions from a survey of marine and freshwater researchers who use mesocosm systems to synthesise their opportunities and limitations for advancing solutions to grand ecological challenges in aquatic ecosystems. While most research utilising mesocosm systems in aquatic ecology has focused on quantifying the effects of environmental threats, there is a largely unexplored potential for using them to test solutions. To overcome spatio-temporal constraints, there are opportunities to scale up the size and time-scales of mesocosm studies, or alternatively, test the outcomes of habitat-scale restoration at a smaller scale. Enhancing connectivity in future studies can help to overcome the limitation of isolation and test an important aspect of ecological recovery. Conducting ‘metacosm' studies: coordinated, distributed mesocosm experiments spanning wide climatic and environmental gradients and utilising more regression-based experimental designs can help to tackle the challenge of context dependent results. Finally, collaboration of theoretical, experimental and applied ecologists and biogeochemists with environmental engineers and technological developers will be necessary to develop and test the tools required to advance solutions to the impacts of human activities on Earth's vulnerable aquatic ecosystems.

• Spilling, Kristian; Vanharanta, Mari; Santoro, Mariano; Villena-Alemany, Cristian; Labrenz, Matthias & Grossart, Hans-Peter [Show all 7 contributors for this article] (2025). Picophytoplankton act as the primary consumers of excess phosphorus after the spring bloom in the eutrophic Baltic Sea. Limnology and Oceanography. ISSN 0024-3590. 70(52), p. 555–568. doi: 10.1002/lno.70027. Full text in Research Archive Show summary

  Eutrophication in the Baltic Sea has caused an imbalance in the inorganic nitrogen (N) to phosphorus (P) ratio, leaving excess phosphate (PO4) after the phytoplankton spring bloom that terminates after N depletion. Using monitoring data, we demonstrated that the PO4 concentration has continued to increase in the outermost Gulf of Finland during past decades. We further investigated the fate of such excess PO4 in a two-week mesocosm (1.2 m3) experiment. The starting concentration of PO4 was 0.66 μM, and treatments included a non-treated control (control), nitrate addition (N-add; 3.6 μM), glucose addition (C-add; 36 μM) and combined nitrate and glucose addition (N + C-add). The addition of N, both in N-add and N + C-add treatments, stimulated nano- and microphytoplankton, while the picophytoplankton abundance increased after N depletion. Also, the copepod biomass was positively affected by the N addition. N2-fixing cyanobacteria were present but in low abundance. Carbon addition did not enhance heterotrophic bacterial uptake of PO4 contrary to our expectations, nor did it affect the phyto- or zooplankton community composition. The PO4 concentration was reduced to ~ 0.4 μM in the control and C-add treatments and to 0.16 μM in the two N-amended treatments, with an inorganic N : P uptake ratio of 6.7. These results underscore the role of picophytoplankton in reducing the excess PO4 pool after the spring bloom, a function traditionally ascribed to bloom-forming filamentous cyanobacteria in the Baltic Sea.

• Suessle, Philipp; Taucher, Jan; Goldenberg, Silvan Urs; Baumann, Moritz; Spilling, Kristian & Ferreira, Andrea Noche [Show all 8 contributors for this article] (2025). Particle fluxes by subtropical pelagic communities under ocean alkalinity enhancement. Biogeosciences. ISSN 1726-4170. 22(1), p. 71–86. doi: 10.5194/bg-22-71-2025. Full text in Research Archive Show summary

  Ocean alkalinity enhancement (OAE) has been proposed as a carbon dioxide removal technology (CDR), allowing for long-term storage of carbon dioxide in the ocean. By changing the carbonate speciation in seawater, OAE may potentially alter marine ecosystems with implications for the biological carbon pump. Using mesocosms in the subtropical North Atlantic, we provide first empirical insights into impacts of carbonate-based OAE on the vertical flux and attenuation of sinking particles in an oligotrophic plankton community. We enhanced total alkalinity (TA) in increments of 300 µmol kg−1, reaching up to ΔTA = 2400 µmol kg−1 compared to ambient TA. We applied a pCO2-equilibrated OAE approach; i.e., dissolved inorganic carbon (DIC) was raised simultaneously with TA to maintain seawater pCO2 in equilibrium with the atmosphere, thereby keeping perturbations of seawater carbonate chemistry moderate. The vertical flux of major elements, including carbon, nitrogen, phosphorus, and silicon, as well as their stoichiometric ratios (e.g., carbon-to-nitrogen ratios), remained unaffected over 29 d of OAE. The particle properties controlling the flux attenuation, including sinking velocities and remineralization rates, also remained unaffected by OAE. However, we observed abiotic mineral precipitation at high OAE levels (ΔTA = 1800 µmol kg−1 and higher) that resulted in a substantial increase in particulate inorganic carbon (PIC) formation. The associated consumption of alkalinity reduces the efficiency of CO2 removal and emphasizes the importance of maintaining OAE within a carefully defined operating range. Our findings suggest that carbon export by oligotrophic plankton communities is insensitive to OAE perturbations using a CO2 pre-equilibrated approach. The integrity of ecosystem services is a prerequisite for large-scale application and should be further tested across a variety of nutrient regimes and for less idealized OAE approaches.

• Goldenberg, Silvan Urs; Spisla, Carsten; Sanchez, Nicolas; Taucher, Jan; Spilling, Kristian & Sswat, Michael [Show all 12 contributors for this article] (2024). Diatom-mediated food web functioning under ocean artificial upwelling. Scientific Reports. 14(1), p. 1–10. doi: 10.1038/s41598-024-54345-w. Full text in Research Archive Show summary

  Enhancing ocean productivity by artificial upwelling is evaluated as a nature-based solution for food security and climate change mitigation. Fish production is intended through diatom-based plankton food webs as these are assumed to be short and efficient. However, our findings from mesocosm experiments on artificial upwelling in the oligotrophic ocean disagree with this classical food web model. Here, diatoms did not reduce trophic length and instead impaired the transfer of primary production to crustacean grazers and small pelagic fish. The diatom-driven decrease in trophic efficiency was likely mediated by changes in nutritional value for the copepod grazers. Whilst diatoms benefitted the availability of essential fatty acids, they also caused unfavorable elemental compositions via high carbon-to-nitrogen ratios (i.e. low protein content) to which the grazers were unable to adapt. This nutritional imbalance for grazers was most pronounced in systems optimized for CO2 uptake through carbon-to-nitrogen ratios well beyond Redfield. A simultaneous enhancement of fisheries production and carbon sequestration via artificial upwelling may thus be difficult to achieve given their opposing stoichiometric constraints. Our study suggest that food quality can be more critical than quantity to maximize food web productivity during shorter-term fertilization of the oligotrophic ocean.

• Bansfield, D.; Spilling, Kristian; Mikola, A. & Piiparinen, J. (2024). An investigation into the ability of three fungi and one yeast to grow and capture nutrients in cheese whey. Bioresource Technology Reports. 26, p. 1–7. doi: 10.1016/j.biteb.2024.101854. Full text in Research Archive
• Borg-Stoveland, Stian; Draganovic, Vukasin; Spilling, Kristian & Gabrielsen, Tove M. (2024). Successful growth of coastal marine microalgae in wastewater from a salmon recirculating aquaculture system. Journal of Applied Phycology. ISSN 0921-8971. 36, p. 2851–2861. doi: 10.1007/s10811-024-03310-1. Full text in Research Archive Show summary

  As global demand for seafood increases, recirculating aquaculture systems (RAS) have gained prominence for sustainable fish rearing. The sustainability of RAS still requires improvement, particularly managing the fish waste. Here we investigated the growth and nutrient removal capabilities of three microalgal species (Isocrysis galbana, Phaeodactylum tricornutum and Skeletonema marinoi) in aquaculture wastewater (AWW) mixed at different concentrations with cultivation medium. All three microalgae showed growth in different concentrations of the AWW obtained from an Atlantic salmon RAS facility in Agder, Norway. The average growth rates for I. galbana, S. marinoi and P. tricornutum at 75% AWW concentration were 0.31± 0.00 day-1, 0.34 ± 0.00 day-1, 0.25 ± 0.02 day-1, respectively. All three species effectively contributed to nitrate, ammonium and phosphate removal. When cultivated in 75% AWW, the microalgae achieved nearly complete removal of nitrite, nitrate, and phosphate, while approximately 90% of ammonium was also removed. Our results confirm the viability of AWW for microalgal cultivation on a laboratory scale suggesting this presents a sustainable route to further develop a circular bioeconomy in aquaculture.

• Vanharanta, Mari; Santoro, Mariano; Villena-Alemany, Cristian; Piiparinen, Jonna; Piwosz, Kasia & Grossart, Hans-Peter [Show all 8 contributors for this article] (2024). Microbial remineralization processes during postspring-bloom with excess phosphate available in the northern Baltic Sea. FEMS Microbiology Ecology. ISSN 0168-6496. 100(8). doi: 10.1093/femsec/fiae103. Full text in Research Archive
• Spilling, Kristian; Heinemann, Malte; Vanharanta, Mari; Baumann, Moritz; Ferreira, Andrea Noche & Suessle, Philipp [Show all 7 contributors for this article] (2023). Respiration rate scales inversely with sinking speed of settling marine aggregates. PLOS ONE. 18(3). doi: 10.1371/journal.pone.0282294. Full text in Research Archive Show summary

  Sinking marine aggregates have been studied for a long time to understand their role in carbon sequestration. Traditionally, sinking speed and respiration rates have been treated as independent variables, but two recent papers suggest that there is a connection albeit in contrasting directions. Here we collected recently formed (<2 days old) aggregates from sediment traps mounted underneath mesocosms during two different experiments. The mesocosms were moored off Gran Canaria, Spain (~ 27.9 N; 15.4 E) in a coastal, sub-tropical and oligotrophic ecosystem. We determined the respiration rates of organisms (mainly heterotrophic prokaryotes) attached to aggregates sinking at different velocities. The average respiration rate of fast sinking aggregates (>100 m d-1) was 0.12 d-1 ± 0.08 d-1 (SD). Slower sinking aggregates (<50 m d-1) had on average higher (p <0.001) and more variable respiration rates (average 0.31 d-1 ± 0.16 d-1, SD). There was evidence that slower sinking aggregates had higher porosity than fast sinking aggregates, and we hypothesize that higher porosity increase the settlement area for bacteria and the respiration rate. These findings provide insights into the efficiency of the biological carbon pump and help resolve the apparent discrepancy in the recent studies of the correlation between respiration and sinking speed.

• Bansfield, D.; Spilling, Kristian; Mikola, A. & Piiparinen, J. (2023). Growth of fungi and yeasts in food production waste streams: a feasibility study. BMC Microbiology. 23(1). doi: 10.1186/s12866-023-03083-6. Full text in Research Archive Show summary

  Food production produces nutrient-rich waste streams which, depending on local legislation, are either sent to wastewater treatment plants or discharged into the environment. In addition to causing environmental harm in the second instance, valuable nutrients are lost. A more circular approach would be to reuse these waste streams. Fungi and yeasts are ideal candidates as they require lots of organic carbon (which is especially high in food production waste streams) for growth, with the potential for producing value-added biomass. Here, we tested the metabolic activity and possible growth of seven fungi and three yeasts in five different food production waste streams. Initial tests were done to find the most promising waste streams for growth and these were chosen for further study. All species were then cultured in these waste streams and oxygen uptake was measured to gauge metabolic activity which we used as a proxy for growth rate. Pelletization’s effect on metabolic rates was tested on the most pellet-forming species, by adding agar to inhibit pellet formation. The most promising waste stream for yeast/fungal growth was cheese whey (Whey). Pellet inhibition (i.e., filamentous growth) resulted in increased metabolic activity of cells in the confectionary bakery waste stream with agar but decreased metabolic activity in Whey with agar. The best-growing species, Geotrichum candidum, has potential commercial value as a producer of enzymes, biochemicals and lipids and could provide added value while improving the circularity of water and nutrients in food production.

• Spilling, Kristian; Piiparinen, Jonna; Achterberg, Eric P.; Arístegui, Javier; Bach, Lennart T. & Camarena-Gómez, Maria T. [Show all 13 contributors for this article] (2023). Extracellular enzyme activity in the coastal upwelling system off Peru: A mesocosm experiment. Biogeosciences. ISSN 1726-4170. 20(8), p. 1605–1619. doi: 10.5194/bg-20-1605-2023. Full text in Research Archive Show summary

  The Peruvian upwelling system is a highly productive ecosystem with a large oxygen minimum zone (OMZ) close to the surface. In this work, we carried out a mesocosm experiment off Callao, Peru, with the addition of water masses from the regional OMZ collected at two different sites simulating two different upwelling scenarios. Here, we focus on the pelagic remineralization of organic matter by the extracellular enzyme activity of leucine aminopeptidase (LAP) and alkaline phosphatase activity (APA). After the addition of the OMZ water, dissolved inorganic nitrogen (N) was depleted, but the standing stock of phytoplankton was relatively high, even after N depletion (mostly > 4 µg chlorophyll a L−1). During the initial phase of the experiment, APA was 0.6 nmol L−1 h−1 even though the PO concentration was > 0.5 µmol L−1. Initially, the dissolved organic phosphorus (DOP) decreased, coinciding with an increase in the PO concentration that was probably linked to the APA. The LAP activity was very high, with most of the measurements in the range of 200–800 nmol L−1 h−1. This enzyme hydrolyzes terminal amino acids from larger molecules (e.g., peptides or proteins), and these high values are probably linked to the highly productive but N-limited coastal ecosystem. Moreover, the experiment took place during a rare coastal El Niño event with higher than normal surface temperatures, which could have affected enzyme activity. Using a nonparametric multidimensional scaling analysis (NMDS) with a generalized additive model (GAM), we found that biogeochemical variables (e.g., nutrient and chlorophyll-a concentrations) and phytoplankton and bacterial communities explained up to 64 % of the variability in APA. The bacterial community best explained the variability (34 %) in LAP. The high hydrolysis rates for this enzyme suggest that pelagic N remineralization, likely driven by the bacterial community, supported the high standing stock of primary producers in the mesocosms after N depletion.

• Spilling, Kristian; Martín, Mirian Arellano San; Granlund, Mira; Schulz, Kai G.; Spisla, Carsten & Vanharanta, Mari [Show all 8 contributors for this article] (2023). Microzooplankton communities and their grazing of phytoplankton under artificial upwelling in the oligotrophic ocean. Frontiers in Marine Science. 10. doi: 10.3389/fmars.2023.1286899. Full text in Research Archive Show summary

  Ocean artificial upwelling has been suggested to boost primary production and increase harvestable resources such as fish. Yet, for this ecosystem-based approach to work, an effective energy transfer up the food web is required. Here, we studied the trophic role of microzooplankton under artificial upwelling via biomass and community composition as well as grazing rates on phytoplankton. Using mesocosms in the oligotrophic ocean, we supplied nutrient-rich deep water at varying intensities (low to high) and addition modes (a Singular large pulse or smaller Recurring pulses). Deep-water fertilization created a diatom-dominated bloom that scaled with the amount of inorganic nutrients added, but also Synechococcus-like cells, picoeukaryotes and nanophytoplankton increased in abundance with added nutrients. After 30 days, towards the end of the experiment, coccolithophores bloomed under recurring upwelling of high intensity. Across all upwelling scenarios, the microzooplankton community was dominated by ciliates, dinoflagellates (mixo- and heterotrophic) and radiolarians. Under the highest upwelling intensity, the average grazing rates of Synechococcus-like cells, picoeukaryotes and nanophytoplankton by microzooplankton were 0.35 d-1 ± 0.18 (SD), 0.09 d-1 ± 0.12 (SD), and 0.11 d-1 ± 0.13 (SD), respectively. There was little temporal variation in grazing of nanophytoplankton but grazing of Synechococcus-like cells and picoeukaryotes were more variable. There were positive correlations between abundance of these groups and grazing rates, suggesting a response in the microzooplankton community to prey availability. The average phytoplankton to microzooplankton ratio (biovolume) increased with added deep water, and this increase was highest in the Singular treatment, reaching ~30 (m3 m-3), whereas the phytoplankton to total zooplankton biomass ratio (weight) increased from ~1 under low upwelling to ~6 (g g-1) in the highest upwelling but without a difference between the Singular and the Recurring mode. Several smaller, recurring upwelling events increased the importance of microzooplankton compared with one large pulse of deep water. Our results demonstrate that microzooplankton would be an important component for trophic transfer if artificial upwelling would be carried out at scale in the oligotrophic ocean.

• Vanharanta, Mari & Spilling, Kristian (2023). Uptake of excess phosphate at low inorganic N:P ratio in a coastal sea afflicted with eutrophication. Marine Ecology Progress Series. ISSN 0171-8630. 718, p. 23–37. doi: 10.3354/meps14391. Full text in Research Archive Show summary

  Following the spring bloom in the northern Baltic Sea, nitrogen limits phytoplankton growth and there is typically a residual phosphate concentration (>0.2 µmol l-1) remaining that is often assumed to induce the recurring blooms of nitrogen-fixing cyanobacteria. However, these cyanobacterial blooms typically occur 2-3 mo later during the summer, when the phosphate concentration has been depleted, and it is unclear what organisms take up the excess phosphate. We studied the removal potential of excess phosphate (0.55 µmol l-1) at different temperatures (10, 13, 16°C) and with or without nitrogen addition in an indoor 20 l tank experiment. In addition, we followed the element pools and plankton community composition. As expected, the phosphate uptake rate was up to 3-fold faster in nitrogen-amended than non-amended tanks, but complete drawdown of phosphate also occurred under severe nitrogen limitation. The uptake ratio of dissolved inorganic nitrogen to phosphorus was 4.6, which is substantially lower than the Redfield ratio (16) and indicates excessive phosphate removal potential relative to nitrogen. A large part of the excess phosphate ended up in the particulate pool, which has a higher potential to sink out from the surface. The nitrogen-fixing cyanobacteria Nodularia spumigena grew close to summer bloom concentrations only in the highest experimental temperature. However, the combined biovolume of all 3 major bloom-forming cyanobacteria accounted for only 5.3% of the total autotropic biovolume, and their potential phosphate uptake was calculated to be <3% of the excess phosphate available at the beginning of the study. Therefore, our results demonstrate that the contribution of filamentous cyanobacteria to the removal of excess phosphate is small.

• Paul, Allanah Joy; Bach, Lennart Thomas; Arístegui, Javier; Esch, Elisabeth Von Der; Hernández-Hernández, Nauzet & Piiparinen, Jonna [Show all 9 contributors for this article] (2022). Upwelled plankton community modulates surface bloom succession and nutrient availability in a natural plankton assemblage. Biogeosciences. ISSN 1726-4170. 19(24), p. 5911–5926. doi: 10.5194/bg-19-5911-2022. Full text in Research Archive Show summary

  Upwelling of nutrient-rich waters into the sunlit surface layer of the ocean supports high primary productivity in eastern boundary upwelling systems (EBUSs). However, subsurface waters contain not only macronutrients (N, P, Si) but also micronutrients, organic matter and seed microbial communities that may modify the response to macronutrient inputs via upwelling. These additional factors are often neglected when investigating upwelling impacts on surface ocean productivity. Here, we investigated how different components of upwelled water (macronutrients, organic nutrients and seed communities) drive the response of surface plankton communities to upwelling in the Peruvian coastal zone. Results from our short-term (10 d) study show that the most influential drivers in upwelled deep water are (1) the ratio of inorganic nutrients (NOx : PO) and (2) the microbial community present that can seed heterogeneity in phytoplankton succession and modify the stoichiometry of residual inorganic nutrients after phytoplankton blooms. Hence, this study suggests that phytoplankton succession after upwelling is modified by factors other than the physical supply of inorganic nutrients. This would likely affect trophic transfer and overall productivity in these highly fertile marine ecosystems.

• Camarena-Gómez, María Teresa; Lähteenmäki-Uutela, Anu & Spilling, Kristian (2022). Macroalgae production in Northern Europe: Business and government perspectives on how to regulate a novel blue bioeconomy. Aquaculture. ISSN 0044-8486. 560. doi: 10.1016/j.aquaculture.2022.738434. Full text in Research Archive Show summary

  Macroalgae biomass production, understood as cultivation and harvesting, is a minor industry in Europe at present, but the sector is recognized as having substantial growth potential. Here, we framed the environmental license as a boundary object between business and authorities and investigated the details of macroalgal licensing procedures in seven Northern European countries (Finland, Estonia, Sweden, Germany, Norway, Ice- land, and Scotland). We conducted surveys and interviews with macroalgae companies and licensing authorities to understand the challenges faced by both sides. Generally, macroalgae production in Northern European countries is regulated by environmental and water laws and is not included in maritime spatial plans. Private actors need to apply for an environmental, water and/ or fishing permit to start operations in this sector, often with several authorities involved. The companies expressed their dissatisfaction with non-specific laws and burdensome licensing procedures that may delay or even prevent the start-up. The authorities highlighted the lack of scientific environmental risk assessments of macroalgae production and the need to resolve possible conflicts with other marine sectors. Companies need the license to access markets while authorities view the license as a tool to enforce environmental legislation. As a boundary object, the license is the result of correspondence and cooperation between companies and authorities. A one-step licensing procedure for macroalgae production should be applied to encourage this business and to facilitate the compliance of legislation. In addition, macroalgae related activities should be recognized in the national maritime spatial plans to facilitate long-term planning.

• Spilling, Kristian; Asmala, Eero; Haavisto, Noora; Haraguchi, Lumi; Kraft, Kaisa & Lehto, Anne-Mari [Show all 14 contributors for this article] (2022). Brownification affects phytoplankton community composition but not primary productivity in eutrophic coastal waters: A mesocosm experiment in the Baltic Sea. Science of the Total Environment. ISSN 0048-9697. 841, p. 1–9. doi: 10.1016/j.scitotenv.2022.156510. Full text in Research Archive Show summary

  Climate change is projected to cause brownification of some coastal seas due to increased runoff of terrestrially derived organic matter. We carried out a mesocosm experiment (15 d) to test the effect of this on the planktonic ecosystem ex- pecting reduced primary production and shifts in the phytoplankton community composition. The experiment was set up in 2.2 m3 mesocosm bags using four treatments, each with three replicates: control (Contr) without any manipula- tion, organic carbon additive HuminFeed (Hum; 2 mg L−1), inorganic nutrients (Nutr; 5.7 μM NH4 and 0.65 μM PO4), and combined Nutr and Hum (Nutr + Hum) additions. Measured variables included organic and inorganic nutrient pools, chlorophyll a (Chla), primary and bacterial production and particle counts by flow cytometry. The bags with added inorganic nutrients developed a phytoplankton bloom that depleted inorganic N at day 6, followed by a rapid decline in Chla. Brownification did not reduce primary production at the tested concentration. Bacterial production was lowest in the Contr, but similar in the three treatments receiving additions likely due to increased carbon available for heterotrophic bacteria. Picoeukaryotes clearly benefited by brownification after inorganic N depletion, which could be due to more effective nutrient recycling, nutrient affinity, light absorption, or alternatively lower grazing pressure. In conclusion, brownification shifted the phytoplankton community composition towards smaller species with potential effects on carbon fluxes, such as sinking rates and export to the sea floor.

• Spilling, Kristian; Asmala, Eero; Haavisto, Noora; Haraguchi, Lumi; Kraft, Kaisa & Lehto, Anne-Mari [Show all 14 contributors for this article] (2022). Dataset from a mesocosm experiment on brownification in the Baltic Sea. Data in Brief. 45(108755). doi: 10.1016/j.dib.2022.108755. Full text in Research Archive Show summary

  Climate change is projected to cause brownification of some coastal seas due to increased runoff of terrestrially derived organic matter. We carried out a mesocosm experiment over 15 days to test the effect of this on the planktonic ecosystem. The experiment was set up in 2.2 m3 plastic bags moored outside the Tvärminne Zoological Station at the SW coast of Finland. We used four treatments, each with three replicates: control (Contr) without any manipulation; addition of a commercially available organic carbon additive called HuminFeed (Hum; 2 mg L−1); addition of inorganic nutrients (Nutr; 5.7 µM NH4 and 0.65µM PO4); and a final treatment of combined Nutr and Hum (Nutr+Hum) additions. Water samples were taken daily, and measured variables included water transparency, organic and inorganic nutrient pools, chlorophyll a (Chla), primary and bacterial production and particle counts by flow cytometry.

• Lähteenmäki-Uutela, Anu; Rahikainen, Moona; Camarena-Gómez, María Teresa; Piiparinen, Jonna; Spilling, Kristian & Yang, Baoru (2021). European Union legislation on macroalgae products. Aquaculture International. ISSN 0967-6120. 29(2), p. 487–509. doi: 10.1007/s10499-020-00633-x. Full text in Research Archive Show summary

  Macroalgae-based products are increasing in demand also in Europe. In the European Union, each category of macroalgae-based products is regulated separately. We discuss EU legislation, including the law on medicinal products, foods including food supplements and food additives, feed and feed additives, cosmetics, packaging materials, fertilizers and biostimulants, as well as biofuels. Product safety and consumer protection are the priorities with any new products. Macroalgae products can be sold as traditional herbal medicines. The novel food regulation applies to macroalgae foods that have not previously been used as food, and organic macroalgae are a specific regulatory category. The maximum levels of heavy metals may be a barrier for macroalgae foods, feeds, and fertilizers. Getting health claims approved for foods based on macroalgae is demanding. In addition to the rules on products, the macroalgae business is strongly impacted by the elements of the general regulatory environment such as agricultural/aquacultural subsidies, maritime spatial planning and aquaculture licensing, public procurement criteria, tax schemes, and trade agreements.

• Spilling, Kristian; Seppälä, Jukka; Schwenk, Dagmar; Rischer, Heiko & Tamminen, Timo (2021). Variation in the fatty acid profiles of two cold water diatoms grown under different temperature, light, and nutrient regimes. Journal of Applied Phycology. ISSN 0921-8971. 33(3), p. 1447–1455. doi: 10.1007/s10811-021-02380-9. Full text in Research Archive
• Baumann, Moritz; Taucher, Jan; Paul, Allanah J.; Heinemann, Malte; Vanharanta, Mari & Bach, Lennart T. [Show all 12 contributors for this article] (2021). Effect of Intensity and Mode of Artificial Upwelling on Particle Flux and Carbon Export. Frontiers in Marine Science. 8. doi: 10.3389/fmars.2021.742142. Full text in Research Archive
• Camarena-Gómez, María Teresa; Ruiz-González, Clara; Piiparinen, Jonna; Lipsewers, Tobias; Sobrino, Cristina & Logares, Ramiro [Show all 7 contributors for this article] (2021). Bacterioplankton dynamics driven by interannual and spatial variation in diatom and dinoflagellate spring bloom communities in the Baltic Sea. Limnology and Oceanography. ISSN 0024-3590. 66(1), p. 255–271. doi: 10.1002/lno.11601. Full text in Research Archive Show summary

  In parts of the Baltic Sea, the phytoplankton spring bloom communities, commonly dominated by diatoms, are shifting toward the co-occurrence of diatoms and dinoflagellates. Although phytoplankton are known to shape the composition and function of associated bacterioplankton communities, the potential bacterial responses to such a decrease of diatoms are unknown. Here we explored the changes in bacterial communities and heterotrophic production during the spring bloom in four consecutive spring blooms across several sub-basins of the Baltic Sea and related them to changes in environmental variables and in phytoplankton community structure. The taxonomic structure of bacterioplankton assemblages was partially explained by salinity and temperature but also linked to the phytoplankton community. Higher carbon biomass of the diatoms Achnanthes taeniata, Skeletonema marinoi, Thalassiosira levanderi, and Chaetoceros spp. was associated with more diverse bacterial communities dominated by copiotrophic bacteria (Flavobacteriia, Gammaproteobacteria, and Betaproteobacteria) and higher bacterial production. During dinoflagellate dominance, bacterial production was low and bacterial communities were dominated by Alphaproteobacteria, mainly SAR11. Our results suggest that increases in dinoflagellate abundance during the spring bloom will largely affect the structuring and functioning of the associated bacterial communities. This could decrease pelagic remineralization of organic matter and possibly affect the bacterial grazers communities.

• Bansfield, Danielle; Spilling, Kristian; Mikola, Anna & Piiparinen, Jonna (2021). Bioflocculation of Euglena gracilis via direct application of fungal filaments: a rapid harvesting method. Journal of Applied Phycology. ISSN 0921-8971. 34, p. 321–334. doi: 10.1007/s10811-021-02651-5. Full text in Research Archive
• Bach, Lennart Thomas; Paul, Allanah Joy; Boxhammer, Tim; Esch, Elisabeth von der; Graco, Michelle & Schulz, Kai Georg [Show all 46 contributors for this article] (2020). Factors controlling plankton community production, export flux, and particulate matter stoichiometry in the coastal upwelling system off Peru. Biogeosciences. ISSN 1726-4170. 17(19), p. 4831–4852. doi: 10.5194/bg-17-4831-2020. Full text in Research Archive
• Piwosz, Kasia; Vrdoljak, Ana; Frenken, Thijs; Gonzalez-Olalla, Juan Manuel; Šantić, Danijela & McKay, R. Michael [Show all 17 contributors for this article] (2020). Erratum for Piwosz et al., “Light and Primary Production Shape Bacterial Activity and Community Composition of Aerobic Anoxygenic Phototrophic Bacteria in a Microcosm Experiment”. mSphere. 5(4). doi: 10.1128/mSphere.00673-20. Full text in Research Archive
• Piwosz, Kasia; Vrdoljak, Ana; Frenken, Thijs; González-Olalla, Juan Manuel; Šantić, Danijela & McKay, R. Michael [Show all 17 contributors for this article] (2020). Light and primary production shape bacterial activity and community composition of aerobic anoxygenic phototrophic bacteria in a microcosm experiment. mSphere. 5(4). doi: 10.1128/MSPHERE.00354-20. Full text in Research Archive

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• Spilling, Kristian; Asmala, Eero; Haavisto, Noora; Haraguchi, Lumi; Kraft, Kaisa & Lehto, Anne-Mari [Show all 14 contributors for this article] (2023). Corrigendum to “Brownification affects phytoplankton community composition but not primary productivity in eutrophic coastal waters: A mesocosm experiment in the Baltic Sea” [Sci. Total Environ. 841 (2022): 156510] (Science of the Total Environment (2022) 841, (S0048969722036075), (10.1016/j.scitotenv.2022.156510)). Science of the Total Environment. ISSN 0048-9697. 871. doi: 10.1016/j.scitotenv.2023.162040. Full text in Research Archive
• Spilling, Kristian; Tedesco, Letizia; Klais, Riina & Olli, Kalle (2019). Editorial: Changing plankton communities: Causes, effects and consequences. Frontiers in Marine Science. 6(May). doi: 10.3389/fmars.2019.00272. Full text in Research Archive

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