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Picophytoplankton (<2 µm diameter), a diverse group of picocyanobacteria and photosynthetic picoeukaryotes, are significant contributors to primary production. Predatory mortality controls picophytoplankton biomass and thereby energy transfer in the marine food web. The 2 major pathways of picophytoplankton mortality are grazing and viral lysis. Grazing passes carbon directly to higher trophic levels, while lysis products are passed into the viral loop. Picophytoplankton are abundant in the Baltic Sea but little is known about their predatory mortality. Using a modification of the dilution approach, we calculated growth and mortality rates of picophytoplankton and studied the effect of predation on community structure during late August and September. The experiments were conducted coinciding with the peak in picophytoplankton abundance (∼105 cells ml-1) at the Linnaeus Microbial Observatory in the Baltic Sea Proper. The results showed that grazing is an important controller of picocyanobacteria and photosynthetic picoeukaryote populations, while no significant viral lysis effect was detected. Grazing on picocyanobacteria was proportional to growth rates, while grazing on photosynthetic picoeukaryotes exceeded growth. Selective grazing of phylogenetically distinct picocyanobacterial clades had a significant effect on community structure, suggesting that grazing has an impact on the seasonal dynamics of co-occurring clades. Picocyanobacteria had a higher carbon transfer contribution to higher trophic levels than photosynthetic picoeukaryotes at the time of the experiments. The study shows that picophytoplankton are important contributors to carbon cycling in the Baltic Sea microbial food web and should be considered for future ecological models.

Picophytoplankton (<2 µm diameter), a diverse group of picocyanobacteria and photosynthetic picoeukaryotes, are significant contributors to primary production. Predatory mortality controls picophytoplankton biomass and thereby energy transfer in the marine food web. The 2 major pathways of picophytoplankton mortality are grazing and viral lysis. Grazing passes carbon directly to higher trophic levels, while lysis products are passed into the viral loop. Picophytoplankton are abundant in the Baltic Sea but little is known about their predatory mortality. Using a modification of the dilution approach, we calculated growth and mortality rates of picophytoplankton and studied the effect of predation on community structure during late August and September. The experiments were conducted coinciding with the peak in picophytoplankton abundance (similar to 10(5) cells ml(-1)) at the Linnaeus Microbial Observatory in the Baltic Sea Proper. The results showed that grazing is an important controller of picocyanobacteria and photosynthetic picoeukaryote populations, while no significant viral lysis effect was detected. Grazing on picocyanobacteria was proportional to growth rates, while grazing on photosynthetic picoeukaryotes exceeded growth. Selective grazing of phylogenetically distinct picocyanobacterial clades had a significant effect on community structure, suggesting that grazing has an impact on the seasonal dynamics of co-occurring clades. Picocyanobacteria had a higher carbon transfer contribution to higher trophic levels than photosynthetic picoeukaryotes at the time of the experiments. The study shows that picophytoplankton are important contributors to carbon cycling in the Baltic Sea microbial food web and should be considered for future ecological models.

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Diaz, B. P., Knowles, B., Johns, C. T., Laber, C. P., Bondoc, K. G. V., Haramaty, L., Natale, F., Harvey, E. L., Kramer, S. J., Bolaños, L. M., Lowenstein, D. P., Fredricks, H. F., Graff, J., Westberry, T. K., Mojica, K. D. A., Haëntjens, N., Baetge, N., Gaube, P., Boss, E., Carlson, C. A., Behrenfeld, M. J., Van Mooy, B. A. S., Bidle, K. D. Seasonal mixed layer depth shapes phytoplankton physiology, viral production, and accumulation in the North Atlantic. Nature Communications, 12(1), (2021): 6634, https://doi.org/10.1038/s41467-021-26836-1., Seasonal shifts in phytoplankton accumulation and loss largely follow changes in mixed layer depth, but the impact of mixed layer depth on cell physiology remains unexplored. Here, we investigate the physiological state of phytoplankton populations associated with distinct bloom phases and mixing regimes in the North Atlantic. Stratification and deep mixing alter community physiology and viral production, effectively shaping accumulation rates. Communities in relatively deep, early-spring mixed layers are characterized by low levels of stress and high accumulation rates, while those in the recently shallowed mixed layers in late-spring have high levels of oxidative stress. Prolonged stratification into early autumn manifests in negative accumulation rates, along with pronounced signatures of compromised membranes, death-related protease activity, virus production, nutrient drawdown, and lipid markers indicative of nutrient stress. Positive accumulation renews during mixed layer deepening with transition into winter, concomitant with enhanced nutrient supply and lessened viral pressure., This work was made possible by NASA’s Earth Science Program in support of the North Atlantic Aerosol and Marine Ecosystem Study (15-RRNES15-0011 and 0NSSC18K1563 to K.D.B.; NNX15AF30G to M.J.B.), as well as with support from the National Science Foundation (OIA-2021032 to K.D.B., OCE-157943 to C.A.C., and OCE-1756254 to B.A.S.V.M.), the Gordon and Betty Moore Foundation (Award# 3789 to K.G.V.B.), and NASA’s Future Investigators in Space Science and Technology program (FINESST; grant #826380 to K.D.B.; graduate support to BD).

The picophytoplankton Synechococcus is a globally abundant autotroph that contributes significantly to primary production in the oceans and coastal areas. These cyanobacteria constitute a diverse genus of organisms that have developed independent niche spaces throughout aquatic environments. Here, we use the 16S V3-V4 rRNA gene region and flow cytometry to explore the diversity of Synechococcus within the picophytoplankton community in the Gullmar Fjord, on the west coast of Sweden. We conducted a station-based 1-year time series and two transect studies of the fjord. Our analysis revealed that within the large number of Synechococcus amplicon sequence variants (ASVs; 239 in total), prevalent ASVs phylogenetically clustered with clade representatives in both marine subcluster 5.1 and 5.2. The near-surface composition of ASVs shifted from spring to summer, when a 5.1 subcluster dominated community developed along with elevated Synechococcus abundances up to 9.3 x 10(4) cells ml(-1). This seasonal dominance by subcluster 5.1 was observed over the length of the fjord (25 km), where shifts in community composition were associated with increasing depth. Unexpectedly, the community shift was not associated with changes in salinity. Synechococcus abundance dynamics also differed from that of the photosynthetic picoeukaryote community. These results highlight how seasonal variations in environmental conditions influence the dynamics of Synechococcus clades in a high latitude threshold fjord.

Seasonal shifts in phytoplankton accumulation and loss largely follow changes in mixed layer depth, but the impact of mixed layer depth on cell physiology remains unexplored. Here, we investigate the physiological state of phytoplankton populations associated with distinct bloom phases and mixing regimes in the North Atlantic. Stratification and deep mixing alter community physiology and viral production, effectively shaping accumulation rates. Communities in relatively deep, early-spring mixed layers are characterized by low levels of stress and high accumulation rates, while those in the recently shallowed mixed layers in late-spring have high levels of oxidative stress. Prolonged stratification into early autumn manifests in negative accumulation rates, along with pronounced signatures of compromised membranes, death-related protease activity, virus production, nutrient drawdown, and lipid markers indicative of nutrient stress. Positive accumulation renews during mixed layer deepening with transition into winter, concomitant with enhanced nutrient supply and lessened viral pressure. Phytoplankton are important primary producers. Here the authors investigate phytoplankton physiological changes associated with bloom phases and mixing regimes in the North Atlantic, finding that stratification and deep mixing shape accumulation rates by altering physiology and viral production.