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1. Inorganic and organic carbon and nitrogen uptake strategies of picoplankton groups in the northwestern Atlantic Ocean

   https://doi.org/10.1002/lno.11909  

   Berthelot, Hugo; Duhamel, Solange; L'Helguen, Stéphane; Maguer, Jean‐François; Cassar, Nicolas (August 2021, Limnology and Oceanography)

   Abstract Picoplankton populations dominate the planktonic community in the surface oligotrophic ocean. Yet, their strategies in the acquisition and the partitioning of organic and inorganic sources of nitrogen (N) and carbon (C) are poorly described. Here, we measured at the single‐cell level the uptake of dissolved inorganic C (C‐fixation), C‐leucine, N‐leucine, nitrate (NO₃⁻), ammonium (NH₄⁺), and N‐urea in pigmented and nonpigmented picoplankton groups at six low‐N stations in the northwestern Atlantic Ocean. Our study highlights important differences in trophic strategies betweenProchlorococcus,Synechococcus, photosynthetic pico‐eukaryotes, and nonpigmented prokaryotes. Nonpigmented prokaryotes were characterized by high leucine uptake rates, nonsignificant C‐fixation and relatively low NH₄⁺, N‐urea, and NO₃⁻uptake rates. Nonpigmented prokaryotes contributed to 7% ± 3%, 2% ± 2%, and 9% ± 5% of the NH₄⁺, NO₃⁻, and N‐urea community uptake, respectively. In contrast, pigmented groups displayed relatively high C‐fixation rates, NH₄⁺and N‐urea uptake rates, but lower leucine uptake rates than nonpigmented prokaryotes.Synechococcusand photosynthetic pico‐eukaryotes NO₃⁻uptake rates were higher thanProchlorococcusones. Pico‐sized pigmented groups accounted for a significant fraction of the community C‐fixation (63% ± 27%), NH₄⁺uptake (47% ± 27%), NO₃⁻uptake (62% ± 49%), and N‐urea uptake (81% ± 35%). Interestingly,Prochlorococcusand photosynthetic pico‐eukaryotes showed a greater reliance on C‐ and N‐leucine thanSynechococcuson average, suggesting a greater reliance on organic C and N sources. Taken together, our single‐cell results decipher the wide diversity of C and N trophic strategies between and within marine picoplankton groups, but a clear partitioning between pigmented and nonpigmented groups still remains. 

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2. Live cell analysis at sea reveals divergent thermal performance between photosynthetic ocean microbial eukaryote populations

   https://doi.org/10.1038/s41396-019-0355-6  

   McInnes, Allison Skinner; Laczka, Olivier F.; Baker, Kirralee G.; Larsson, Michaela E.; Robinson, Charlotte M.; Clark, Jennifer S.; Laiolo, Leonardo; Alvarez, Marco; Laverock, Bonnie; Kremer, Colin T.; et al (January 2019, The ISME Journal)

   Abstract Experimentation at sea provides insight into which traits of ocean microbes are linked to performance in situ. Here we show distinct patterns in thermal tolerance of microbial phototrophs from adjacent water masses sampled in the south-west Pacific Ocean, determined using a fluorescent marker for reactive oxygen species (ROS). ROS content of pico-eukaryotes was assessed after 1, 5 and 25 h of incubation along a temperature gradient (15.6–32.1 °C). Pico-eukaryotes from the East Australian Current (EAC) had relatively constant ROS and showed greatest mortality after 25 h at 7 °C below ambient, whereas those from the Tasman Sea had elevated ROS in both warm and cool temperature extremes and greatest mortality at temperatures 6–10 °C above ambient, interpreted as the outcome of thermal stress. Tracking of water masses within an oceanographic circulation model showed populations had distinct thermal histories, with EAC pico-eukaryotes experiencing higher average temperatures for at least 1 week prior to sampling. While acclimatization and community assembly could both influence biological responses, this study clearly demonstrates that phenotypic divergence occurs along planktonic drift trajectories. 

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3. Time series of phytoplankton net primary production reveals intense interannual variability and size‐dependent chlorophyll‐specific productivity on a continental shelf

   https://doi.org/10.1002/lno.12749  

   Fontaine, Diana N; Marrec, Pierre; Menden‐Deuer, Susanne; Sosik, Heidi M; Rynearson, Tatiana A (January 2025, Limnology and Oceanography)

   Abstract Phytoplankton community size structure influences the production and fate of organic carbon in marine food webs and can undergo strong seasonal shifts in temperate regions. As part of the Northeast US Shelf (NES) Long‐Term Ecological Research program, we measured net primary production (NPP) rates and chlorophylla(Chla) concentrations in three phytoplankton size classes (< 5, 5–20, and > 20 μm) during winter and summer for 3 yr along a coastal‐to‐offshore transect. Mean depth‐integrated NPP was 37% higher in summer than winter, with limited cross‐shelf differences because of significant interannual variability. When averaged across the shelf, depth‐integrated NPP was dominated by the > 20 μm size class in winter and generated equally by the three size fractions in summer because of substantial contributions from cells > 20 μm at the Chlamaximum depth. Furthermore, the relationship between Chlaand NPP, in terms of relative contributions, varied by size class. Variations in this relationship have implications for models of primary productivity on the NES and beyond. In comparison to historical NPP data, we identified equivalent levels of winter NPP but observed a 25% decrease in summer NPP, suggesting a potential reduction in the seasonality of NPP on the NES. Together, our results highlight seasonal shifts in NPP rates of different phytoplankton size classes, with implications for food web structure and export production. These data emphasize the importance of quantifying size‐fractionated NPP over time to constrain its variability and better predict the fate of organic carbon in coastal systems under environmental change. 

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4. Biological composition and microbial dynamics of sinking particulate organic matter at abyssal depths in the oligotrophic open ocean

   https://doi.org/10.1073/pnas.1903080116  

   Boeuf, Dominique; Edwards, Bethanie R.; Eppley, John M.; Hu, Sarah K.; Poff, Kirsten E.; Romano, Anna E.; Caron, David A.; Karl, David M.; DeLong, Edward F. (January 2019, Proceedings of the National Academy of Sciences)

   Sinking particles are a critical conduit for the export of organic material from surface waters to the deep ocean. Despite their importance in oceanic carbon cycling and export, little is known about the biotic composition, origins, and variability of sinking particles reaching abyssal depths. Here, we analyzed particle-associated nucleic acids captured and preserved in sediment traps at 4,000-m depth in the North Pacific Subtropical Gyre. Over the 9-month time-series, Bacteria dominated both the rRNA-gene and rRNA pools, followed by eukaryotes (protists and animals) and trace amounts of Archaea. Deep-sea piezophile-like Gammaproteobacteria, along with Epsilonproteobacteria, comprised >80% of the bacterial inventory. Protists (mostly Rhizaria, Syndinales, and ciliates) and metazoa (predominantly pelagic mollusks and cnidarians) were the most common sinking particle-associated eukaryotes. Some near-surface water-derived eukaryotes, especially Foraminifera, Radiolaria, and pteropods, varied greatly in their abundance patterns, presumably due to sporadic export events. The dominance of piezophile-like Gammaproteobacteria and Epsilonproteobacteria, along with the prevalence of their nitrogen cycling-associated gene transcripts, suggested a central role for these bacteria in the mineralization and biogeochemical transformation of sinking particulate organic matter in the deep ocean. Our data also reflected several different modes of particle export dynamics, including summer export, more stochastic inputs from the upper water column by protists and pteropods, and contributions from sinking mid- and deep-water organisms. In total, our observations revealed the variable and heterogeneous biological origins and microbial activities of sinking particles that connect their downward transport, transformation, and degradation to deep-sea biogeochemical processes. 

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5. Differential clearance rates of microbial phylotypes by four appendicularian species

   https://doi.org/10.3354/meps14233  

   Dadon-Pilosof, A; Conley, K; Lombard, F; Sutherland, KR; Genin, A; Richter, M; Glöckner, FO; Yahel, G (February 2023, Marine Ecology Progress Series)

   Appendicularians are abundant planktonic filter feeders that play a significant role in the pelagic food web due to their high clearance rates. Their diet and feeding rates have typically been measured as bulk chlorophyll or cell removal, with some attention given to prey size but no differentiation between the microbial phylotypes. Using a combination of in situ and laboratory incubations with flow cytometry and next-generation sequencing, we found species-specific differences in clearance rates and diet compositions of 4 common species: Oikopleura albicans , O. fusiformis , O. longicauda , and O. dioica . While O. albicans most efficiently removed nano-eukaryotic algae, the other smaller species preferentially removed micron-sized pico-eukaryotic algae. Pico- and nano-eukaryotic cells constituted the major food source of the studied appendicularians despite their occurrence in oligotrophic water dominated by prokaryotic cells. Across species, pico- and nano-planktonic microalgae biomass comprised 45 to 75% of the appendicularian diets. Although non-photosynthetic bacteria were removed at lower rates than all other prey groups, their total contribution to the appendicularian diet was not trivial, representing 5 to 19% of the planktonic carbon in the appendicularian diet; pico-cyanobacteria contributed an additional 9 to 18%. Removal rates and efficiencies of pico-eukaryotes were higher than those of prokaryotes of similar size. Strikingly different clearance rates were observed for different prokaryotic phylotypes, indicating that factors other than size are involved in determining the capturability of the cells. Collectively, our findings provide additional evidence for differential retention of microbial prey among mucous-mesh grazers and its substantial effect on the upper-ocean microbial community. 

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