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

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 between Prochlorococcus, 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. Synechococcus and photosynthetic pico‐eukaryotes NO3− uptake rates were higher than Prochlorococcus ones. 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, Prochlorococcus and photosynthetic pico‐eukaryotes showed a greater reliance on C‐ and N‐leucine than Synechococcus on 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.

Selective and differential feeding on marine prokaryotes by mucous mesh feeders

“…This study shows that mucous mesh grazers are a source of mortality for a variety of ubiquitous and numerically abundant bacteria and archaea in the surface waters of the Gulf Stream. Marine picocyanobacteria Prochlorococcus and Synechococcus were a component of the prokaryotic prey in salp guts from the Gulf Stream, which are waters representative of major subtropical ocean gyres where these taxa make substantial contributions to primary production (Berthelot et al, 2021). The ubiquitous marine heterotroph Pelagibacter was also in gut samples of both pteropods and salps.…”

Phago-mixotrophy of small eukaryotic phytoplankton might alleviate iron limitation in HNLC Southern Ocean

“…Jardillier et al (2010) and Irion et al (2021) suggest the carbon fixation by small phytoplankton (<20 µm) is dominated by cells with a diameter of 2-3 µm, slightly smaller than nanoeukaryotes size in this study (average diameter of 3.61 µm), but nevertheless indicate intermediate sizes of phytoplankton have higher contribution toward NPP compared to picophytoplankton. Although Synechococcus had a higher cell abundance compared to picoeukaryotes, the relative contribution of photosynthetic cyanobacteria ( Prochlorococcus and Synechococcus ) and picoeukaryotes to productivity are similar across a variety of open ocean conditions, including tropical and subtropical latitudes with oligotrophic and mesotrophic conditions (Berthelot et al 2021; Jardillier et al 2010; Li 1994; Rii et al 2016), and iron-limited SA waters of this study (Figure 2B).…”

Switches between nitrogen limitation and nitrogen–phosphorus co‐limitation in the subtropical North Atlantic Ocean

“…This calculation does not include concentrations of ammonium (not determined), but assuming an upper ammonium concentration of 20 nM (Clark et al 2008) and adding this to concentrations of nitrate, does not greatly change these values (P* range 3–5 nM). Regardless of other accessible organic forms of nitrogen (Zubkov et al 2003, 2008; Berthelot et al 2021), from a stoichiometric perspective based on a fixed N : P requirement of 16 (acknowledging the substantial potential variability in this ratio; Klausmeier et al 2004; Martiny et al 2013), dissolved P was therefore in excess of phytoplankton requirements relative to N. In line with the measured nutrient concentrations, the “standard” bioassay showed very similar responses to those previously conducted in the region (Fig. 1).…”

“…One hypothesis is that because Prochlorococcus is less nutrient limited in situ than Synechococcus , this group had invested less resources into ammonium uptake transporters (García‐Fernández et al 2004) and therefore could not respond as rapidly to increases in intracellular Chl a (and by extension, growth rates). Alternatively, Prochlorococcus may have been optimized to use more abundant organic forms of nitrogen such as urea (Painter et al 2008; Shilova et al 2017) and amino acids (Zubkov et al 2003; Berthelot et al 2021).…”