Differential Responses of Eukaryotic Phytoplankton to Nitrogenous Nutrients in the North Pacific Subtropical Gyre

Abstract

Fixed inorganic nitrogen (N) is persistently scarce in the well-lit regions of the subtropical ocean gyres and its supply plays an important role in controlling phytoplankton productivity. In a series of experiments conducted in the North Pacific Subtropical Gyre (NPSG), we examined changes in primary productivity and eukaryotic phytoplankton community structure in response to additions of nitrate (NO − 3 ), ammonium (NH + 4 ), or urea in conjunction with phosphate and silicic acid. Overall, we observed large increases in rates of 14 C-primary productivity (∼2-to 27-fold) and concentrations of chlorophyll a (∼2-to 7-fold) following N addition. Much of the physiological response was due to larger (>3 µm) phytoplankton, whose contributions to primary productivity increased ∼2-fold while picophytoplankton (0.2-3 µm) contributions decreased by a similar proportion. Five experiments, conducted in the spring, summer and winter, revealed apparent seasonally-dependent responses in phytoplankton community structure to N availability. During the summer, pennate diatoms increased significantly following N addition as evidenced by both photosynthetic pigment analyses and high-throughput sequencing of 18S rDNA. For example, following the addition of N substrates, concentrations of fucoxanthin (a diatom pigment biomarker) increased between 23-and 49-fold, and relative abundances of rRNA genes belonging to Pseudo-nitzschia increased from negligible (∼0.3%) to 30-60% of the >3 µm phytoplankton assemblage. However, unlike the diatom-driven responses observed in the summer, experiments conducted in the spring and winter demonstrated large increases in concentrations of 19 ′ -butanoyloxyfucoxanthin (a pelagophyte pigment biomarker) together with increases in the relative abundance of rRNA genes clustering among Pelagomonas. Overall, our findings revealed differences in the responses of major taxa during experiments conducted in different times of the year, with concomitant impacts on patterns of phytoplankton diversity. In addition, the overall responses in chlorophyll a, 14 C-primary production, and eukaryotic phytoplankton community composition appeared largely independent of the type of N substrate added. Our results highlight seasonal-scale differences on the role of N availability in shaping eukaryotic phytoplankton diversity in the surface waters of the oligotrophic NPSG.

NanoSIMS single cell analyses reveal the contrasting nitrogen sources for small phytoplankton

“…3) suggests regulation by light availability, as previously hypothesized [10,74]. In addition, the nature of the N sources available are suspected to affect planktonic community structure, particularly in regions where N limits primary production such as in the NPSG [75][76][77][78]. Our results highlight group specific N strategies that are in line with this theory.…”

Cross-basin differences in the nutrient assimilation characteristics of induced phytoplankton blooms in the subtropical Pacific waters

“…The induced phytoplankton blooms in this study were accompanied by changes in community structure. Several nutrient enrichment experiments conducted in the subtropical oceans have demonstrated significant increases in eukaryotic phytoplankton following nutrient enrichments, particularly of N (Bonnet et al, 2008;Moore et al, 2008;Mahaffey et al, 2012;Shilova et al, 2017;Rii et al, 2018;Lampe et al, 2019;Robidart et al, 2019). Similar blooms dominated by eukaryotic phytoplankton were also observed at most stations in this study as evidenced by the Tfuco increases (Fig.…”

Response of pico-nano-eukaryotes to inorganic and organic nutrient additions

“…As previously reported, the response of the microbial community to controlled nutrient manipulation largely differs depending of the sampling period Rii et al, 2018;Teira et al, 2015Teira et al, , 2011. The 200 m prefiltration to exclude large zooplankton probably altered the small eukaryote community composition during the incubations because of the strong grazing pressure over the smallest plankton components (Burkill et al, 1987;Caron et al, 2000;Escaravage and Prins, 2002;Landry and Hassett, 1982;Lignell et al, 2003;.…”