Phosphorus is an obligate requirement for the growth of all organisms; major biochemical reservoirs of phosphorus in marine plankton include nucleic acids and phospholipids. However, eukaryotic phytoplankton and cyanobacteria (that is, 'phytoplankton' collectively) have the ability to decrease their cellular phosphorus content when phosphorus in their environment is scarce. The biochemical mechanisms that allow phytoplankton to limit their phosphorus demand and still maintain growth are largely unknown. Here we show that phytoplankton, in regions of oligotrophic ocean where phosphate is scarce, reduce their cellular phosphorus requirements by substituting non-phosphorus membrane lipids for phospholipids. In the Sargasso Sea, where phosphate concentrations were less than 10 nmol l-1, we found that only 1.3 +/- 0.6% of phosphate uptake was used for phospholipid synthesis; in contrast, in the South Pacific subtropical gyre, where phosphate was greater than 100 nmol l-1, plankton used 17 6% (ref. 6). Examination of the planktonic membrane lipids at these two locations showed that classes of sulphur- and nitrogen-containing membrane lipids, which are devoid of phosphorus, were more abundant in the Sargasso Sea than in the South Pacific. Furthermore, these non-phosphorus, 'substitute lipids' were dominant in phosphorus-limited cultures of all of the phytoplankton species we examined. In contrast, the marine heterotrophic bacteria we examined contained no substitute lipids and only phospholipids. Thus heterotrophic bacteria, which compete with phytoplankton for nutrients in oligotrophic regions like the Sargasso Sea, appear to have a biochemical phosphorus requirement that phytoplankton avoid by using substitute lipids. Our results suggest that phospholipid substitutions are fundamental biochemical mechanisms that allow phytoplankton to maintain growth in the face of phosphorus limitation.

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Abstract. Due to the low atmospheric input of phosphate into the open ocean, it is one of the key nutrients that could ultimately control primary production and carbon export into the deep ocean. The observed trend over the last 20 years has shown a decrease in the dissolved inorganic phosphate (DIP) pool in the North Pacific gyre, which has been correlated to the increase in di-nitrogen (N2) fixation rates. Following a NW-SE transect, in the Southeast Pacific during the early austral summer (BIOSOPE cruise), we present data on DIP, dissolved organic phosphate (DOP) and particulate phosphate (PP) pools along with DIP turnover times (TDIP) and N2 fixation rates. We observed a decrease in DIP concentration from the edges to the centre of the gyre. Nevertheless the DIP concentrations remained above 100 nmol L−1 and T DIP was more than 6 months in the centre of the gyre; DIP availability remained largely above the level required for phosphate limitation to occur and the absence of Trichodesmium spp and low nitrogen fixation rates were likely to be controlled by other factors such as temperature or iron availability. This contrasts with recent observations in the North Pacific Ocean at the ALOHA station and in the western Pacific Ocean at the same latitude (DIAPALIS cruises) where lower DIP concentrations (<20 nmol L−1) and T DIP <50 h were measured during the summer season in the upper layer. The South Pacific gyre can be considered a High Phosphate Low Chlorophyll (HPLC) oligotrophic area, which could potentially support high N2 fixation rates and possibly carbon dioxide sequestration, if the primary ecophysiological controls, temperature and/or iron availability, were alleviated.

International audienceIn the oligotrophic ocean characterized by nitrate (NO − 3) depletion in surface waters, dinitrogen (N 2) fixation and dissolved organic nitrogen (DON) can represent significant nitrogen (N) sources for the ecosystem. In this study, we deployed large in situ mesocosms in New Caledonia in order to investigate (1) the contribution of N 2 fixation and DON use to primary production (PP) and particle export and (2) the fate of the freshly produced particulate organic N (PON), i.e., whether it is preferentially accumulated and recycled in the water column or exported out of the system. The mesocosms were fertilized with phosphate (PO 3− 4) in order to prevent phosphorus (P) limitation and promote N 2 fixation. The diazotrophic community was dominated by diatom–diazotroph associations (DDAs) during the first part of the experiment for 10 days (P1) followed by the unicel-lular N 2-fixing cyanobacteria UCYN-C for the last 9 days (P2) of the experiment. N 2 fixation rates averaged 9.8 ± 4.0 and 27.7 ± 8.6 nmol L −1 d −1 during P1 and P2, respectively. NO − 3 concentrations (< 0.04 µmol L −1) in the mesocosms were a negligible source of N, indicating that N 2 fixation was the main driver of new production throughout the experiment. The contribution of N 2 fixation to PP was not significantly different (p > 0.05) during P1 (9.0 ± 3.3 %) and P2 (12.6 ± 6.1 %). However, the e ratio that quantifies the efficiency of a system to export particulate organic carbon (POC export) compared to PP (e ratio = POC export / PP) was significantly higher (p < 0.05) during P2 (39.7 ± 24.9 %) than during P1 (23.9 ± 20.2 %), indicating that the production sustained by UCYN-C was more efficient at promoting C export than the production sustained by DDAs. During P1, PON was stable and the total amount of N provided by N 2 fixation (0.10 ± 0.02 µmol L −1) was not significantly different (p > 0.05) from the total amount of PON exported (0.10 ± 0.04 µmol L −1), suggesting a rapid and probably direct export of the recently fixed N 2 by the DDAs. During P2, both PON concentrations and PON export increased in the mesocosms by a factor 1.5–2. Unlike in P1, this PON production was not totally explained by the new N provided by N 2 fixation. The use of DON, whose concentrations decreased significantly (p < 0.05) from 5.3 ± 0.5 µmol L −1 to 4.4 ± 0.5 µmol L −1 , appeared to be the missing N source. The DON consumption (∼ 0.9 µmol L −1) during P2 is higher Published by Copernicus Publications on behalf of the European Geosciences Union. 4100 H. Berthelot et al.: Dinitrogen fixation and dissolved organic nitrogen fueled primary production than the total amount of new N brought by N 2 fixation (∼ 0.25 µmol L −1) during the same period. These results suggest that while DDAs mainly rely on N 2 fixation for their N requirements, both N 2 fixation and DON can be significant N sources for primary production and particulate export following UCYN-C blooms in the New Caledonia lagoon and by extension in the N-limited oceans where similar events ...