Abstract
Here we present, for the first time, the elemental concentration, including C, N and O, of single phytoplankton cells collected from the sea. Plankton elemental concentration and stoichiometry are key variables in phytoplankton ecophysiology and ocean biogeochemistry, and are used to link cells and ecosystems. However, most field studies rely on bulk techniques that overestimate carbon and nitrogen because the samples include organic matter other than plankton organisms. Here we used X-ray microanalysis (XRMA), a technique that, unlike bulk analyses, gives simultaneous quotas of C, N, O, Mg, Si, P, and S, in single-cell organisms that can be collected directly from the sea. We analysed the elemental composition of dinoflagellates and diatoms (largely Chaetoceros spp.) collected from different sites of the Catalan coast (NW Mediterranean Sea). As expected, a lower C content is found in our cells compared to historical values of cultured cells. Our results indicate that, except for Si and O in diatoms, the mass of all elements is not a constant fraction of cell volume but rather decreases with increasing cell volume. Also, diatoms are significantly less dense in all the measured elements, except Si, compared to dinoflagellates. The N:P ratio of both groups is higher than the Redfield ratio, as it is the N:P nutrient ratio in deep NW Mediterranean Sea waters (N:P = 20–23). The results suggest that the P requirement is highest for bacterioplankton, followed by dinoflagellates, and lowest for diatoms, giving them a clear ecological advantage in P-limited environments like the Mediterranean Sea. Finally, the P concentration of cells of the same genera but growing under different nutrient conditions was the same, suggesting that the P quota of these cells is at a critical level. Our results indicate that XRMA is an accurate technique to determine single cell elemental quotas and derived conversion factors used to understand and model ocean biogeochemical cycles.
Highlights
The C:N:P:Si ratio as well as nutrient quotas or concentrations in marine phytoplankton are routinely used in ocean biogeochemistry models to explain global patterns of plankton distribution and to predict primary production both qualitatively and quantitatively
This study presents the first data of C, N, O, Mg, Si, P and S elemental concentration and stoichiometry of single dinoflagellate and diatom cells collected from the sea
These values have been obtained using X-ray microanalysis, the only single-cell method that can simultaneously identify and quantify all these elements in individual cells. We have validated this new methodology by putting the data obtained from individual cells from the NW Mediterranean Sea within the frame of historical data on phytoplankton elemental composition and stoichiometry
Summary
The C:N:P:Si ratio as well as nutrient quotas or concentrations in marine phytoplankton are routinely used in ocean biogeochemistry models to explain global patterns of plankton distribution and to predict primary production both qualitatively (in terms of elemental and biochemical composition) and quantitatively. These parameters are of critical importance to study, understand, model and predict ocean biogeochemical cycles [1, 2, 3]. XRMA can overcome this problem, because, unlike other single-cell methods, it allows the simultaneous identification and quantification of all the elements (C, N, O, Na, Mg, Al, Si, P, S, Cl, K and Ca) present in the cell
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