Phytoplankton elemental quotas of key elements are traditionally measured using bulk analysis methods. Using mono-cultures, and correcting for dissolved organic components, bulk techniques provide reliable elemental quotas, however, not all species can be cultured and not all cultures can be axenic. On the other hand, bulk samples from the ocean neither can be corrected for the presence of extraneous material (e.g. detritus, dead cells), nor allow to study the elemental content at a species level. Advancements in energy dispersive spectrometry (EDS) allow now the simultaneous identification and quantification of the elemental content (C, N, O, Na, Mg, Al, Si, P, S, Cl, K and Ca) of single phytoplankton cells of a wide range of sizes (Norland et al. 1995; Segura-Noguera et al. 2012). We have used a scanning electron microscope operating at low incident beam energy (15-20 kV), coupled with an X-ray detector, to study the elemental composition and stoichiometry of microphytoplankton cells from the NW Mediterranean Sea (Segura-Noguera et al. 2016), as well as smaller cultured cyanobacteria and diatoms growing in nutrient replete and P-limited media. In this presentation I will first explain the advantages and drawbacks of this new method, and after, I will summarize the most relevant findings about the elemental composition at a single-cell level revealed by X-ray microanalysis: differences between diatoms and dinoflagellates; intra-specific variations due to environmentaly-related physiological differences, leading to differences in the resulting stoichiometry; and the contribution to the study of carbon and nutrient cycles in the NW Mediterranean Sea (water mass nutrients concentration and deviations from the Redfield ratio).
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Segura-Noguera M, Blasco D, Fortuño J-M. 2012. An improved energy-dispersive X-ray microanalysis method for analyzing simultaneously carbon, nitrogen, oxygen, phosphorus, sulfur, and other cation and anion concentrations in single natural marine microplankton cells. Limnol Oceanogr Methods.10(9):666–80.
Segura-Noguera M, Blasco D, Fortuño J. 2016. Taxonomic and Environmental Variability in the Elemental Composition and Stoichiometry of Individual Dinoflagellate and Diatom Cells from the NW Mediterranean Sea. PLoS ONE 11(4): e0154050. doi:10.1371/journal.pone.0154050