Taxonomic and Environmental Variability in the Elemental Composition and Stoichiometry of Individual Dinoflagellate and Diatom Cells from the NW Mediterranean Sea

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

Analytical detection of parasite infection of Dinophysis norvegica using FISH probes

Parasites of the genus Amoebophrya infect several freelivingdinoflagellates, including harmful species. Whereas advanced infection can be easily detected, earlier stages of infection are more difficult to establish. rRNA-based fluorescent in situ hybridisation (FISH) probes specific to Amoebophrya sp. infecting Dinophysis norvegica in the Baltic Sea were combined with 4’,6-diamidino-2-phenylindole (DAPI) staining to study this host-parasite system in a series of samples collected over a 24h period in August 1998. Small forms of Amoebophrya sp. (6–8ìm) within D. norvegica, undetected by DAPI staining, were revealed by the FISH probe. As a result, the average infection estimated by FISH (7.8%) was 6.5 times higher than the estimate based on DAPI alone. Multiple infections were observedin 75% of the infected D. norvegica cells, indicating that this may be a common feature in this host-parasite system. FISH probes therefore provide a valuable tool to assess parasite infection of dinoflagellates in fieldsamples