Drivers of Microbial Carbon Fluxes Variability in Two Oligotrophic Mediterranean Coastal Systems.

The carbon fluxes between phytoplankton and heterotrophic bacterioplankton were studied in two coastal oligotrophic sites in the NW Mediterranean. Phytoplankton and bacterial production rates were measured under natural conditions using different methods. In the Bay of Villefranche, the temporal variability revealed net heterotrophy in July-October and net autotrophy in December-March. The spatial variability was studied in the Bay of Palma, showing net autotrophic areas in the west and heterotrophic areas in the east. On average bacterial respiration, represented 62% of the total community respiration. Bacterial growth efficiency (BGE) values were significantly higher in autotrophic conditions than in heterotrophic ones. During autotrophic periods, dissolved primary production (DPP) was enough to sustained bacterial metabolism, although it showed a positive correlation with organic carbon stock (DOC). Under heterotrophic conditions, DPP did not sustain bacterial metabolism but bacterial respiration correlated with DPP and bacterial production with DOC. Temperature affected positively, DOC, BGE, bacterial respiration and production when the trophic status was autotrophic. To summarize, the response of bacterial metabolism to temperature and carbon sources depends on the trophic status within these oligotrophic coastal systems.

Night and day morphologies in a planktonic dinoflagellate.

We describe remarkable diurnal changes in the morphology of the planktonic dinoflagellate Ceratium ranipes. The species is distinguished by the unusual appendages, known as toes or fingers, protruding from its horns. Varieties have been described based on the characteristics of the fingers. We discovered that cultures, maintained on a 12:12 photoperiod, when examined during the dark period were composed of 'finger-less' cells. Monitoring of isolated cells revealed a diurnal cycle of distinct changes in morphology with daytime cells showing appendages, well-stocked with chlorophyll, and the absence of the fingers at the end of the photoperiod. Fingers are absorbed at the end of the light period and re-grown at the end of the dark period. Sampling the Bay of Villefranche, we found the distinct morphologies in a natural population at night and during the day.