Trophic Interactions between Phagotrophic Protozoa and Bacterioplankton in Coastal Waters

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It is now firmly established that heterotrophic microbes, bacteria and phagotrophic protozoa are responsible for a largepart of carbon/energy flow in marine food webs. Using fluorescently labeled bacteria prepared from naturalbacterioplankton assemblages, it has been demonstrated that 20um ciliates, from waters in a salt marsh estuary and fromcoastal waters in the NWMediterranean Sea, could grazebacteria at high clearance rates. And these ciliatespreferentially ingest freely suspended, rather that particle-attached, bacteria. Previous studies have provided indirectevidence for a positive feedback effect of bacterivorousprotozoa on bacterial growth. Continued investigations will beconducted on protozoa-bacteria trophic interactions in coastal pelagic food webs emphasizing the: 1) evaluation of short-termand fine spatial scale variations in both bacterioplanktonproductivity and protozoan bacterivory within discrete watermasses; 2) examination of the extent of resource partitioningamong bacterivorous protozoa, especially with respect to the size spectra of bacteria grazed by flagellates and ciliates; 3)determination of the efficiency of conversion of ingestedbacteria into protozoan biomass; and 4) testing the hypothesisthat decoupling estuarine bacteria from their protozoan grazersresults in a change in bacterial metabolism.