Changes in Marine Prokaryotic Community Induced by Varying Types of Dissolved Organic Matter and Subsequent Grazing Pressure

We analysed changes in the abundance, biomass, activity and composition of coastal marine prokaryotic communities after the addition of organic substrates, such as glucose, leucine and yeast extract, and the effect of grazing pressure exerted by nanoflagellates. The addition of a carbon source (i.e. glucose) promoted the growth of Gammaproteobacteria, while a combined source of C and N (i.e. leucine) favoured the development of Alphaproteobacteria. The addition of yeast extract, a complex substrate rich in N and growth factors, promoted the proliferation of Alphaproteobacteria and Gammaproteobacteria. Grazing pressure exerted by nanoflagellates produced marked differences on the size structure of the prokaryotic biomass. A pronounced tendency to filamentation and aggregation was observed in the glucose treatment, while in the case of yeast extract, small and mainly freely dispersed prokaryotes were maintained throughout the incubations. Thus, the final community in the yeast extract treatment showed a high percentage of edible biomass, while an important fraction of potentially grazing-resistant prokaryotes (more than 50% of total prokaryotic biomass) was detected in the microcosms enriched with glucose. These results suggest a marked effect of DOM sources on the development of grazing-resistant prokaryotes.