Spatial distribution of protists in the presence of macroaggregates in a marine system

The spatial distribution of marine protistan communities in the presence of organic macroaggregates, formed from natural seawater, was studied in seve ral microcosm experiments. The presence of macroaggregates had two main eff ects. First, the size of the communities of bacteria, flagellates and cilia tes increased, as these communities were three orders of magnitude higher i n the aggregates than in the microcosm water. In addition, it brought the d iversification on the niches accessible to planktonic microorganisms, as th ree phases were formed: water, aggregates and aggregate-water interphase. S ome of the detected protistan taxa were only found in the water, and theref ore they can be: considered as truly free-swimming protists. Others quickly colonised the aggregates, and finally, some of them showed a preference fo r the aggregate-water interphase. We discuss this spatial structuring of th e protistan community on the basis of their Feeding strategies and structur al and behavioural characteristics. (C) 2000 Federation of European Microbi ological Societies. Published by Elsevier Science B.V. All rights reserved.