Bacterivory in Oceanic Environments: Relative Importance ofthe Heterotrophic Microbial Loop

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The role of pelagic microzooplankton in food webs of the sea has been explored sketchily in field work. Small, delicate organisms are difficult to isolate and study in the field situation. One method of studying their role is to compare food chains of widely different marine environments. The goal of this research program is to determine the relative importance of carbon transformation through the microbial loop of an oligotrophic and euptrophic oceanic community, including especially the consumers of bacterial biomass. The hypothesis to be tested is that a greater percentage of the total amount of carbon fixed by primary productivity is consumed and respsired by heterotrophic microorganisms, i.e. bacteria and protozoa, in oceanic ecosystems than in coastal ecosystems. Primary productivity, bacterial productivity, and bactivory will be measured at a coastal site and a Sargasso Sea site during three cruises in the western North Atlantic. The total amount of bacterial carbon produced and the percentage of primary production that this bacterial carbon represents will be determined in these oceanic environments and compared to the patterns of microbial carbon tranformation in two neritic environments, Chesapeake Bay and Vineyard Sound. Size fractionation and single cell isolation studies of common species of larger protozoa and microm;etazoa will be performed using radiolabelled natural bacteria in order to identify the primary consumers of bacterial biomass in these communities. %%% Much of the carbon produced in the sea is in the form of microorganisms too small to see or easily manipulate. In recent years it has become clear that these organisms are quite crucial to the food chain of the seas from the poles to the equator. One way to evaluate the importance of the microorganisms is to compare their growth rates in several different environments and to examine which organisms are able to feed on the microorganisms. This study will measure rates of growth of several types of microorganisms in contrasting environments, the relatively impoverished Sargasso Sea and the relatively fertile Georges Bank. It will also identify which organisms consume the bacteria. The expectation is that microbial production is much more important in the food chain of the Sargasso Sea than it is in the Georges Bank area.