Discontinuous Grazing Pressure by Microzooflagellates on Marine Synechococcus

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Small phytoplankton play a major role in the primary production of the oceans. The unicellular cyanobacterium Synechococcus is a prominent and important member of this community that can reach cell densities of more than 100 thousands cells per liter, achieve in situ growth rates of 3 doubling per day and as a result may contribute between 5 and 25% of the total primary productivity in the temperate and tropical oceans. Studies to date indicate that Synechococcus is grazed at rates comparable to its growth rate and that microflagellates are the major group responsible for turning over Synechococcus biomass. Synechococcus has a characteristic discontinuous pattern of diel growth both in nature and in culture that can be monitored by direct and indirect methods. Results of experiments measuring both growth and grazing rates of Synechococcus in the Sargasso Sea led to the hypothesis that grazing of Synechococcus by microflagellates can be periodic, occurring primarily at night. To test this hypothesis, Dr. Waterbury will isolate and characterize the microflagellates responsible for Synechococcus turnover and document their impact on the temporal patterns of Synechococcus abundance. The project will be conducted in both inshore and open-ocean waters and will experimentally employ culture techniques, direct and indirect measurements of in situ growth and grazing rates using direct cell counts, and C-14 bicarbonate incorporation. The significance of this study extends beyond simply furthering our understanding of the ecology of marine Synechococcus spp. This unicellular photosynthetic bacterium is an important component of the marine microbial food web. Insights concerning the identity of Synechococcus grazers and patterns and rates of grazing on Synechococcus can act as a model for and provide important insights into the workings of the "microbial loop".//