Department Seminar: Michael Landry

Title: Unraveling the complexities of microbial food-web dynamics in the southern California Current Ecosystem

Abstract: The microbial component of pelagic marine food webs, comprising heterotrophic prokaryotes, pico-sized phytoplankton (Prochlorococcus, Synechococcus and picoeukaryotes) and their protistan and viral consumers, is projected to increase in importance with climate change, but little is known about the details or complexities of microbial trophic interactions that will determine the outcomes. In the California Current Ecosystem (CCE), picophytoplankton biomass and abundance peak in waters of intermediate productivity and decrease at higher production. Using experimental data from eight cruises crossing the pronounced CCE trophic gradient, we tested the hypothesis that these declines are driven by intensified grazing on heterotrophic bacteria passed to similarly sized picophytoplankton via shared predators. Results confirm previously observed distributional patterns as well as significant increases in bacterial abundance, cell growth rate and grazing mortality with primary production. Mortalities of picophytoplankton, however, diverge from the bacterial mortality trend such that relative grazing rates on Synechococcus compared to heterotrophic bacteria decline by 12-fold between low and high productivity waters. The large shifts in mortality rate ratios are not explained by size variability, but rather suggest high selectivity of grazer assemblages or tightly coupled tradeoffs in microbial growth advantages and grazing vulnerabilities. These findings challenge the long-held view that protistan grazing mainly determines overall biomass of microbial communities while viruses uniquely regulate diversity by “killing the winners” and have motivated testing of a molecular approach to resolve taxon-specific differences in growth and grazing dynamics within microbial communities.