Impact of mesozooplankton grazing on the microbial community revealed by denaturing gradient gel electrophoresis (DGGE)

Although grazer inclusion incubation experiments are commonly used to evaluate mesozooplankton grazing mortality of phytoplankton, few studies have employed molecular methods to examine the changes in the composition of the microbial community across different grazing treatments. We used denaturing gradient gel electrophoresis (DGGE) to study the impact of mesozooplankton-mediated trophic cascades on microbial food web components at two subtropical coastal sites with contrasting trophic conditions. Reciprocal changes in microbial food web components resulting from trophic cascades were observed, with decreases in > 3 µm eukaryotes, increases in < 3 µm eukaryotes and decreases in bacterioplankton. At the estuarine station, diatoms and dinoflagellates accounted for the majority of the micro and nanoplankton, and they, particularly dinoflagellates, were selectively grazed by mesozooplankton. Although the total band numbers of picoeukaryotes did not vary significantly, the relative abundance of different phylogenetic groups changed with the appearance of mesozooplankton grazers. Both total band brightness and species richness and diversity of bacterioplankton declined sharply under mesozooplankton grazing at the coastal site, but not at the estuarine site, suggesting that mesozooplankton-mediated trophic cascade effect was less pronounced in eutrophic estuarine waters. We concluded that DGGE could be a useful tool to track the community shifts of microbial food web in experimental settings and we have demonstrated in this study such shifts in both eukaryotic and prokaryotic microbial communities induced by mesozooplankton grazing. The magnitude of mesozooplankton-mediated trophic cascade effect transferred to picoplankton appears to be stronger at the mesotrophic coastal water than at the eutrophic estuarine water.