The importance of trophic interactions in controlling the abundance and composition of heterotrophic bacteria in aquatic food webs was studied using a series of microcosm experiments. In particular, the relationship between phytoplankton, bacterioplankton, and eukaryotic grazers was examined for two contrasting environments from the tidally influenced portion of the James River (Virginia, USA). Separate experiments were conducted using the microbial community from: (1) a riverine habitat, dominated by primary production; and (2) an estuarine habitat, dominated by allocthanous organic inputs. To uncouple the links within the microbial food web, the communities from each site were manipulated into four treatment groups based on light availability and the presence or absence of grazers. After a 72-hour incubation, nutrient concentrations, phytoplankton levels, and grazer abundance and diversity were monitored to confirm the success of each experimental manipulation.  Several attributes of the bacterial community were also examined including microscopic and culture-based estimates of abundance, as well community composition (via RAPD fingerprinting of whole-community DNA extracts).

Results/Conclusions

The response of the bacterial component of the microbial food web to the removal of phytoplankton, grazers, or both, varied depending on the initial trophic structure of the system.  In the estuarine system, the size and composition of the bacterial community was not influenced by any of the experimental treatments. However, manipulations of the riverine system showed a strong influence of both top-down and bottom-up trophic controls on bacterioplankton. Specifically, bacterial abundance was negatively correlated with both grazer and phytoplankton abundance. Community composition was significantly different in each of the experimental treatments; the greatest changes were associated with the removal of phytoplankton from the system. Together, these results indicate that the importance of trophic relationships in the determining of bacterial community structure can vary depending on microbial loop dynamics as well as the energy flow and nutrient cycling in aquatic ecosystems.