Life in Brown Waters: Aquatic Bacteria Respond to Increased Terrestrial Carbon Loading

Bacteria are vital for the functioning of natural and managed ecosystems due to their role in biogeochemical processes; however, it is unclear how bacteria will respond to environmental changes. For example, anthropogenic influences are increasing the transport of dissolved organic carbon (DOC) from terrestrial into aquatic ecosystems. Terrestrial DOC is generally regarded as refractory to aquatic bacteria, yet ecosystem level measures, such as CO₂ flux and bacterial respiration, indicate that at least a subset of the community must use the resource. We explored how terrestrial DOC supply affects the diversity and composition of aquatic bacterial communities. We manipulated the carbon loading rates in ten experimental ponds at the Kellogg Biological Station by adding a terrestrial carbon analog (SuperHume), comprised of humic and fulvic acids, to each pond along a gradient of loading rates (0 g/m² yr^-1 – 212 g/m² yr^-1). We collected bacterial biomass for community DNA and RNA coextractions, and amplified the V3-V5 region of the 16S rRNA gene to analyze the bacterial community diversity and composition. By analyzing both the DNA and RNA pools we are able to distinguish if our manipulation influenced the total community (DNA) or the active members of the community (RNA).  DOC loading rates significantly reduced the taxa richness of active bacteria in the ponds, but had no significant effect on the total community. Likewise, we found major compositional changes in the active community as a response to carbon loading, but not in the total community. Together, these results suggest that altered DOC loading influenced the active members of the microbial community by selecting for a small subset of the total community. This subset of the total community should be those members who are able to successfully process the increased carbon resources. Furthermore, our work implies that environmental changes may impact ecosystem functions through alterations in the microbial community composition and function.