Copper and chromium exposure affect chironomid larval microbiota composition.

Chironomids are aquatic insects that are known to be pollution tolerant. We have recently demonstrated that endogenous chironomid microbiota protects its host from toxic metals. Following these findings, we hypothesized that under different environmental conditions, a different bacterial consortium will evolve. Our aim was to explore the change in chironomid larval microbiota composition triggered by exposure to toxic copper and hexavalent chromium. Chironomid larvae were collected from the environment and treated in the laboratory with copper, hexavalent chromium, and no metal (control). After six days, the microbial composition of the surviving larvae was examined. We found a significant change in larval microbiota composition between the three treatments and for different copper concentrations. The abundance of specific taxa varied significantly between the treatments. At the genus level, the abundance of some genera (e.g. Yersinia, Acinetobacter) increased in the presence of copper, and some genera (e.g. Yersinia, Dysgonomonas, Delftia, Enterococcus) increased in the presence of hexavalent chromium, compared to the control. The change in the larval microbiota composition was rapid and metal-specific. We suggest that each larva hosts a consortium of bacterial species that can proliferate under a specific environmental change and thus, protect the insect under unstable environmental conditions.