Response of water column microbial communities to sudden exposure to deltamethrin in aquatic mesocosms

Sudden exposure of an aquatic system to an insecticide can have significant effects on populations other than susceptible organisms. Although this is intuitively obvious, little is actually known about how such exposure might affect bacterial communities and their relative metabolic activity in ecosystems. Here, we assessed small sub-unit (ssu)-RNA levels in open and shaded 9 m3 aquatic mesocosms (16 units - 2 × 2 factorial design in quadruplicate) to examine the effects of sudden addition of deltamethrin to the units. When deltamethrin was added, a cascade of bacterial then phytoplankton “blooms” occurred over time. The bacterial bloom, which most likely included organisms from the plastid/cyanobacterial phylogenetic guild, was almost immediate (within hours), whereas the phytoplankton (algal) bloom lagged by about 4 days. This sequential response can be explained by an apparent sudden release of nutrients consequent to arthropod death that triggered a series of responses in the microbial loop. Interestingly, bacterial blooms were noted in both open and shaded mesocosms, whereas the algal bloom was only seen in open units, suggesting that both deltamethrin addition (and presumptive nutrient release) and an adequate light supply was required for the phytoplankton response. Overall, this work shows that microbial activities as reflected by ssu-rRNA levels can respond dramatically via apparently indirect effects following insecticide application.