Nitrate amendment reduces biofilm biomass and shifts microbial communities in remote, oligotrophic ponds

Abstract

AbstractHumans have increased the amount of reactive N available in the environment by over an order of magnitude since the industrial revolution. Most studies have been conducted in ecosystems with pervasive anthropogenic nutrient inputs, so little is understood about how naive biofilm communities respond to elevated nutrients. Our nutrient-diffusing substrate (NDS) experiments, which were conducted in Alaskan freshwater ponds with very little anthropogenic nutrient inputs, suggest that P limits biofilm photoautotrophs. However, despite low water-column nutrient concentrations, overall biofilm biomass was not enhanced by the addition of N or P. Rather, we observed an ~60% biomass reduction with NO3– amendment in 15 oligotrophic ponds across 2 y. This widespread biomass reduction was accompanied by changes in microbial communities, but these trends were not observed with NH4+ or P amendment. Nonamended communities (i.e., no nutrient amendment other than lysogeny broth agar) were characterized by anaerobic...