Shifts in microbial community structure and function in light- and dark-grown biofilms driven by warming

Biofilms are dynamic players in biogeochemical cycling in running waters and are subjected to environmental stressors like those provoked by climate change. We investigated whether a 2 degrees C increase in flowing water would affect prokaryotic community composition and heterotrophic metabolic activities of biofilms grown under light or dark conditions. Neither light nor temperature treatments were relevant for selecting a specific bacterial community at initial phases (7-day-old biofilms), but both variables affected the composition and function of mature biofilms (28-day-old). In dark-grown biofilms, changes in the prokaryotic community composition due to warming were mainly related to rotifer grazing, but no significant changes were observed in functional fingerprints. In light-grown biofilms, warming also affected protozoan densities, but its effect on prokaryotic density and composition was less evident. In contrast, heterotrophic metabolic activities in light-grown biofilms under warming showed a decrease in the functional diversity towards a specialized use of several carbohydrates. Results suggest that prokaryotes are functionally redundant in dark biofilms but functionally plastic in light biofilms. The more complex and self-serving light-grown biofilm determines a more buffered response to temperature than dark-grown biofilms. Despite the moderate increase in temperature of only 2 degrees C, warming conditions drive significant changes in freshwater biofilms, which responded by finely tuning a complex network of interactions among microbial populations within the biofilm matriThis study was funded by the Spanish Ministry of Economy and Competitiveness, project FLUMED-HOTSPOTS (CGL2011-30151-C02-01). V. Diaz Villanueva participated in this project supported by FONCYT (Argentina) (PICT-2007-01747) and CONICET (PIP 114-201101-00304