Daily cycle of bacterial active community structure

Microbial communities at high altitude ecosystem spring sites, are subjected along the day to extreme changes in irradiation and temperature. Herein, we determined the composition of actively transcribing bacteria from spring waters through 16S rRNA gene tag pyrosequencing (cDNA) experimentally exposed along the day (morning, noon and afternoon) to variable solar radiation and light quality, and evaluated their influence on nutrients recycling. Irradiation, temperature and nutrients rates of change were associated with changes in the active bacterial community structure, predominantly conformed by Cyanobacteria, Verrucomicrobia, Proteobacteria, and other 35 Phyla including also the recently described Candidate Phyla Radiation (e.g., Parcubacteria, Gracilibacteria, OP3, TM6, SR1). An increase of diversity was observed at noon when the highest irradiances were measured (3.3 -3.9 H’, 1125 W m-2) compared to morning and afternoon (0.6-2.8 H’). These increase was associated with a decrease of Cyanobacteria and an enhancement of Proteobacteria and initially low frequently and rare bacteria phyla (<0.5% sequences contribution). UV radiation compared to full sun radiation indicates the presence of a higher number of stimulatory compared to inhibitory effects, including contrasting responses of some Orders, e.g., positive effects were found for Cyanobacteria, Rhodobacterales, Rhodospirillales, Bacteroidales; whereas negative effects for the orders Oceanospirillales, Fibrobacterales, Alteromonadales and Sphingobacteriales. Nutrient recycling was influenced by microbial composition changes, particularly for Cyanobacteria, nitrifying bacteria and Chlorobi. In total, our results indicate that phototrophic bacteria from high altitude aquatic ecosystems were affected but potentially resilient to the negative effect of high solar radiation influencing nutrient recycling in these ecosystems.