Singlet oxygen, a neglected but important environmental factor: short-term and long-term effects on bacterioplankton composition in a humic lake

Photolysis of dissolved organic matter (DOM) leads to contrasting effects on bacterioplankton dynamics, i.e. stimulation and inhibition of bacterial activity. In particular, the role of short-lived reactive oxygen species (ROS), e.g. singlet oxygen (¹O₂), in altering microbial activity and species composition has scarcely been investigated. Therefore, we have artificially increased the natural rate of ¹O₂ formation in short-term (∼4 h) in situ and long-term (72 h) laboratory incubations of surface water samples from a humic acid-rich lake. Denaturing gradient gel electrophoresis (DGGE) patterns revealed significant changes in occurrence of abundant bacterioplankton phylotypes upon ¹O₂ exposure. Cluster analysis of DGGE patterns showed that a moderate increase in ¹O₂ exposure leads to similar changes in different years indicating the establishment of bacterial communities adapted to ¹O₂ exposure. Bacterioplankton phylotypes favoured under these conditions belonged to Betaproteobacteria of the beta II cluster (e.g. Polynucleobacter necessarius) and the beta I cluster related to Limnohabitans (R-BT subcluster) as well as Alphaproteobacteria affiliated to Novosphingobium acidiphilum. In contrast, Actinobacteria of the freshwater acI-B cluster were sensitive even against moderate ¹O₂ exposure. We conclude that ¹O₂ exposure due to DOM photolysis represents an important natural selective factor affecting bacterial species dynamics in aquatic ecosystems in many ways.