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

P>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 (1O(2)), in altering microbial activity and species composition has scarcely been investigated. Therefore, we have artificially increased the natural rate of 1O(2) formation in short-term (similar to 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 1O(2) exposure. Cluster analysis of DGGE patterns showed that a moderate increase in 1O(2) exposure leads to similar changes in different years indicating the establishment of bacterial communities adapted to 1O(2) 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 1O(2) exposure. We conclude that 1O(2) exposure due to DOM photolysis represents an important natural selective factor affecting bacterial species dynamics in aquatic ecosystems in many ways.