The role of singlet oxygen and humic acid toxicity for bacterioplankton activity and community structure

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In surface waters of aquatic habitats ROS and low molecular weight compounds are generated by the photolysis of DOM. Both, stimulating and inhibitory effects of DOM photolysis on bacterioplankton activity and growth have been observed. However, the interaction of those contrasting effects has not been investigated with respect to their influence on bacterioplankton species composition. Beside the direct negative effects of ROS generation (oxidative stress), bacteria compete with chemical mineralization by ROS for low molecular weight DOM compounds. In particular, information on the effects of the very reactive 1O2 is scarce. Therefore, we will investigate the metabolic activity and species composition of bacterioplankton in a humic acid rich lake with respect to changes in 1O2 exposure. This approach will help to unravel to which extent direct effects (oxidative stress) and indirect effects (chemical mineralization of DOM) of 1O2 generation influence selection and growth dynamics of bacterioplankton species. Toxic effects of humic acids on organisms appear to be correlated to ROS and in particular 1O2 generation. Hence, abundant bacterioplankton species isolated previously will be used to investigate bacterial defense mechanisms against humic acid toxicity and the role of 1O2 in this process.