Abstract
Light excitation of humic matter generates reactive oxygen species (ROS) in surface waters of aquatic ecosystems. Abundant ROS generated in humic matter rich lakes include singlet oxygen (1O2) and hydrogen peroxide (H2O2). Because these ROS differ in half-life time and toxicity, we compared their effects on microbial activity (14C-Leucine incorporation) and bacterial community composition (BCC) in surface waters of humic Lake Grosse Fuchskuhle (North-eastern Germany). For this purpose, experiments with water samples collected from the lake were conducted in July 2006, September 2008 and August 2009. Artificially increased 1O2 and H2O2 concentrations inhibited microbial activity in water samples to a similar extent, but the effect of the respective ROS on BCC varied strongly. BCC analysis by 16S rRNA gene clone libraries and RT-PCR DGGE revealed ROS specific changes in relative abundance and activity of major bacterial groups and composition of dominating phylotypes. These changes were consistent in the three experiments performed in different years. The relative abundance of Polynucleobacter necessarius, Limnohabitans-related phylotypes (Betaproteobacteria), and Novosphingobium acidiphilum (Alphaproteobacteria) increased or was not affected by photo-sensitized 1O2 exposure, but decreased after H2O2 exposure. The opposite pattern was found for Actinobacteria of the freshwater AcI-B cluster which were highly sensitive to 1O2 but not to H2O2 exposure. Furthermore, group-specific RT-PCR DGGE analysis revealed that particle-attached P. necessarius and Limnohabitans-related phylotypes exhibit higher resistance to 1O2 exposure compared to free-living populations. These results imply that 1O2 acts as a factor in niche separation of closely affiliated Polynucleobacter and Limnohabitans-related phylotypes. Consequently, oxidative stress caused by photochemical ROS generation should be regarded as an environmental variable determining abundance, activity, and phylotype composition of environmentally relevant bacterial groups, in particular in illuminated and humic matter rich waters.
Highlights
Dissolved organic matter (DOM) is the major carbon and energy source for heterotrophic bacteria in aquatic ecosystems [1]
By applying the furfuryl alcohol (FFA) method [24] we observed similar in situ [1O2]SS concentrations on all three experiment days that were in the range of 11.2 to 14.1610214 M in the surface water layer of the lake (Table 1)
Hydrogen peroxide concentrations were low in all water samples
Summary
Dissolved organic matter (DOM) is the major carbon and energy source for heterotrophic bacteria in aquatic ecosystems [1]. Humic lakes are characterized by a high content of allochthonous DOM with recalcitrant high-molecular-weight poly-aromatic compounds. Photochemical transformations of these compounds generate low-molecular-weight substances and thereby stimulate microbial activity and growth [2,3]. ROS generation in aquatic ecosystems occurs by light excitation of DOM, in particular humic matter, and subsequent formation of triplet excited states in poly-aromatic compounds [8]. Light-excited DOM transfers energy or electrons to molecular oxygen. Thereby, the transfer of energy generates singlet oxygen (1O2) and the incomplete reduction of oxygen leads to the formation of hydrogen peroxide (H2O2) and other ROS. Recent experiments strongly suggest that distinct structures in humic matter are linked to the formation of 1O2 or H2O2 [9] and that the reaction of 1O2 with DOM generates small amounts of H2O2 [9,10]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
