Phototrophic Chloroflexus-like bacteria and their role in hypersaline microbial mats

Abstract

Chloroflexus-like bacteria (CLB) are filamentous anoxygenic phototrophic bacteria possessing BChla and sometimes BChlc as major photosynthetic pigments. Their ecological function in nature is still largely unknown due to their unique versatile physiology that allows both photoautotrophy, photoheterotrophy and heterotrophy under different conditions. This physiological flexibility increases the ability to compete with other bacteria, probably one reason why they are often encountered as quantitative important microbial mat community members. Most CLB characterized so far originate from hyperthermal lakes and indeed most knowledge regarding the In situ behavior of CLB is from thermophilic species. Nevertheless, CLB are known to populate other marine and hypersaline environments as well as other aquatic ecosystems, though knowledge about them is scarce. In this work CLB from the natural hypersaline athalassic Lake Chiprana (NE Spain) were investigated with respect to eco-physiological properties and phylogenetic diversity to determine their ecological role in this specific environment. For the first time, aerobic respiration of hypersaline CLB community members was assessed by a novel In situ method, using near-infrared (NIR) light of 740 nm and oxygen microsensor techniques. These studies revealed that CLB respire oxygen when NIR light is absent but immediately switch to anoxygenic photosynthesis upon its presence. It was concluded that CLB play a major role in microbial mat community aerobic respiration in the absence of NIR light, as a switch to NIR light illumination resulted under some conditions to a 50% increase in oxygen in the mat. The effect of NIR light illumination and the role of CLB in mat community aerobic respiration was further quantified and mathematically modeled. In addition to NIR light dependent functional properties also structural, i.e. spatial, properties of CLB in the mat were investigated in order to better understand its impact on mat community physiology. FISH studies confirmed that members of the Roseiflexus and Chloroflexus genera were confined to the upper photic zone of the mat in close proximity tocyanobacteria at a depth of ~2-4 mm. In this specific depth zone which is fully oxygenated during the day, CLB may profit from cyanobacterial organic excretion products while facing no competition with the strict anaerobic green sulfur bacteria as occurs in deeper parts of the mat. Further 16S rRNA gene clone library and photopigment analysis revealed that members of the genera Oscillochloris and Candidatus Chlorothrix also occurred in deeper mat layers where they probably depend less on excreted photosynthates but more on the presence of free sulfide produced by sulfate-reducing bacteria. A further 16S rRNA gene clone library study in which four hypersaline mats from three different continents were compared revealed a number of unique environmental CLB sequences (< 97% homology) in each mat suggesting a relatively high as well as endemic species diversity. As in all four mats, however, the majority of retrieved sequences were most closely related to the previously isolated species Candidatus Chlorothrix halophila, it was concluded that this bacteriochlorophyll c-producing species is pandemic and dominates the Chloroflexaceae community in hypersaline microbial mats. The overall conclusion of these studies is that despite the fact that CLB phylogeny as well as In-situ physiology needs still further elucidation, these bacteria play a significant and important role community carbon cycling and should therefore be given more consideration in future microbial mat studies.