Diversity and function of Chloroflexus-like bacteria in a hypersaline microbial mat: phylogenetic characterization and impact on aerobic respiration

Max-Planck-Institute for Marine Microbiology, Celsiusstrasse 1, D-28359 Bremen, Germany; CNRS and Université Montpellier II UMR 5119, Case 093, Place Eugène Bataillon F-34095 Montpellier Cedex 05, France; Delft University of Technology, Postbox 5048, NL-2600 GA Delft, The Netherlands

^* To whom correspondence should be addressed. Email: h.m.jonkers{at}tudelft.nl.

	Abstract

We studied the diversity of Chloroflexus-like bacteria (CLB) in a hypersaline phototrophic microbial mat and assayed their near-infrared (NIR) light dependent oxygen respiration rates. PCR with primers that were reported to specifically target the 16S rRNA gene from members of the phylum Chloroflexi resulted in the recovery of 49 sequences and 16 phylotypes (sequences of the same phylotype share more than 96% similarity) of which ten sequences (four phylotypes) appeared related to filamentous anoxygenic phototrophic members of the Family Chloroflexaceae. Photopigment analysis revealed the presence of BChlc, BChld and -carotene, pigments known to be produced by phototrophic CLB. Oxygen microsensor measurements on intact mats revealed a NIR (710-770 nm) light-dependent decrease in aerobic respiration, a phenomenon that we also observed in an axenic culture of Chloroflexus auranticaus. The metabolic characteristic of phototrophic CLB to switch from anoxygenic photosynthesis under NIR illumination to aerobic respiration under non-NIR illumination was further used to estimate their contribution to mat community respiration. Steady-state oxygen profiles under dark conditions and visible (VIS; 400 - 700 nm), NIR (710-770 nm) and VIS + NIR were compared. NIR light illumination led to a substantial increase in oxygen concentration in the mat. The observed impact on oxygen dynamics shows that CLB play a significant role in the cycling of carbon in this hypersaline microbial mat ecosystem. This study further demonstrates that the applied method, the combination of microsensor techniques and VIS/NIR illumination, allows a rapid establishment of the presence and significance of CLB in environmental samples.