Competition for inorganic carbon between oxygenic and anoxygenic phototrophs in a hypersaline microbial mat, Guerrero Negro, Mexico Niko Finke, 1,2 * † Tori M. Hoehler, 2 Lubos Polerecky, 3 Benjamin Buehring 4 and Bo Thamdrup 1 1 Nordic Center for Earth Evolution, Institute of Biology, University of Southern Denmark, Odense, Denmark. 2 Exobiology Branch, NASA Ames Research Center, Moffett Field, CA, USA. 3 Microsensorgroup, Max-Planck-Institute for Marine Microbiology, Bremen, Germany. 4 Marum, University of Bremen, Bremen, Germany. Summary While most oxygenic phototrophs harvest light only in the visible range (400–700 nm, VIS), anoxygenic phototrophs can harvest near infrared light (> 700 nm, NIR). To study interactions between the photosynthetic guilds we used microsensors to measure oxygen and gross oxygenic photosynthesis (gOP) in a hypersaline microbial mat under full (VIS + NIR) and VIS illumination. Under normal dis- solved inorganic carbon (DIC) concentrations (2 mM), volumetric rates of gOP were reduced up to 65% and areal rates by 16–31% at full compared with VIS illu- mination. This effect was enhanced (reduction up to 100% in volumetric, 50% in areal rates of gOP) when DIC was lowered to 1 mM, but diminished at 10 mM DIC or lowered pH. In conclusion, under full-light illu- mination anoxygenic phototrophs are able to reduce the activity of oxygenic phototrophs by efficiently competing for inorganic carbon within the highly oxy- genated layer. Anoxygenic photosynthesis, calcu- lated from the difference in gOP under full and VIS illumination, represented between 10% and 40% of the C-fixation. The DIC depletion in the euphotic zone as well as the significant C-fixation by anoxygenic phototrophs in the oxic layer influences the carbon isotopic composition of the mat, which needs to be taken into account when interpreting isotopic biosig- nals in geological records. Introduction In photosynthetic microbial mats, full geochemical cycles occur over a small scale, allowing us to study ecosystem dynamics, particularly microbial interactions and their geochemical consequences, under controlled conditions. Thus, microbial mats can serve as a model system for studying complex biogeochemical interactions on larger scales. Photosynthetic hypersaline microbial mats usually show a high photosynthetic activity but very little growth, a result typically attributed to nutrient limitation (e.g. Can- field and Des Marais, 1993; Grötzschel et al., 2002; Ludwig et al., 2006). Studies of different hypersaline microbial mats have revealed various types of limitation. In mats from a permanent hypersaline lake Chiprana, Spain, that contain unusually high concentrations of inor- ganic carbon, oxygenic photosynthesis was limited by the supply of high-quality organic nitrogen (amino acids) and phosphate (Ludwig et al., 2006), whereas mats with a lower inorganic carbon concentration showed limitation of oxygenic photosynthesis by inorganic carbon (Jensen and Revsbech, 1989; Grötzschel et al., 2002). Thus, besides light availability, net growth of photosynthetic microbial mats is controlled by the diffusion of nutrients and inorganic carbon in and out of the mat. Photosynthetic communities in microbial mats usually comprise two major groups of organisms: oxygenic and anoxygenic phototrophs. Oxygenic phototrophs, such as cyanobacteria, utilize light in the visible range (VIS, 400– 700 nm), whereas anoxygenic phototrophs, such as Chloroflexus-like bacteria and green and purple sulfur bacteria, can use near infrared light (NIR, 700–1100 nm) for photosynthesis. Some cyanobacteria, i.e. those con- taining chlorophyll d, can also use NIR light (720 nm) for oxygenic photosynthesis (Kühl et al., 2005). While oxy- genic phototrophs use H 2O as electron donor, anoxygenic phototrophs require alternative reduced substrates such as hydrogen sulfide, hydrogen or reduced organic carbon. Judging from their presence in almost all photosynthetic mats, anoxygenic phototrophs are expected to have a significant impact on the overall ecology of the mat ecosystems, including the distribution and activities of oxygenic phototrophs. However, their role and activity, and particularly their contribution to geochemical fluxes Received 23 October, 2012; accepted 24 October, 2012. *For corre- spondence. E-mail nfinke@web.de; Tel. (+1) 706 542 2034; Fax (+1) 706 542 5888. † Present address: Department of Marine Sciences, University of Georgia, Athens, GA, USA. Environmental Microbiology (2013) doi:10.1111/1462-2920.12032 © 2012 Society for Applied Microbiology and Blackwell Publishing Ltd