A hypersaline microbial mat from the Pacific Atoll Kiritimati: insights into composition and carbon fixation using biomarker analyses and a 13 C-labeling approach S. I. BU ¨ HRING, 1,2* R. H. SMITTENBERG, 1,3* D. SACHSE, 1,4* J. S. LIPP, 2 S. GOLUBIC, 5 J. P. SACHS, 1,6* K.-U. HINRICHS 2 AND R. E. SUMMONS 1 1 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA 2 MARUM – Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany 3 Geological Institute, ETH, Zurich, Switzerland 4 Leibniz Center for Surface Process and Climate Studies, University of Potsdam, Potsdam, Germany 5 Biology Department, Boston University, Boston, MA, USA 6 School of Oceanography, University of Washington, Seattle, WA, USA ABSTRACT Modern microbial mats are widely recognized as useful analogs for the study of biogeochemical processes relevant to paleoenvironmental reconstruction in the Precambrian. We combined microscopic observations and investigations of biomarker composition to investigate community structure and function in the upper layers of a thick phototrophic microbial mat system from a hypersaline lake on Kiritimati (Christmas Island) in the Northern Line Islands, Republic of Kiribati. In particular, an exploratory incubation experiment with 13 C-labeled bicarbonate was conducted to pinpoint biomarkers from organisms actively fixing carbon. A high relative abundance of the cyanobacterial taxa Aphanocapsa and Aphanothece was revealed by micro- scopic observation, and cyanobacterial fatty acids and hydrocarbons showed 13 C-uptake in the labeling experiment. Microscopic observations also revealed purple sulfur bacteria (PSB) in the deeper layers. A cyclic C 19:0 fatty acid and farnesol were attributed to this group that was also actively fixing carbon. Back- ground isotopic values indicate Calvin–Benson cycle-based autotrophy for cycC 19:0 and farnesol-producing PSBs. Biomarkers from sulfate-reducing bacteria (SRB) in the top layer of the mat and their 13 C-uptake pat- terns indicated a close coupling between SRBs and cyanobacteria. Archaeol, possibly from methanogens, was detected in all layers and was especially abundant near the surface where it contained substantial amounts of 13 C-label. Intact glycosidic tetraether lipids detected in the deepest layer indicated other archaea. Large amounts of ornithine and betaine bearing intact polar lipids could be an indicator of a phos- phate-limited ecosystem, where organisms that are able to substitute these for phospholipids may have a competitive advantage. Received 29 October 2008; accepted 22 February 2009 Corresponding author: S. I. Bu¨ hring. Tel.: +49 421 21865744; fax: +49 421 21865715; e-mail: solveig. buehring@uni-bremen.de INTRODUCTION Modern cyanobacterial mats are prevalent in diverse envi- ronments including desert crusts, coastal lagoons, hot springs and aquatic hypersaline settings (e.g. Potts, 1994; Jahnke et al., 2001; Jonkers et al., 2003; Wieland et al., 2003). Extreme environmental conditions suppress the activity of grazing organisms (e.g. Cornee et al., 1992; Fenchel, 1998a) and enable development of mats that may reach gigantic proportions taking the minute scale of their main contributors, cyanobacteria, into account. Photo- trophic microbial mats are characterized by daily fluctua- tions in redox status due to the physiology of the cyanobacteria and associated microorganisms (Canfield et al., 2005). Diatoms are prevalent in the surface layers of many modern mats, possibly because they are well adapted to high irradiance during the day. Oxygen supersaturation is common within and above mats during daytime because *Present addresses Ó 2009 The Authors Journal compilation Ó 2009 Blackwell Publishing Ltd 1 Geobiology (2009), 7, 1–16 DOI: 10.1111/j.1472-4669.2009.00198.x