Teske1 Abstract Spatial Structure and Activity of Sedimentary Microbial Communities Underlying a Beggiatoa spp. Mat in a Gulf of Mexico Hydrocarbon Seep Karen G. Lloyd1*, Daniel B. Albert1, Jennifer F. Biddle1, Jeffrey P. Chanton 2 , Oscar Pizarro3, Andreas 1 Department of Marine Sciences, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America, 2 Department of Oceanography, Florida State University, Tallahassee, Florida, United States of America, 3 Australian Centre for Field Robotics, The University of Sydney, Sydney, Australia Background: Subsurface fluids from deep-sea hydrocarbon seeps undergo methane- and sulfur-cycling microbial transformations near the sediment surface. Hydrocarbon seep habitats are naturally patchy, with a mosaic of active seep sediments and non-seep sediments. Microbial community shifts and changing activity patterns on small spatial scales from seep to non-seep sediment remain to be examined in a comprehensive habitat study. Methodology/Principal Findings: We conducted a transect of biogeochemical measurements and gene expression related to methane- and sulfur-cycling at different sediment depths across a broad Beggiatoa spp. mat at Mississippi Canyon 118 (MC118) in the Gulf of Mexico. High process rates within the mat (,400 cm and ,10 cm from the mat’s edge) contrasted with sharply diminished activity at ,50 cm outside the mat, as shown by sulfate and methane concentration profiles, radiotracer rates of sulfate reduction and methane oxidation, and stable carbon isotopes. Likewise, 16S ribosomal rRNA, dsrAB (dissimilatory sulfite reductase) and mcrA (methyl coenzyme M reductase) mRNA transcripts of sulfate-reducing bacteria (Desulfobacteraceae and Desulfobulbaceae) and methane-cycling archaea (ANME-1 and ANME-2) were prevalent at the sediment surface under the mat and at its edge. Outside the mat at the surface, 16S rRNA sequences indicated mostly aerobes commonly found in seawater. The seep-related communities persisted at 12–20 cm depth inside and outside the mat. 16S rRNA transcripts and V6-tags reveal that bacterial and archaeal diversity underneath the mat are similar to each other, in contrast to oxic or microoxic habitats that have higher bacterial diversity. Conclusions/Significance: The visual patchiness of microbial mats reflects sharp discontinuities in microbial community structure and activity over sub-meter spatial scales; these discontinuities have to be taken into account in geochemical and microbiological inventories of seep environments. In contrast, 12–20 cm deep in the sediments microbial communities performing methane-cycling and sulfate reduction persist at lower metabolic rates regardless of mat cover, and may increase activity rapidly when subsurface flow changes. Citation: Lloyd KG, Albert DB, Biddle JF, Chanton JP, Pizarro O, et al. (2010) Spatial Structure and Activity of Sedimentary Microbial Communities Underlying a Beggiatoa spp. Mat in a Gulf of Mexico Hydrocarbon Seep. PLoS ONE 5(1): e8738. doi:10.1371/journal.pone.0008738 Editor: Ching-Hong Yang, University of Wisconsin-Milwaukee, United States of America Received July 23, 2009; Accepted December 16, 2009; Published January 15, 2010 Copyright: 2010 Lloyd et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This study was supported by the National Aeronautics and Space Administration (NASA) Astrobiology Institutes ‘‘Environmental genomes’’ (NCC 2- 1054) and ‘‘Subsurface Biospheres’’ (NCC 2-1275) (A.T) and an Environmental Protection Agency (EPA) STAR fellowship # 91671401-0 (K. G. L.) and through a grant from the University of Mississippi Gulf of Mexico Hydrate Research Consortium and Department of Energy. This portion of the Gulf of Mexico Gas Hydrate Research Consortium cruise was funded by United States Minerals Management Services. Tagged sequencing was supported by the W.M. Keck Foundation through the International Census of Marine Microbes. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: karen.lloyd@biology.au.dk Introduction In deep-sea hydrocarbon seeps, fluids that originate from thermal maturation of deeply buried fossil organic carbon seep into the upper sediment column, where they often solidify into methane-rich hydrates and may contribute to global climate forcing in episodic releases [1,2]. Hydrocarbon seeps are not evenly distributed, but are found at localized hot spots dictated by the location of underlying conduits and fracture zones that vary through space and time [3,4,5,6,7]. Temporal shifts in hydrocar- bon seeps result from relocation of subsurface conduits or from the temperature-driven destabilization of subsurface gas hydrates. Deep-source fluids and hydrates are transformed in surface sediments by highly active, benthic microbial ecosystems, which determine gas emissions and drive carbonate formation through methanogenesis, or sulfate reduction coupled to hydrocarbon oxidation [8,9,10]. The products of these anaerobic microbial processes, such as sulfide, incompletely oxidized organic com- pounds or dissolved inorganic carbon (DIC), are suitable substrates for sulfide-oxidizing Beggiatoa spp. These large, filamentous bacteria can be white, yellow, or orange and form extensive microbial mats with diameters of up to several meters, which cover PLoS ONE | www.plosone.org 1 January 2010 | Volume 5 | Issue 1 | e8738