<jats:title>ABSTRACT</jats:title> <jats:p> In this study we determined the composition and biogeochemistry of novel, brightly colored, white and orange microbial mats at the surface of a brine seep at the outer rim of the Chefren mud volcano. These mats were interspersed with one another, but their underlying sediment biogeochemistries differed considerably. Microscopy revealed that the white mats were granules composed of elemental S filaments, similar to those produced by the sulfide-oxidizing epsilonproteobacterium “ <jats:italic>Candidatus</jats:italic> Arcobacter sulfidicus.” Fluorescence in situ hybridization indicated that microorganisms targeted by a “ <jats:italic>Ca</jats:italic> . Arcobacter sulfidicus”-specific oligonucleotide probe constituted up to 24% of the total the cells within these mats. Several 16S rRNA gene sequences from organisms closely related to “ <jats:italic>Ca</jats:italic> . Arcobacter sulfidicus” were identified. In contrast, the orange mat consisted mostly of bright orange flakes composed of empty Fe(III) (hydr)oxide-coated microbial sheaths, similar to those produced by the neutrophilic Fe(II)-oxidizing betaproteobacterium <jats:italic>Leptothrix ochracea</jats:italic> . None of the 16S rRNA gene sequences obtained from these samples were closely related to sequences of known neutrophilic aerobic Fe(II)-oxidizing bacteria. The sediments below both types of mats showed relatively high sulfate reduction rates (300 nmol·cm <jats:sup>−3</jats:sup> ·day <jats:sup>−1</jats:sup> ) partially fueled by the anaerobic oxidation of methane (10 to 20 nmol·cm <jats:sup>−3</jats:sup> ·day <jats:sup>−1</jats:sup> ). Free sulfide produced below the white mat was depleted by sulfide oxidation within the mat itself. Below the orange mat free Fe(II) reached the surface layer and was depleted in part by microbial Fe(II) oxidation. Both mats and the sediments underneath them hosted very diverse microbial communities and contained mineral precipitates, most likely due to differences in fluid flow patterns. </jats:p>