Mats of psychrophilic thiotrophic bacteria associated with cold seeps of the Barents Sea

S. Grünke,V. Salman,A. Boetius,D. de Beer,A. Ramette,J. Felden,A. Lichtschlag,H. N. Schulz-Vogt

doi:10.5194/bg-9-2947-2012

Abstract

Abstract. This study investigated the bacterial diversity associated with microbial mats of polar deep-sea cold seeps. The mats were associated with high upward fluxes of sulfide produced by anaerobic oxidation of methane, and grew at temperatures close to the freezing point of seawater. They ranged from small patches of 0.2–5 m in diameter (gray mats) to extensive fields covering up to 850 m2 of seafloor (white mats) and were formed by diverse sulfide-oxidizing bacteria differing in color and size. Overall, both the dominant mat-forming thiotrophs as well as the associated bacterial communities inhabiting the mats differed in composition for each mat type as determined by microscopy, 16S rRNA gene sequencing and automated ribosomal intergenic spacer analysis. While the smaller gray mats were associated with a highly diverse composition of sulfide oxidizers, the larger white mats were composed of only 1–2 types of gliding Beggiatoa filaments. Molecular analyses showed that most of the dominant mat-forming sulfide oxidizers were phylogenetically different from, but still closely related to, thiotrophs known from warmer ocean realms. The psychrophilic nature of the polar mat-forming thiotrophs was tested by visual observation of active mats at in situ temperature compared to their warming to >4 °C. The temperature range of mat habitats and the variation of sulfide and oxygen fluxes appear to be the main factors supporting the diversity of mat-forming thiotrophs in cold seeps at continental margins.

Highlights

At cold seeps, hydrocarbon-rich muds, brines and gases are transported from the subsurface to the seafloor and into the hydrosphere (Milkov, 2000; Dimitrov, 2002; Judd et al, 2002; Hovland et al, 2005; Cathles et al, 2010)
The microbial mats investigated in this study were associated with three deep-sea cold seep habitats, i.e. Storegga, Nyegga and the Hakon Mosby mud volcano (HMMV), located at water depths between 720–1250 m at temperatures close to the freezing point of seawater (−0.7 to 0.2 ◦C)
900 km northeast of the Storegga Slide, the Hakon Mosby mud volcano is located on the southwest Barents Sea slope and hosts a circular structure of chemosynthetic habitats associated with different levels of fluid flow

Summary

Introduction

Hydrocarbon-rich muds, brines and gases are transported from the subsurface to the seafloor and into the hydrosphere (Milkov, 2000; Dimitrov, 2002; Judd et al, 2002; Hovland et al, 2005; Cathles et al, 2010). Thiotrophic bacteria can exploit the chemical energy from the oxidation of sulfide or sulfur with oxygen or nitrate for their cell metabolism and growth. At cold seeps, they occur abundantly as chemosynthetic symbionts of bivalves and tube worms (Dubilier et al, 2008), and as free-living bacteria forming conspicuous mats on the seafloor. They occur abundantly as chemosynthetic symbionts of bivalves and tube worms (Dubilier et al, 2008), and as free-living bacteria forming conspicuous mats on the seafloor These microbial mats can show different colors and shapes and mark biogeochemical hotspots at cold seeps, often overlying gas hydrates or accumulations of free gas (Larkin and Henk, 1996; Boetius and Suess, 2004; Joye et al, 2004; Niemann et al, 2006; Omoregie et al, 2008; Grunke et al, 2011).

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Conclusion