Molecular Characterization of Methanotrophic and Chemoautotrophic Communities at Cold Seeps

Abstract

Cold seeps are complex ecosystems based on chemosynthesis. Sulfide and methane are available in high concentrations and support a variety of highly adapted microorganisms and symbiont-bearing invertebrates. The anaerobic oxidation of methane (AOM) is a key biogeochemical process at cold seeps and is assumed to be mediated by a consortium of anaerobic methanotrophic archaea (ANME) and sulfate-reducing bacteria. In this thesis I used 16S rRNA-based molecular methods to identify and quantify methanotrophic and chemoautotrophic, sulfur-oxidizing communities at two cold seeps and collaborated with scientists from other disciplines to correlate the molecular results with biogeochemical, geological, and physical data sets. Two types of cold seeps were studied, the Arctic Haakon Mosby Mud Volcano (HMMV; 1250 m water depth) and the Hydrate Ridge at the Cascadia Margin (700 m water depth) off the coast of Oregon. The actively methane-seeping HMMV hosts novel clades of aerobic and anaerobic methanotrophs. The distribution of the methanotrophic guilds is controlled by fluid flow and bioirrigation activities of marine invertebrates. The center of the volcano is characterized by high upward fluid flow. High numbers of novel aerobic methanotrophs were detected in the upper millimeters of the sediment. These dominated the microbial community in surface sediments of HMMV. At sites with decreased upward fluid flow, a new group of ANME archaea (ANME-3) was identified, dominating the zone of AOM in 1-4 cm sediment depth. In this zone, the microbial biomass was almost entirely comprised by ANME-3 archaea which form consortia with sulfate-reducing bacteria of the Desulfobulbus branch. In bioirrigated sediments populated by siboglinid tubeworms, sulfate is transported deeper into the sediment and here the AOM consortia were found at the base of the tubeworm roots. The symbioses between bacteria and the tubeworms at HMMV were characterized. Two species of tubeworms coexist at the same site and represent the dominant megafauna at HMMV. The symbionts of Sclerolinum contortum were identified as chemoautotrophic sulfur oxidizers and are closely related to symbionts of vestimentiferan tubeworms. The symbionts of Oligobrachia haakonmosbiensis represent a novel symbiont lineage. The molecular results in combination with stable carbon isotope analysis suggest that this species may harbor both chemoautotrophic sulfur-oxidizing and methane-oxidizing symbionts. Another focus of the thesis was to compare microbial communities in gas hydrate bearing shallow subsurface sediments with surface communities at Hydrate Ridge. The microbial diversity in these habitats was similar. Cells of the ANME-1 clade dominated the archaeal community in shallow subsurface sediments with highest numbers in depths directly above two gas hydrate layers. A novel type of microbial consortium was detected consisting of ANME-1 archaea and sulfate-reducing bacteria of the Desulfosarcina/Desulfococcus branch. The microbial community in gas hydrate melts was dominated by either ANME-1 or ANME-2 archaea. These cells had very low rRNA contents, indicating that they may have been inactive for extended periods. In contrast, ANME cells in sediments surrounding gas hydrates had high rRNA contents even at 1 m below the seafloor suggesting that sulfate and methane are available for the microbial community in distinct subsurface horizons.