Description: The ultramafic-hosted Logatchev hydrothermal field (LHF) is characterized by vent fluids, which are enriched in dissolved hydrogen and methane compared with fluids from basalt-hosted systems. Thick sediment layers in LHF are partly covered by characteristic white mats. In this study, these sediments were investigated in order to determine biogeochemical processes and key organisms relevant for primary production. Temperature profiling at two mat-covered sites showed a conductive heating of the sediments. Elemental sulfur was detected in the overlying mat and metal-sulfides in the upper sediment layer. Micro-profiles revealed an intensive hydrogen sulfide flux from deeper sediment layers. Fluorescence in situ hybridization showed that filamentous and vibrioid, Arcobacter-related Epsilonproteobacteria dominated the overlying mats. This is in contrast to sulfidic sediments in basalt-hosted fields where mats of similar appearance are composed of large sulfur-oxidizing Gammaproteobacteria. Epsilonproteobacteria (7-21%) and Deltaproteobacteria (20-21%) were highly abundant in the surface sediment layer. The physiology of the closest cultivated relatives, revealed by comparative 16S rRNA sequence analysis, was characterized by the capability to metabolize sulfur components. High sulfate reduction rates as well as sulfide depleted in (34)S further confirmed the importance of the biogeochemical sulfur cycle. In contrast, methane was found to be of minor relevance for microbial life in mat-covered surface sediments. Our data indicate that in conductively heated surface sediments microbial sulfur cycling is the driving force for bacterial biomass production although ultramafichosted systems are characterized by fluids with high levels of dissolved methane and hydrogen

Description: Lilliput was discovered in 2005 as the southernmost known hydrothermal field along the Mid-Atlantic Ridge. It is exceptional in that it lacks high-temperature venting probably because of a thickened crust. The absence of thermophilic and hyperthermophilic prokaryotes in emissions supports the argument against the presence of a hot subsurface at Lilliput, as is typically suggested for diffuse emissions from areas of high-temperature venting. The high phylogenetic diversity and novelty of bacteria observed could be because of the low-temperature influence, the distinct location of the hydrothermal field or the Bathymodiolus assemblages covering the sites of discharge. The low-temperature fluids at the Lilliput are characterized by lowered pH and slightly elevated hydrogen (16 nM) and methane (∼2.6 μM) contents compared with ambient seawater. No typical hydrogen and methane oxidizing prokaryotes were detected. The higher diversity of reverse tricarboxylic acid genes and the form II RubisCO genes of the Calvin Benson-Bassham (CBB) cycle compared with the form I RubisCO genes of the CBB cycle suggests that the chemoautotrophic community is better adapted to low oxygen concentrations. Thiomicrospira spp. and Epsilonproteobacteria dominated the autotrophic community. Sulfide is the most abundant inorganic energy source (0.5 mM). Diverse bacteria were associated with sulfur cycling, including Gamma-, Delta- and Epsilonproteobacteria, with the latter being the most abundant bacteria according to fluorescence in situ hybridization. With members of various Candidate Divisions constituting for 25% of clone library sequences we suggest that their role in vent ecosystems might be more important than previously assumed and propose potential mechanisms they might be involved in at the Lilliput hydrothermal field.