ALBERT

Description: Among the most extreme habitats on Earth, dark, deep, anoxic brines host unique microbial ecosystems that remain largely unexplored. As the terminal step of anaerobic degradation of organic matter, methanogenesis is a potentially significant but poorly constrained process in deep-sea hypersaline environments. We combined biogeochemical and phylogenetic analyses as well as incubation experiments to unravel the origin of methane in hypersaline sediments of Orca Basin in the northern Gulf of Mexico. Substantial concentrations of methane (up to 3.4 mM) coexisted with high concentrations of sulfate (16-43 mM) in two sediment cores retrieved from the northern and southern parts of Orca Basin. The strong depletion of 13C in methane (-77 to -89 per mill) pointed towards a biological source. While low concentrations of competitive substrates limited the significance of hydrogenotrophic and acetoclastic methanogenesis, the presence of non-competitive methylated substrates (methanol, trimethylamine, dimethyl sulfide, dimethylsulfoniopropionate) supported the potential for methane generation through methylotrophic methanogenesis. Thermodynamic calculations demonstrated that hydrogenotrophic and acetoclastic methanogenesis were unlikely to occur under in situ conditions, while methylotrophic methanogenesis from a variety of substrates was highly favorable. Likewise, carbon isotope relationships between methylated substrates and methane supported methylotrophic methanogenesis as the major source of methane. Stable isotope tracer and radiotracer experiments with 13C bicarbonate, acetate and methanol as well as 14C-labeled methylamine indicated that methylotrophic methanogenesis was the predominant methanogenic pathway. Based on 16S rRNA gene sequences, halophilic methylotrophic methanogens related to the genus Methanohalophilus dominated the benthic archaeal community in the northern basin but also occurred in the southern basin. High abundances of methanogen lipid biomarkers such as intact polar and polyunsaturated hydroxyarchaeols were detected in sediments from the northern basin, with lower abundances in the southern basin. Strong 13C-depletion of saturated and monounsaturated hydroxyarchaeol were consistent with methylotrophic methanogenesis as the major methanogenic pathway. Collectively, the availability of methylated substrates, thermodynamic calculations, experimentally determined methanogenic activity as well as lipid and gene biomarkers strongly suggested methylotrophic methanogenesis as predominant pathway of methane formation in the presence of sulfate in Orca Basin sediments.

Description: The Guaymas Basin hydrothermal vents in the Gulf of California have been visited during RV Atlantis mission AT37-06 in December 2016. Aim of this mission was to understand element fluxes and microbially driven reactions at the diffusive vent sites. The submarine ALVIN and push cores were used to sample undisturbed sediments at specific locations covering different temperature and geochemical regimes. From these cores porewater was sampled using Rhizons. Here we present concentrations of select anions in the porewater. We measured the concentrations of sulfate, sulfide, chloride, ammonia, phosphate and silicate.

Description: The Guaymas Basin hydrothermal vents in the Gulf of California (Mexico) were visited with the research vessel RV Atlantis in November to December 2018 (mission AT 42-05). Due to its proximity to land, this area is covered with thick organic-rich sediments. In the subsurface heated fluids enrich in hydrocarbons gases, leading to complex biogeochemical reactions in surface sediments (Teske et al., 2016; doi:10.3389/fmicb.2016.00075). We used the submarine ALVIN to collect sediments with push cores at sites covered with different mat types and surrounding areas. Aim of this effort was to describe the biogeochemistry and the fluxes of dissolved species in these sediments. We collected porewater from these push cores using Rhizon technology (Seeberg-Elverfeldt et al., 2005; doi:10.4319/lom.2005.3.361). Porewater sampling was performed in anoxic tents to preserve oxygen-sensitive dissolved metal species. Samples were preserved according to established protocols. In the home laboratory we measured concentrations of dissolved inorganic carbon, chloride, sulfate, sulfide, dissolved inorganic carbon, nitrate, nitrite, ammonium, phosphate and silicate and concentrations of dissolved iron using two approaches and manganese were assessed. Sediment cores is the direct vicinity of these cores were taken as replicates for the measurement of dissolved gases. Cores were sectioned and 2 ml sediments were sampled into glass-vials filled with 5 ml sodium hydroxide solutions and were sealed gas-tight. Concentrations of the hydrocarbon gases methane, ethane, propane, i-butane and butane were measured using gas chromatography coupled to flame ionization detection.