Anaerobic Bacteria in the Sediment Oxidaze Methane Present in the Sediment and Sulfate

Lara Beth Ainley

Miguel Simões Baptista

Buga Berković

student no. 37492

student no. 37574

student no. 37483

EMBC

May 2009

Faro

Summary

Large quantities of methane lie beneath the seafloor dissolved in pore fluids, crystallized in solid phase gas hydrates, or as a free gas. Little of this methane reaches the oxic water column, given that most of it is oxidized to CO2 by microbes that inhabit anoxic marine sediments. Cold seeps are the areas of the seafloor where hydrogen rich fluids leach from the sediment. Methane is the primary seepage fluid characterising cold seeps, but often hydrogen sulphides and hydrocarbons are also present.Generally, light independent extremophiles, chemotrophs, archaea and bacteria dominate the biological assemblage of cold seep environments. Cold seeps are usually found at both geologically active and passive plate margins where the interstitial waters of the sediment are enriched with methane that is forced upwards and out of the sediment due to pressure gradients.

There are a number of processes responsible for the formation of methane in cold seep sediments. Methane can form by the decomposition and thermogenic degradation of organic matter. Geologically, methane hydrates form due to mineral dehydration and gas migration. Overpressuring of the sediment causes these hydrates to migrate along geological fault lines. Finally, Anaerobic Methane Oxidation (AOM) is an important biochemical process responsible for the formation of methane in the sediment. Also a form of methanogenesis, AOM is mediated by the respiration of anaerobic microbial organisms. This process occurs in anoxic sediments where the methane is oxidised by a syntrophic consortium of anaerobic methanotrophics, typically Archaea, and often occurs in conjunction with sulphate reduction by symbiotic bacteria. AOM and sulphate reduction lead to the formation of authigenic carbonates and hydrogen sulphide in the sediments.

Processes of chemosynthesis related to cold seeps are due to the bacterial assemblages in these habitats. Besides free living bacteria, which can form extensive mats, there can be found animals having symbiotic bacteria that oxidise sulphur compounds.Such chemotrophic bacteria found in the symbiosis with marine invertebrates, need reduced inorganic compounds and oxygen. In the case of cold seeps, reduced compounds can be both sulphide and methane. But the oxygen can usually only be found in narrow boundary zone between oxic and anoxic environments, due to the rapid oxidation. Hosts of chemoautotrophic bacteria, in this case clams and worms, serve to gap that boundary to provide everything necessary to their symbiotic bacteria.Anaerobic bacteria are found where they can oxidise the methane present in the sediment. Simultaneously sulfate rich water diffuses into the surrounding sediments and is also used by the same group of the bacteria along with the methane. As methane and sulfate are processed, large amounts of hydrogen sulfide and carbon dioxide are produced. Hydrogen sulfide is then used along with the oxygen from the water by the symbiotic bacteria. Clams found in the area differ from other clams found in the oceans, because they have to take up the oxygen and carbon dioxide through the gills and sulfide through the foot, which is in the sediment, to meet the needs of their symbiotic bacteria.

Although a growing amount of research is focusing on the microbial presence around cold seeps, still many questions remain unanswered regarding the biochemical pathways, microbes involved and the physiological interactions regulating AOM. Recent genetic studies supported the existence of several archaeal groups involved in AOM. The existence of distinct compounds found in lipid biomarkers and the diversity of archaeal 16S rRN genes found in methane seep sediments both indicate the presence of several groups of putative methane-oxidizing Archaea, including ANME-1 and ANME-2. Furthermore, the existence of microbial consortia coupling AOM with sulfate reduction has been confirmed upon the observation of archaeal group ANME-2 as being physically associated with Desulfosarcina spp, a sulfate-reducing bacteria. In each microbial aggregate, a central core of ANME-2 is surrounded by a shell of its sulfate-reducing partner. In the reported case, ANME-2 bacteria is believed to mediate “reverse methanogenesis” and Desulfosarcina spp is assumed to couple the oxidation of incompletely oxidized byproducts of AOM (e.g. H2, acetate or formate) to the reduction of sulfate. Also, it has been found that in some regions over 90% of the archaeal and sulfate-reducing bacterial biomass are caused by the above mentioned consortia, indicating that this type of association dominates the anaerobic methanotrophication activity at methane cold seeps. Still, circumstantial evidence indicates the possibility of additional methane-oxidizing archaeal types. Both ANME-2 and ANME-1 archaea also appear to exist aggregated with bacterial partners other than Desulfosarcina and as monospecific aggregates in some circumstances, both enforcing the belief that they are actively involved in AOM. ANME-1 however, more frequently occur as monospecific mats or single filaments and appear to be less dependent on the activities of a closely associated bacterial partner. ANME-1 and ANME-2 archaea play a crucial role in the establishment and success of methane-seep communities through the conversion of methane into more readily accessible carbon and energy substrates.

Other studies used fluorescence in situ hybridization (FISH) and detected sulphide-oxidizing Beggiatoa that generally occur as thick mats. These archaea, however, were also found coupled with delta-proteobacteria (Desulfosarcina and Desulfococcus) that surrounded a cluster of Beggiatoa. These consortia where highly abundant in surface sediments at sulphide concentrations < 10mM, and are believed to mediate the anaerobic oxidation of methane. This process is supposed to be a reversal of methane formation that involves methanogens and a sulphate reducing partner. These consortia should occur as a result of the benefits of a highly efficient transfer of intermediates by molecular diffusion.

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COLD SEEPS IN THE OCEANS

By Romana Roje (37491), Shauna Narine (38546), Victoria Kessareva (36095) and Jeff Bogart Abroguena (37489)

Cold seeps are deep-sea environments where methane gas or hydrogen sulfide is released from large storage of the seafloor by slow diffusion. Fluid flow is driven by pressure gradients created through tectonic compaction, unsteady sedimentation and diagenetic processes. Cold seeps are mainly located along subduction zones or continental margins and function as habitat and energy source for chemosynthetic primary production, particularly for unique symbiotic marine communities inhabiting depth about from15m to more than 7,500m. They are considered the most recent marine habitat first discovered on the Florida Encampment, Gulf of Mexico in 1984. Brine pools or ‘seafloor lakes’ are hypersaline and methane-rich areas in the deep sea environment and are linked to cold seep’s symbiotic communities of chemosynthetic microbes and macrofauna inhabiting the pool edge. Brines evolve by fluid migration through faults which are established above salt deposits. Mud volcanoes, also associated with seeps are active areas of fluid seepage, discovered in the 1990s. Chemoherms are massive carbonate structures where discharged methane is converted to carbonate. Gas hydrates consist of water and methane and are only stable under elevated pressure and low temperature, may evolve together with oil leakages from hydrocarbon deposits and from decomposition of organic matter. More than 550 species have been identified in cold seeps which are characterized by considerable variations in the concentrations of sulfide, methane and other chemical constituents and mechanisms regulating fluid flow. Methane is formed in the marine environment by anaerobic decay of organic matter in sediments and the water column, and in the form of gas hydrates and at cold water seeps in ocean sediments. Growth and metabolism of the associated macrofauna are based on a chemoautotrophic endosymbiotic association with the bacteria which has the ability to chemosynthetically derive energy from hydrogen sulfide when converting to sulphate. Dominant seep macrofauna (usually also endemic) consists of bivalve families as Vesicomyidae (Calyptogena), Mytilidae, Thyasiridae, Lucinidae and Solemyidae (Solemya and Acharax); agreggates of Vestimentiferan tube worms (siboglinid polychaetes) including the most common genus Lamellibrachia; ice worms, pogonophora worms, and sponges as Cladorhizidae and Hymedesmiidae. But there are also a lot of visiting scavenging and predatory fish (hakes, pancake bat fish) and crustaceans, deposit-feeding gastropods and holothurians, suspension-feeding polychaetes and anemones. Reproductive patterns of species occurring at vents and seeps are similar to those of species from the same phyla found in non-chemosynthetic environments. The most common species, Lamellibrachia sp., grows very slowly (averaging 0.77 cm/year) and commonly reaches lengths over 2 m. It was calculated that individuals in mature aggregations are a minimum of 100 years old. Three types of bacteria are found ii cold seeps aerobic symbiotic (in gill of clam) or free living on the surface of sediment (depend on H2S) that make mats, or anaerobic bacteria in the sediment that produce methane and sulfide. Several archaeal assemblages (dependant on specific environmental conditions) are involved in the anaerobic consumption of methane. Microbial mediated anaerobic oxidation of methane (AOM) includes methane oxidation with sulfate and yielding equimolar amounts of carbonate and sulfide. AOM can be mediated by structured consortia consisting of methanotrophic archaea (ANME group 2) belonging to the order Methanosarcinales (or a second archaeal group (ANME-1) distantly related to the Methanosarcinales and Methanomicrobiales) and sulfate-reducing bacteria (SRB) belonging to the Desulfosarcina-Desulfococcus branch (DSS) of the Deltaproteobacteria, as syntrophic partners. Anaerobic methane oxidation is coupled to sulfate reduction: CH4 + SO42- → HCO3- + HS- + H2O.At methane seeps authigenic carbonates may be generated from the biogeochemical turnover yielding hydrogen sulfide and bicarbonate ions which subsequently react with ions derived from pore waters and the water column to form sulfidic and carbonaceous minerals, accompanied by an increase in pore water alkalinity, performed by various archaeal assemblages working in syntrophic co-operation with sulfate-reducing bacteria. AOM processes are important for transferring hydrocarbon-derived energy and carbon to higher trophic levels via grazing or symbiotic interactions, and reducing the accumulation of toxic petroleum hydrocarbon compounds. And AOM is the major biological sink of the greenhouse gas methane in marine sediments, consuming up to 90 % of the methane produced there.

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