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A correlation of anaerobic methane oxidizing archaea with geochemical gradients in coastal Californian marine sediments Constan, Lea

Abstract

The anaerobic oxidation of methane (AOM) is a globally important process estimated to consume over 384.2 teragrams of the greenhouse gas methane in sediments per year. At least three anaerobic methane oxidizing archaeal groups, ANME-1, -2, -3, which further partition into subgroups, have been implicated in this process. However, none of these organisms have been isolated in pure culture and the ecological and functional dynamics of AOM remain poorly understood. The ANME express a homologue of methyl coenzyme M reductase (Mcr), known to catalyze methane formation in methane-producing archaea. These and other findings suggest that the ANME are capable of oxidizing methane through a reversal of methanogenesis. Although small subunit ribosomal DNA (SSU) is a good proxy for the identification of ANME groups, it is not sufficiently divergent to easily distinguish between ANME subgroups. The gene for the alpha subunit of Mcr (mcrA), on the other hand, is divergent enough to resolve fine scale ANME subgroup distributions and is a functional marker for AOM, as well. The objective of this study was to evaluate the role of geochemical parameters in the partitioning of ANME-1 and ANME-2 subgroups in coastal seep sediments. Using a set of quantitative polymerase chain reaction (qPCR) assays to target the mcrA subgroups, samples were quantified within and between three separate seep sediment sites off the coast of California and correlated to methane, sulfate, sulfide, ammonia and alkalinity measurements. The analysis indicates that methane porewater concentrations dictate the upper limit of total ANME abundance, suggesting it is the sole ANME energy source. ANME-2 population structure was strongly influenced by the defined environmental variables, with differential subgroup partitioning corresponding to sulfate and ammonia gradients. In contrast, the relationship of ANME-1 with chemistry was not well resolved, indicating that other factors, such as competition may be the prime determinants of ANME-1 population structure. The study establishes a defined set of parameters governing ANME subgroup partitioning, which provides a robust quantitative framework for inferring the ecological dynamics of methane oxidizing archaeal communities in a wide variety of settings.

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