Methanogenesis at extremely haloalkaline conditions in the soda lakes of Kulunda Steppe (Altai, Russia)

Abstract

Microbial methanogenesis at extreme conditions of saline alkaline soda lakes has, so far, been poorly investigated. Despite the obvious domination of sulfidogenesis as the therminal anaerobic process in the hypersaline soda lakes of Kulunda Steppe (Altai, southwestern Siberia), high concentrations of methane were detected in the anaerobic sediments. Potential activity measurements with different substrates gave results significantly deviating from what is commonly found in hypersaline habitats with neutral pH. In particular, not only a non-competitive methylotrophic pathway was active, but also lithotrophic and, in some cases, even acetate-dependent methanogenesis was found to be present in hypersaline soda lake sediments. All three pathways were functioning exclusively within the alkaline pH range between 8 and 10.5, while the salt concentration was the key factor influencing the activity. Methylotrophic and, to a lesser extent, lithotrophic methanogenesis were active up to soda-saturating conditions (4 M total Na(+)). Acetate-dependent methanogenesis was observed at salinities below 3 M total Na(+). Detection of methanogens in sediments using the mcrA gene as a functional marker demonstrated domination of methylotrophic genera Methanolobus and Methanosalsum and lithotrophic Methanocalculus. In a few cases, acetoclastic Methanosaeta was detected, as well as two deep lineage methanogens. Cultivation results corresponded well to the mcrA-based observations. Enrichments for natronophilic methylotrophic methanogens resulted in isolation of Methanolobus strains at moderate salinity, while at salt concentrations above 2 M Na(+) a novel member of the genus Methanosalsum was dominating. Enrichments with H2 or formate invariably resulted in domination of close relatives of Methanocalculus natronophilus. Enrichments with acetate at low salt concentration yielded two acetoclastic alkaliphilic Methanosaeta cultures, while at salinity above 1 M Na(+) syntrophic associations were apparently responsible for the observed acetate conversion to methane. Overall, the results indicated the presence of functionally structured and active methanogenic populations in Siberian hypersaline soda lakes.