Investigation of the methanogen population structure and activity in a brackish lake sediment.

The methanogen community in sediment from the edge of a small brackish lake connected to the Beaulieu Estuary (Hampshire, UK) was investigated by analysis of 16S rRNA gene diversity using new methanogen-specific primers plusArchaea-specific primers. 16S rRNA gene primers previously used for polymerase chain reaction (PCR) detection of methanogenicArchaeafrom a variety of environments were evaluated byin silicotesting. The primers displayed variable coverage of the four main orders of methanogens, highlighting the importance of this type of primer evaluation. Three PCR primer sets were designed using novel reverse primers to facilitate specific amplification of the ordersMethanomicrobiales/ Methanosarcinales,MethanobacterialesandMethanococcales. Diversity of the methanogen functional gene, methyl coenzyme M reductase (mcrA), was also studied. All gene libraries constructed from this sediment indicated thatMethanomicrobialesandMethanosarcinaleswere the only methanogens detected. There was good agreement between the relative sequence abundances in the methanogen-specific 16S rRNA gene library and terminal restriction fragment length polymorphism (T-RFLP) profiling, suggesting that the population was dominated by putative H2/CO2 utilizingMethanomicrobiales, although acetate-utilizing methanogens were also present. The methanogen population analyses were in agreement with methanogenic activity measurements, which indicated that bicarbonate methanogenesis was higher than acetate methanogenesis at all depths measured and overall there was a significant difference (P = 0.001) between the rates of the two pathways. This study demonstrates the utility of new 16S rRNA gene PCR primers targeting specific methanogenic orders, and the combined results suggest that the CO2 reduction pathway dominates methanogenesis in the brackish sediment investigated. [ABSTRACT FROM AUTHOR]