Vertical distribution of structure and function of the methanogenic archaeal community in Lake Dagow sediment

Abstract

Detailed studies on the relation of structure and function of microbial communities in a sediment depth profile scarcely exist. We determined as functional aspect the vertical distribution of the acetotrophic and hydrogenotrophic CH₄ production activity by measuring production rates and stable ¹³C/¹²C‐isotopic signatures of CH₄ in the profundal sediment of Lake Dagow. The structural aspect was determined by the composition of the methanogenic community by quantifying the abundance of different archaeal groups using ‘real‐time’ polymerase chain reaction and analysis of terminal restriction fragment length polymorphism (T‐RFLP). Methane production rates in the surface sediment (0–3 cm depth) were higher in August than in May, but strongly decreased with depth (down to 20 cm). The δ¹³C of the produced CH₄ and CO₂ indicated an increase in isotopic fractionation with sediment depth. The relative contribution of hydrogenotrophic to total methanogenesis, which was calculated from the isotopic signatures, increased with depth from about 22% to 38%. Total numbers of microorganisms were higher in August than in May, but strongly decreased with depth. The increase of microorganisms from May to August mainly resulted from Bacteria. The Archaea, on the other hand, exhibited a rather constant abundance, but also decreased with depth from about 1 × 10⁸ copies of the archaeal 16S rRNA gene per gram of dry sediment at the surface to 4 × 10⁷ copies per gram at 15–20 cm depth. T‐RFLP analysis combined with phylogenetic analysis of cloned sequences of the archaeal 16S rRNA genes showed that the methanogenic community consisted mainly of Methanomicrobiales and Methanosaetaceae. The relative abundance of Methanosaetaceae decreased with depth, whereas that of Methanomicrobiales slightly increased. Hence, the vertical distribution of the functional characteristics (CH₄ production from acetate versus H₂/CO₂) was reflected in the structure of the community consisting of acetotrophic (Methanosaetaceae) versus hydrogenotrophic (Methanomicrobiales) phenotypes.