Page 93 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Desulfovibrio and Bacillus possess the capacity to couple organic compound oxidation to iron reduction
The amplicon data at 17 and 25 months revealed that Desulfovibrio accounted for 23% and 26% of the bacterial reads, respectively, whereas metagenome 16S rRNA reads indicated lower relatively lower abundance (10%) (Fig. 2). 16S rRNA and phylogenetic analysis of genes encoding sulfite reductase (DsvAB, the Dsr of Desulfovibrio vulgaris) revealed that D. vulgaris was the dominant organism. Desulfovibrio vulgaris belongs to a group of sulfate reducers that break down organic acids incompletely and produce acetate as end-product (Heidelberg et al. 2004). However, sulfate was not detected after the first three months (< 0.02 mM) and FeS had accumulated in the sediment (data not shown). Upon sulfate depletion, D. vulgaris can switch to Fe(III) reduction, as well as to the reduction of Cr(VI) and U(VI) that occur as trace elements in sub-bituminous coal (Querol, Fernández-Turiel and López-Soler 1995; Heidelberg et al. 2004). However, whether this strategy can be used by D. vulgaris to compete for oxidized iron and to simultaneously produce acetate as substrate for acetoclastic methanogenesis should be further investigated.
Baccillaceae were nearly absent at 17 months (0.7%) and more abundant at 25 months (2% and 5% of amplicon and metagenome 16S rRNA reads, respectively, Fig. 2) with high 16S rRNA gene sequence identity to Bacillus subterraneus, a species that can use Fe(III), Mn(IV), nitrate, nitrite and fumarate as electron acceptors (Kanso, Greene and Patel 2002; Strąpoć et al. 2008). A review by Nealson and Saffarini (1994) indicated that the iron and manganese reduction capacity of Bacillus species was more widespread (Nealson and Saffarini 1994). Further work to assess coupling of metal reduction to organic compounds degradation and growth in Bacillus species is required.
Acetoclastic Methanothrix (formerly: Methanosaeta) species are responsible for methanogenesis
Both at 17 and 25 months, Methanosaetaceae were the dominant archaeal family (≥ 99%). Based on the relative abundance data from the metagenome, they constitute 1% of the total microbial community (Fig. 2). Methane concentration measurements indicated highest methanogenic activity between 12 and 19 months after amendment. All members of the family Methanosaetaceae are strictly acetoclastic methanogens, indicating that hydrogenotrophic
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