A Win–Loss Interaction on Fe(0) Between Methanogens and Acetogens From a Climate Lake

Diverse physiological groups congregate into environmental corrosive biofilms, yet the interspecies interactions between these corrosive physiological groups are seldom examined. We, therefore, explored Fe(0)-dependent cross-group interactions between acetogens and methanogens from lake sediments. On Fe(0), acetogens were more corrosive and metabolically active when decoupled from methanogens, whereas methanogens were more metabolically active when coupled with acetogens. This suggests an opportunistic (win–loss) interaction on Fe(0) between acetogens (loss) and methanogens (win). Clostridia and Methanobacterium were the major candidates doing acetogenesis and methanogenesis after four transfers (metagenome sequencing) and the only groups detected after 11 transfers (amplicon sequencing) on Fe(0). Since abiotic H(2) failed to explain the high metabolic rates on Fe(0), we examined whether cell exudates (spent media filtrate) promoted the H(2)-evolving reaction on Fe(0) above abiotic controls. Undeniably, spent media filtrate generated three- to four-fold more H(2) than abiotic controls, which could be partly explained by thermolabile enzymes and partly by non-thermolabile constituents released by cells. Next, we examined the metagenome for candidate enzymes/shuttles that could catalyze H(2) evolution from Fe(0) and found candidate H(2)-evolving hydrogenases and an almost complete pathway for flavin biosynthesis in Clostridium. Clostridial ferredoxin-dependent [FeFe]-hydrogenases may be catalyzing the H(2)-evolving reaction on Fe(0), explaining the significant H(2) evolved by spent media exposed to Fe(0). It is typical of Clostridia to secrete enzymes and other small molecules for lytic purposes. Here, they may secrete such molecules to enhance their own electron uptake from extracellular electron donors but indirectly make their H(2)-consuming neighbors—Methanobacterium—fare five times better in their presence. The particular enzymes and constituents promoting H(2) evolution from Fe(0) remain to be determined. However, we postulate that in a static environment like corrosive crust biofilms in lake sediments, less corrosive methanogens like Methanobacterium could extend corrosion long after acetogenesis ceased, by exploiting the constituents secreted by acetogens.