The effect of trace elements on the metabolism of methanogenic consortia

Trace metals are essential for the growth and metabolism of anaerobic microorganisms, duo to their roles in key enzymes or cofactors of metabolic pathways. The requirement of trace metals has been recognized. But, proper dosing of these metals in anaerobic treatment system as nutrient still is a great challenge, since dosing of a metal at a high concentration is toxic for growth of microorganisms, and dosing of a specific metal may lead out-compete of one group of microorganisms by the other. In order to obtain knowledge for optimization of metal dosing of anaerobic treatment system, the influence of trace metals, like cobalt, nickel, tungsten and molybdenum on the conversion of methanol and propionate were studied in this research.By using cobalt-sufficient medium, a methanogenic enrichment culture was enriched from a thermophilic lab-scale UASB reactor fed with methanol as carbon and energy source. From which a novel thermophilic obligate methylotrophic methanogenicarchaeon, strain L2FAW ^T , was isolated and characterized as Methanomethylovoransthermophila . The growth of strain L2FAW ^T on methanol is stimulated by the addition of cobalt; the optimal cobalt concentration is 0.5 to 2 M. therefore, cobalt is important for direct methanol conversion by this methanogen.On the other hand, a syntrophiccocultureof methanol degradation was enriched from the same sludge by using cobalt deficient medium, which consisted of a homoacetogen and ahydrogenotrophicmethanogen. Thiscoculturedegrades methanol partially to acetate and partially to methane, depending on the presence of cobalt. Acetate is the main product when cobalt is presence at high concentration; otherwise methane is formed as dominant products. Therefore, cobalt plays a role in the regulation of the pathway of methanol conversion. The optimal cobalt concentration of thecoculturefor complete methanogenesis from methanol is about 0.1 M. A thermophilic spore-forming bacteria, strain AMP, was isolated from thecoculture, and it is most closely related to Moorellathermoaceticabased on 16S rRNA analysis. Despite its high DNA-DNA homology with M.thermoacetica , strain AMP differs from M.thermoacetica on its inability to use glucose, formate and H ₂ /CO ₂ , and its uniquehydrogenogenicgrowth on CO. Moreover, strain AMP can grow on formate in acoculturewith ahydrogenotrophicmethanogen. It is described for the first time that a bacterium can grow on the conversion of formate to H ₂ and bicarbonate provided that hydrogen is consumed by a methanogen.The effect of cobalt and nickel on the corrinoid and F430 content and on growth of Methanosarcina barkeri on methanol was studied. Cobalt and nickel limitation was achieved and competition between cobalt and nickel uptake was observed. Uptake efficiency of cobalt was high at low cobalt concentration and decreased when the cobalt concentration in the medium was increased. Corrinoid and F430 content correlated positively with the cell content of the corresponding metal, but incorporation in the corrinoid and F430 was significant less at low cell metal contents, ranging from 35% to 80% for corrinoid and 5% to 15% for F430.The trace elements tungsten and molybdenum play an essential role in the growth of anaerobic microorganisms. Depletion of tungsten and/or molybdenum in the media did not affect axenic growth of Syntrophobacterfumaroxidans onpropionate+fumarate, indicating under these conditions this organism does not have a high tungsten or molybdenum requirement. However, growth of Methanospirillumhungatei on either formate or hydrogen and carbon dioxide required tungsten, and molybdenum can replace tungsten to some extent. Growth of the Syntrophobacter-Methanospirillumcocultureon propionate is significantly affected by the addition of these two metals. Measurement of enzyme levels in cell extracts of syntrophically grown cells indicated that the levels of hydrogenaseandformatedehydrogenaseactivity were correlated with the methane formation rates by thecocultures, which suggests both hydrogen and formate play important role in syntrophic propionate oxidation.