Importance of cobalt for individual trophic groups in an anaerobic methanol-degrading consortium

Methanol is an important anaerobic substrate in industrial wastewater treatment and the natural environment. Previous studies indicate that cobalt greatly stimulates methane formation during anaerobic treatment of methanolic wastewaters. To evaluate the effect of cobalt in a mixed culture, a sludge with low background levels of cobalt was cultivated in an upflow anaerobic sludge blanket reactor. Specific inhibitors in batch assays were then utilized to study the effect of cobalt on the growth rate and activity of different microorganisms involved in the anaerobic degradation of methanol. Only methylotrophic methanogens and acetogens were stimulated by cobalt additions, while the other trophic groups utilizing downstream intermediates, H₂-CO₂ or acetate, were largely unaffected. The optimal concentration of cobalt for the growth and activity of methanol-utilizing methanogens and acetogens was 0.05 mg liter^-1. The higher requirement of cobalt is presumably due to the previously reported production of unique corrinoid- containing enzymes (or coenzymes) by direct utilizers of methanol. This distinctly high requirement of cobalt by methylotrophs should be considered during methanolic wastewater treatment. Methylotroph methanogens presented a 60-fold-higher affinity for methanol than acetogens. This result in combination with the fact that acetogens grow slightly faster than methanogens under optimal cobalt conditions indicates that acetogens can outcompete methanogens only when reactor methanol and cobalt concentrations are high, provided enough inorganic carbon is available.