Two-membered mixed cultures of methanogenic and aerobic bacteria in O2-limited chemostats

Three different co-cultures composed of a methanogenic and a strictly aerobic bacterium were grown under O2-limitation in continuous cultures. The combinations used were (1) Methanobacterium formicicum with the aerobic heterotroph Comamonas testosteroni; (2) M. formicicum with a methanotrophic Methylocystis species, and (3) Methanosarcina barkeri with C. testosteroni. Although true steady-states were not obtained, growth and metabolic activity of the methanogenic and aerobic organisms occurred during O2-limited growth of these mixed cultures over extended periods of time. Co-cultures with C. testosteroni were considerably more stable than those with Methylocystis. Co-cultures with M. barkeri were less O2-sensitive than those with M.formicicum. C. testosteroni exhibited a higher O2-affinity than Methylocystis, resulting in a lower dissolved oxygen tension and a superior protection of the methanogenic bacteria against O2-poisoning than in mixed cultures with Methylocystis. The dissolved O2-concentrations in the mixed cultures were below the detection limit of the O2-probes used (0.2 muM). Calculations based on growth properties of pure cultures of C. testosteroni, M. barkeri and M. formicicum suggested that the dissolved O2-concentrations in the mixed cultures, as well as the O2-inhibition constants (apparent K(i)O2) of the methanogens were in the nanomolar range.