Ammonia toxicity in pure cultures of methanogenic bacteria

Summary The toxicity of ammonia to the growth of several methanogenic bacteria was evaluated in terms of an ammonia/potassium exchange reaction and in terms of inhibition of methanogenesis. Growth of Methanobrevibacter smithii, Methanobrevibacter arboriphilus and Methanobacterium strain G2R was normal in media containing up to 400 mM NH 4 Cl, and neither the exchange reaction nor inhibition of methane synthesis occurred. Exchange and inhibition of CH 4 synthesis by ammonia was found in shortterm studies with cell suspensions of Methanospirillum hungatei, Methanosarcina barkeri and Methanothrix concilii . Certain cations countered the toxic effects which ammonia had on methane synthesis, notably Ca 2+ in M. concilii and Na + in M. barkeri . During growth in media containing increasing amounts of NH 4 Cl, the cytoplasmic K + concentration declined dramatically only in M. hungatei , but this was not the mechanism of toxicity. Evidently the presence of counterions in the growth medium caused an enhanced tolerance of certain strains to ammonia. Methanobacterium bryantii displayed an appreciable decline in cytoplasmic K + content and grew slowly in media containing 300 mM NH 4 Cl. Ammoniatoxicity to the growth of all methanogenic bacteria tested can be correlated to alterations in their K + /NH 3 content and/or to inhibitions of CH 4 synthesis.