A MEDIUM OPTIMIZATION STRATEGY FOR IMPROVEMENT OF GROWTH AND METHANE PRODUCTION BY METHANOBACTERIUM-THERMOAUTOTROPHICUM

Methanobacterium thermoautotrophicum, strain Hveragerdi, has been cult ivated in a completely defined mineral salts medium, under strictly an aerobic conditions with CO2 and H-2 as sole carbon and energy source, respectively. During optimization of the medium an iron limitation was identified that could not be overcome by the simple addition of iron- -or iron complexed with nitrilotriacetate (NTA)--to the medium, due to the formation of insoluble FeS complexes. In order to define a medium optimization strategy, and to avoid laborious empirical optimization procedures, a theoretical model has been developed in order to describ e the solubility of iron and other mineral species in the medium as a function of the concentration of sulfur and NTA. This model may be app lied for the optimization of any medium component. With this informati on sulfide has been replaced by a combination of cysteine and thiosulp hate in conjunction with a non-toxic reducing agent (titanium (III) ci trate). Using this defined medium precipitation was avoided and an iro n limitation was overcome resulting in a 5-fold improvement of the fin al biomass concentration from 2-3 g l(-1) to 11.2 g l(-1) together wit h a 2-fold increase (from 45 to almost 100%) in the conversion of CO2 and H-2 to CH4, even at gas flow rates as high as 6 1 min(-1).