Biochemistry and genetics of organoautotrophy in Alcaligenes eutrophus

Abstract

Different metabolic capabilities have led to the distinction of two major groups of autotrophic organisms: (i) obligate and (ii) facultative autotrophs. The former are confined to the assimilation of CO₂ as the main source of cell carbon, whereas the latter are able to utilize many organic compounds as alternative or additional carbon and/or energy sources. The nutritional versatility varies considerably among the different subgroups of aerobic, facultatively autotrophic bacteria. Simultaneous utilization of inorganic and organic carbon and/or energy substrates for growth is designated mixotrophy and a rather common feature of these organisms (Barton et al. 1991). In this context the use of C₁ and related compounds such as methanol, formate and methylated sulfides are of particular interest. These compounds or their carbon moieties are oxidized to CO₂ for conservation of energy and generation of reducing power required for CO₂ assimilation (Quayle 1972; Kelly, Baker 1990). Such special type of mixotrophy is termed organoautotrophy as opposed to the conventional lithoautotrophic metabolism in these organisms that is based on the oxidation of inorganic energy-yielding substrates. In all bacteria studied so far for this property, CO₂ fixation involves the Calvin-Benson-Bassham carbon reduction pathway (Calvin cycle).