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).