Microbial oxidation and utilization of orthophosphite during growth.

In the soil there exist cycles of biological oxidation and reduction for inorganic compounds of sulfur and nitrogen. We have been interested in the possibility that a similar cycle might exist for inorganic compounds of phosphorus. In particular, we have been interested in one step of such a cycle, that of biological oxidation of orthophosphite to orthophosphate. There are few reports in the literature pertaining to the utilization of phosphite phosphorus by microorganisms. Dox (1911), in reporting studies on the growth of Aspergillus niger on media containing various phosphorus sources, concluded that phosphite phosphorus was neither available nor toxic to the fungus. No growth occurred when phosphite was the only phosphorus source but growth did occur if phosphate also was present in the medium. Robertson and Boyer (1956a, b) considered orthophosphite and various organic phosphite compounds to be inert in biological systems. In fact, these workers proposed that orthophosphite be used as a buffer for biological studies in the range of pH 5.5 to 7.5. A later report (Bulen and Frear, 1957) has shown, however, that for some biological systems phosphite cannot be used as a buffer in this pH range. In this instance it was found that phosphite inhibited growth of Azotobacter agilis strain 0 (A. vinelandii) when cultured in Burk's medium with gaseous nitrogen as the nitrogen source. Inhibition was not observed when ammonium acetate served a similar function. Furthermore, these workers found that phosphite inhibited oxygen consumption by washed cells fixing nitrogen and that under these conditions the nitrogen fixation also was inhibited. Orthophosphate had little effect on these reactions. The most convincing report of microbial oxidation of orthophosphite to orthophosphate