Acetate Assimilation by Nitrobacter agilis in Relation to Its “Obligate Autotrophy”

Abstract

Acetate (1 to 10 m m ) had no effect on the rate of nitrite oxidation or exponential growth by Nitrobacter agilis . However, acetate- 1 - 14 C and - 2 - 14 C were both assimilated by growing cultures, and acetate carbon contributed 33 to 39% of newly synthesized cell carbon. Carbon from acetate was incorporated into all of the major cell constituents, including most of the amino acids of cell protein and poly-β-hydroxybutyrate (PHB). Cultures grown in the presence of acetate showed a significant increase in turbidity, attributable in part to protein synthesis and the accumulation of PHB in the “post-exponential phase,” when the supply of nitrite was completely exhausted. Cell suspensons of N. agilis assimilated acetate in the absence of bicarbonate and even in the absence of nitrite. However, the addition of nitrite increased the rate of acetate assimilation by cell suspensions. The distribution of 14 C-acetate incorporated by cell suspensions was qualitatively similar to that found with growing cultures. Cell suspensions of N. agilis slowly oxidized acetate to CO 2 . Addition of nitrite suppressed CO 2 production from acetate but increased the assimilation of acetate carbon into cell material. N. agilis contained all the enzymes of the tricarboxylic acid cycle. Growth of N. agilis in the presence of acetate did not significantly affect the levels of the enzymes of the tricarboxylic acid cycle, but did result in a 100-fold increase in the specific activity of isocitratase. In contrast, carboxydismutase was partially repressed. N. agilis was grown heterotrophically through seven transfers on a medium containing acetate and casein hydrolysate. The addition of nitrite increased the rate of heterotrophic growth. Heterotrophically grown organisms still retained their ability to grow autotrophically with nitrite. However, these organisms oxidized nitrite at a slower rate. Organisms from autotrophic and heterotrophic cultures were analyzed to determine the mean guanine plus cytosine content of their deoxyribonucleic acid; in both cases this mean was 61.2 ± 1%. We concluded that N. agilis is not an obligate autotroph; it appears to be a facultative autotroph which resembles the novel facultative autotroph, Thiobacillus intermedius , very closely.