D-lactate metabolism and the obligate requirement for CO2 during growth on nitrite by the facultative lithoautotroph Nitrobacter hamburgensis

Nitrobacter hamburgensis X14 is a facultative lithoautotroph that conserves energy from the oxidation of nitrite (NO(2)(-)) and fixes carbon dioxide (CO(2)) as its sole source of carbon. The availability of the N. hamburgensis X14 genome sequence initiated a re-examination of its mixotrophic and organotrophic potential, as genes encoding three flavin-dependent oxidases were identified that may function to oxidize lactate, providing energy and carbon for growth. The response of N. hamburgensis to D- and L-lactate in the presence (mixotrophy) and absence (organotrophy) of NO(2)(-) was examined. L-Lactate did not support organotrophic growth or stimulate mixotrophic growth. In contrast, D-lactate enhanced the growth rate and yield of N. hamburgensis in the presence of NO(2)(-), and served as the sole carbon and energy source for growth in the absence of NO(2)(-) with ammonium as the sole nitrogen source. Lithoautotrophically grown cells immediately consumed D-lactate, suggesting that a lactate metabolic pathway is constitutively expressed. Nevertheless, a physiological adaptation to lactate occurred, as D-lactate-grown cells consumed and assimilated lactate at a faster rate than NO(2)(-)-grown cells, and the D-lactate-dependent O(2) uptake rate was significantly greater in cells grown either organotrophically or mixotrophically compared with cells grown lithoautotrophically. Although D-lactate was assimilated and metabolized to CO(2) in the presence or absence of NO(2)(-), exposure to atmospheric CO(2) or the addition of 0.75 mM sodium carbonate was required for mixotrophic growth and for optimum organotrophic growth on D-lactate.