Abstract:
Nitrobacter hamburgensis X14 is a facultative lithoautotroph that conserves energy from the
oxidation of nitrite (NO
2 ) and fixes carbon dioxide (CO2) 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 Dlactate-
grown cells consumed and assimilated lactate at a faster rate than NO
2 -grown cells, and
the D-lactate-dependent O2 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 CO2 in the presence or absence of NO
2 , exposure
to atmospheric CO2 or the addition of 0.75 mM sodium carbonate was required for mixotrophic
growth and for optimum organotrophic growth on D-lactate.
