Using an enrichment technique avoiding the dominating fast-growing spe
cies Rhodobacter capsulatus and Rhodobacter sphaeroides, a new strain
(designated B2c; DSM 10 139) of Rhodoferax fermentans, a facultatively
phototrophic member of the beta subclass of Proteobacteria, was isola
ted. In contrast to the type strain of this species, strain B2c was ca
pable of reducing nitrate. Strain B2c contained a highly active nitrat
e reductase [0.3-0.4 mu mol min(-1) (mg protein)(-1)] which, in extrac
ts from spheroplasts prepared by ultrasonic treatment, was associated
with the membrane fraction. The physiological role of nitrate reductas
e depended on the growth conditions. The enzyme enabled the strain to
grow with nitrate as a nitrogen source and to maintain redox balance i
n a culture medium with a highly reduced carbon compound.