Organic carbon utilization by obligately and facultatively autotrophic beggiatoa strains in homogeneous and gradient cultures.

Marine Beggiatoa strains MS-81-6 and MS-81-1c are filamentous gliding bacteria that use hydrogen sulfide and thiosulfate as electron donors for chemolithotrophic energy generation. They are known to be capable of chemolithoautotrophic growth in sulfide gradient media; here we report the first successful bulk cultivation of these strains in a defined liquid medium. To investigate their nutritional versatilities, strains MS-81-6 and MS-81-1c were grown in sulfide-oxygen gradient media supplemented with single organic compounds. Respiration rates and biomass production relative to those of controls grown in unsupplemented sulfide-limited media were monitored to determine whether organic compounds were utilized as sources of energy and/or cell carbon. With cells grown in sulfide gradient and liquid media, we showed that strain MS-81-6 strongly regulates two enzymes, the tricarboxylic acid cycle enzyme 2-oxoglutarate dehydrogenase and the Calvin cycle enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, in response to the presence of organic carbon (acetate) in the growth medium. In contrast, strain MS-81-1c lacked 2-oxoglutarate dehydrogenase activity and regulated ribulose-1,5-bisphosphate carboxylase/oxygenase activity only slightly in response to organic substrates. Tracer experiments with radiolabeled acetate showed that strain MS-81-1c did not oxidize acetate to CO(inf2) but could synthesize approximately 20% of its cell carbon from acetate. On the basis of these results, we conclude that Beggiatoa strain MS-81-1c is an obligate chemolithoautotroph, while strain MS-81-6 is a versatile facultative chemolithoautotroph.