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 rep ort the first successful bulk cultivation of these strains in a define d liquid medium, To investigate their nutritional versatilities, strai ns MS-81-6 and MS-81-1c were grown in sulfide-oxygen gradient media su pplemented with single organic compounds, Respiration rates and biomas s production relative to those of controls grown in unsupplemented sul fide-limited media were monitored to determine whether organic compoun ds were utilized as sources of energy and/or cell carbon, With cells g rown 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 or ganic carbon (acetate) in the growth medium, In contrast, strain MS-81 -1c lacked 2-oxoglutarate dehydrogenase activity and regulated ribulos e-1,5-bisphosphate carboxylase/oxygenase activity only slightly in res ponse to organic substrates, Tracer experiments with radiolabeled acet ate showed that strain MS-81-1c did not oxidize acetate to CO, but cou ld synthesize approximately 20% of its cell carbon from acetate, On th e basis of these results, we conclude that Beggiatoa strain MS-81-1c i s an obligate chemolithoautotroph, while strain MS-81-6 is a versatile facultative chemolithoautotroph.