A seventh CO2-fixation pathway has recently been described in Desulfovibrio desulfuricans, a sulfate-reducing bacterium (see section 20.9). Attempts to culture D. desulfuricans in autotrophic conditions began after it was observed that the organism could grow in co-culture with a methanogen using only formate as its carbon and energy source. Formate dehydrogenase, present and active in D. desulfuricans, converts CO2 to formate. The researchers reasoned that if the bacteria could grow without their co-culture partner, they could demonstrate autotrophic growth. Their first step was to culture D. desulfuricans in a minimal medium containing sulfate, H2, and CO2. Growth was observed, but only after an extended incubation. The researchers postulated several explanations: (1) a long lag phase due to a shift in nutrients, (2) the appearance of a genetic mutation to adapt to the new medium, and (3) contamination in the culture. How could they distinguish these possible explanations for the observed growth?
With the D. desulfuricans genome sequence, the researchers could look for the genes that encode the enzymes found in known CO2-fixation pathways. Which genes would indicate the use of the Calvin-Benson cycle? MICROBIOLOGY PRESCOTT