Question

3. The growth rate of the phototrophic purple bacterium Rhodobacter is about twice as fast when the bug is grown phototrophically in a medium with malate as the carbon source as when it is grown with carbon dioxide as the carbon source (with H2 as the electron donor). Discuss why this is true and list the nutritional class in which we would place Rhodobacter in these two different conditions.

Answer 1

Malate (a citric acid cycle intermediate) can be used as a source of carbon for Rhodobacter by using the ATP formed from cyclic photophosphorylation for growth (no need to reduce C02 to cell materiatis). It also accelerate growth rates of the organism and the Rhodobacter can be said to grow PHOTOHETEROTROPHICALLY.

In the absence of Malate, it uses CO2 and H2 for growth, thereby growing PHOTOAUTOTROPHICALLY. Unlike when malate is used, ATP is used for CO2 fixation, which slows the organism cellular growth rate.