Answer
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.