Chlorobaculum tepidum regulates chlorosome structure and function in response to temperature and electron donor availability

Green sulfur bacteria (GSB) rely on the chlorosome, a light-harvesting apparatus comprised almost entirely of self-organizing arrays of bacteriochlorophyll (BChl) molecules, to harvest light energy and pass it to the reaction center. In Chlorobaculum tepidum, over 97% of the total BChl is made up of a mixture of four BChl c homologs in the chlorosome that differ in the number and identity of alkyl side chains attached to the chlorin ring. C. tepidum has been reported to vary the distribution of BChl c homologs with growth light intensity, with the highest degree of BChl c alkylation observed under low-light conditions. Here, we provide evidence that this functional response at the level of the chlorosome can be induced not only by light intensity, but also by temperature and a mutation that prevents phototrophic thiosulfate oxidation. Furthermore, we show that in conjunction with these functional adjustments, the fraction of cellular volume occupied by chlorosomes was altered in response to environmental conditions that perturb the balance between energy absorbed by the light-harvesting apparatus and energy utilized by downstream metabolic reactions.