Calcium and the ecology of photosynthesis in purple sulfur bacteria

The ecological success of purple sulfur bacteria (PSB) is linked to their ability to collect near-infrared solar energy by membrane-integrated, pigment–protein photocomplexes. These include a Core complex containing both light-harvesting 1 (LH1) and reaction centre (RC) components (called the LH1–RC photocomplex) present in all PSB and a peripheral light-harvesting complex present in most but not all PSB. In research to explain the unusual absorption properties of the thermophilic purple sulfur bacterium Thermochromatium tepidum, Ca²⁺ was discovered bound to LH1 polypeptides in its LH1–RC; further work showed that calcium controls both the thermostability and unusual spectrum of the Core complex. Since then, Ca²⁺ has been found in the LH1–RC photocomplexes of several other PSB, including mesophilic species, but not in the LH1–RC of purple non-sulfur bacteria. Here we focus on four species of PSB—two thermophilic and two mesophilic—and describe how Ca²⁺ is integrated into and affects their photosynthetic machinery and why this previously overlooked divalent metal is a key nutrient for their ecological success.