Coexistence of organisms competing for the same substrate: An example among the purple sulfur bacteria.

The purpose of this study was to find a possible explanation for the coexistence of large and small purple sulfur bacteria in natural habitats. Experiments were carried out withChromatium vinosum SMG 185 andChromatium weissei SMG 171, grown in both batch and continuous cultures. The data may be summarized as follows: (a) In continuous light, with sulfide as growth rate-limiting substrate, the specific growth rate ofChr. vinosum exceeds that ofChr. weissei regardless of the sulfide concentration employed. Consequently,Chr. weissei is unable to compete successfully and is washed out in continuous cultures. (b) With intermittant light-dark illumination, the organisms showed balanced coexistence when grown in continuous cultures. The "steady-state" abundance ofChr. vinosum was found to be positively related to the length of the light period, and that ofChr. weissei to the length of the dark period. (c) Sulfide added during darkness is rapidly oxidized on subsequent illumination, resulting in the intracellular storage of reserve substances, which are later utilized for growth. The rate of sulfide oxidation/mg cell N/hr was found to be over twice as high inChr. weissei as inChr. vinosum. The observed coexistence may be explained as follows. In the light, with both strains growing, most of the sulfide will be oxidized byChr. vinosum [see (a)]. In the dark, sulfide accumulates. On illumination, the greater part of the accumulated sulfide will be oxidized byChr. weissei [see (c)]. A changed light-dark regimen should then have the effect as observed [see (b)]. These observations suggest that intermittant illumination may, at least in part explain the observed coexistence of both types of purple sulfur bacteria in nature.