SPATIAL RELATIONSHIPS AND PHYSICAL DYNAMICS OF CHLOROBACULUM TEPIDUM AND EXTRACELLULAR S(0) GLOBULES

The green sulfur bacteria, Chlorobi, are anoxygenic phototrophs that produce solid, extracellular elemental sulfur globules as an intermediate step in the oxidation of sulfide to sulfate. These organisms must export sulfur while preventing encrustation during globule formation; during globule degradation they must mobilize the sulfur for intracellular oxidation to sulfate. To understand how the Chlorobi address these challenges, we characterized the spatial relationships and physical dynamics of Chlorobaculum tepidum cells and S(0) globules by light and electron microscopy. Cba. tepidum commonly formed S(0) globules at a distance from cells. Soluble polysulfides detected during globule production may allow for remote nucleation of globules. Polysulfides were also detected during globule degradation, likely produced as an intermediate of sulfur oxidation by attached cells. These polysulfides could feed unattached cells, which made up over 80% of the population and had comparable growth rates to attached cells. Time-lapse microscopy and a genome reanalysis show that Cba. tepidum is capable of motility, thus explaining its observed behavior. Our results show how Cba. tepidum is able to avoid mineral encrustation and benefit from globule degradation even when not attached. In the environment, microbial communities may also benefit from soluble sulfur species produced during globule production and degradation.