Mechanisms of extracellular So globule production and degradation in Chlorobaculum tepidum via dynamic cell globule interactions

The Chlorobiales 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 cell encrustation during S-o globule formation; during globule degradation they must find and mobilize the sulfur for intracellular oxidation to sulfate. To understand how the Chlorobiales address these challenges, we characterized the spatial relationships and physical dynamics of Chlorobaculum tepidum cells and S-o globules by light and electron microscopy. Cba. tepidum commonly formed 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, probably produced as an intermediate of sulfur oxidation by attached cells. Polysulfides could feed unattached cells, which made up over 80% of the population and had comparable growth rates to attached cells. Given that S-o is formed remotely from cells, there is a question as to how cells are able to move toward S-o in order to attach. Time-lapse microscopy shows that Cba. tepidum is in fact capable of twitching motility, a finding supported by the presence of genes encoding type IV pili. Our results show how Cba. tepidum is able to avoid mineral encrustation and benefit from globule degradation even when not attached. In the environment, Cba. tepidum may also benefit from soluble sulfur species produced by other sulfur-oxidizing or sulfur-reducing bacteria as these organisms interact with its biogenic S-o globules.