Cellular content of storage inclusions in purple sulfur bacteria determined by ultrathin sections.

Phototrophic sulfur bacteria accumulate storage inclusions as a mechanism to adapt to several types of environmental stress. We compare the content of elemental sulfur and poly-beta-hydroxybutyrate (PHB) found in cultures growing under laboratory conditions with the content found in microorganisms in natural environments. Since natural communities are extremely heterogeneous in composition (they do not contain only phototrophic sulfur bacteria) and physiological state, it was not possible to apply conventional chemical analyses for the determination of the contents of sulfur and PHB. The study was performed by means of transmission electron microscopy, which turned out to be an excellent tool for this purpose. The results indicate that, in natural environments, cells have an extremely high content of storage inclusions, much higher than their laboratory grown counterparts, probably as a consequence of less favorable environmental conditions.