Sticking Your Sulphur on the Outside

The ability to obtain energy from sunlight through photosynthesis is a feature of a range of bacterial lineages, using a number of different processes and growing under a variety of different conditions. Perhaps the best known such group is the oxygen-producing blue-green algae or Cyanobacteria, but there are other photosynthetic bacteria that do not release oxygen. One such group is the purple sulphur bacteria of the Ectothiorhodospiraceae.

Individual Ectothiorhodospira mobilis, from Trüper (1968).

Purple sulphur bacteria are members of the hyperdiverse array of bacteria known as the Proteobacteria, specifically of a subgroup known as the Gammaproteobacteria (other notable Gammaproteobacteria include such luminaries as the various Pseudomonas species, the plant-attacking Xanthomonas, and perhaps the single most intensely studied bacterial species of all, Escherichia coli). They are found growing under anoxic conditions, using light energy to assimilate carbon via the oxidation of sulphides to organic sulphur, which is in turn further oxidated to sulphate. Purple sulphur bacteria can be divided between two families, the Chromatiaceae and Ectothiorhodospiraceae, that may be distinguished by how they deposit the sulphur globules produced during photosynthesis. In Chromatiaceae, the sulphur globules are retained within the cell membrane but in Ectothiorhodospiraceae, they are deposited externally (one species of Ectothiorhodospiraceae, Thiorhodospira sibirica, does deposit sulphur both internally and externally, but the internal globules are restricted to the peripheral region of the cell within the periplasmic space). Photosynthetic pigments are bacteriochlorophyll a or b, together with carotenoids; these pigments are attached to intracellular membranes appearing as lamellar stacks. Ectothiorhodospiraceae species are found in marine and other saline habitats, often in environments with a more or less alkaline pH. Species of the genus Halorhodospira, which have been found in salt and soda lakes, require exceedingly saline conditions, being unable to grow at total salt concentrations below 10%.

Cross section of Ectothiorhodospira mobilis showing photosynthetic membrane stack, from Trüper (1968).

Though the genera Halorhodospira and Thiorhodospira are strictly anaerobic and invariably photosynthetic, some species of the genus Ectothiorhodospira are able to obtain energy heterotrophically from organic compounds, and may grow under microaerobic conditions in the dark. The range of organics they can utilise in this way is limited to relatively simple compounds: they generally cannot break down carbohydrates, for instance, but they can grow on organic acids such as acetates. Molecular analysis has also indicated the inclusion in the Ectothiorhodospiraceae of a number of non-photosynthetic bacteria. The species Arhodomonas aquaeolei was originally isolated from brine from a subterranean oil reservoir; it breaks down organic compounds using oxygen or nitrate but has the same limitations to simple molecules as Ectothiorhodospira. It seems easy to imagine it evolving from an Ectothiorhodospira-like ancestor but losing its photosynthetic capabilities due to its subterranean habitat, like an animal in a cave losing its eyesight. The genera Nitrococcus and Thioalkalivibrio are lithoautotrophs (that is, they synthesis organic compounds like photosynthetic forms but use the energy from chemical reactions using minerals rather than from light) and generally aerobic (at least one strain of the species Thioalkalivibrio denitrificans is facultatively anaerobic). Thioalkalivibrio species oxidise sulphur, sulphides and other sulphur-containing compounds whereas Nitrococcus convert nitrate to nitrite. Despite their lack of photosynthetic abilities, Nitrococcus still carry indications of their photosynthetic ancestry in the presence of tubular intracellular membranes, the repurposed derivatives of the original stacks.

REFERENCE

Brenner, D. J., N. R. Krieg & J. R. Staley (eds) 2005. Bergey’s Manual of Systematic Bacteriology 2^nd ed. vol. 2. The Proteobacteria pt B. The Gammaproteobacteria. Springer.