The Rhodospirillales: It's Photosynthesis, But Not As You Know It

Colony of Rhodospirillum rubrum, from here.
For today's post, I'm going to take a look at the Rhodospirillales. This is a clade within the larger bacterial group known as the Alphaproteobacteria, other members of which include the disease-causing rickettsiae (thought to be close relatives of the ancestors of eukaryote mitochondria) and the nitrogen-fixing bacteria found in legume nodules. Rhodospirillales species are commonly found in fresh water, though some significant species have been found in other habitats. Many (but far from all) species of Rhodospirillales contain a chlorophyll-like pigment called bacteriochlorophyll a that functions like chlorophyll in catalysing photosynthesis. They also often contain other photosynthetic pigments, carotenoids, that are red in colour: hence the common use of prefixes such as 'rhodo-' and 'roseo-' in genus names in this group. Most photosynthetic Rhodospirillales differ from plants in being photoheterotrophs rather than photoautotrophs: that is, they cannot use carbon dioxide alone as the source of carbon in photosynthesis, and require other carbon sources (Rhodospirillum can grow photoautotrophically if conditions are optimal). Rather than using water as an electron donor, photosynthetic Rhodospirillales generally use other donors such as sulfide or hydrogen.

Individual of Magnetospirillum magneticum, showing the row of magnetosomes, photographed by Richard B. Frankel.
The members of the Rhodospirillales are divided between two families, the Rhodospirillaceae and Acetobacteraceae (Garrity et al. 2005). The majority of members of the Rhodospirillaceae are spiral in shape and grow in anoxic or microaerobic conditions. Acetobacteraceae, in contrast, are most often coccus- or rod-shaped and aerobic. Non-photosynthetic members of Rhodospirillaceae include the nitrogen-fixing genus Azospirillum, often found living in the soil or in association with plants (though, unlike the more familiar nitrogen-fixing genus Rhizobium, Azospirillum is not found specifically in association with root nodules). Also noteworthy is the genus Magnetospirillum, specimens of which possess a row of magnetite-containing bodies called magnetosomes down one side of the cell. These magnetic bodies are presumed to aid the bacterium in navigation relative to the earth's magnetic field. Magnetospirillum can be drawn from sediment through the use of a bar magnet; when observed under a microscope close to a magnet, they can be seen to swim parallel to the magnetic field lines. A cell suspension dropped onto a magnetic stirrer will be seen to flicker, as the cells change their direction of movement (and hence the direction of light scattering) to follow the spinning field.

Dividing cell of Stella vacuolata, from Garrity et al. (2005). Scale bar equals 1 µm.
Non-photosynthetic members of the Acetobacteraceae include the acetic acid bacteria (hence the name of the family). These gain energy by converting ethanol to acetic acid or, in more general terms, alcohol to vinegar. Some genera, such as Acetobacter and Acidomonas, are able to further oxidise the acetate produced to water and carbon dioxide; others, such as Gluconobacter, do not do so. One particularly distinctive genus in the Acetobacteraceae is Stella, species of which are found in soil and animal faecal matter. Cells of Stella are flat and, as suggested by their name, star-shaped with six points. Division of Stella involves the growth of a cross-wall down the midline of the cell, with each daughter cell retaining three of the parent's points to which they add three new points.

REFERENCES

Garrity, G. M., D. J. Brenner, N. R. Krieg & J. T. Staley. 2005. Bergey's Manual of Systematic Bacteriology, 2nd ed., vol. 2. The Proteobacteria, part C. The Alpha-, Beta-, Delta-, and Epsilonproteobacteria. Springer.