Purple bacteria

Purple bacteria are anoxygenic phototrophs widely spread in nature, but especially in aquatic environments, where there are anoxic conditions that favor the synthesis of their pigments.

All these classes also contain numerous non-photosynthetic numbers, such as the nitrogen-fixing Rhizobium and the human gut bacterium Escherichia coli.

The important distinction that remains from these two different metabolisms is that: any S0 formed by purple nonsulfur bacteria is not stored intracellularly but is deposited outside the cell[8] (even if there are exception for this as Ectothiorhodospiraceae).

Since pigment synthesis does not take place in presence of oxygen, phototrophic growth only occurs in anoxic and light conditions.

In fact they can be mixotrophs, capable of anaerobic and aerobic respiration or fermentation[10] basing on the concentration of oxygen and availability of light.

These are integral membrane protein complexes consisting of monomers of α- and β-apoproteins, each one binding molecules of bacteriochlorophyll and carotenoids non-covalently.

[14] Moreover, the photosynthetic unit in Purple Bacteria shows great plasticity, being able to adapt to the constantly changing light conditions.

In fact these microorganisms are able to rearrange the composition and the concentration of the pigments, and consequently the absorption spectrum, in response to light variation.

[22] Finally, even if the purple sulfur bacteria are typically photoautotrophic, some of them are photoheterotrophic and use different carbon sources and electron donors such as organic acids.

On the other hand, purple nonsulfur bacteria are strong photoheterotrophs, even if they are capable of photoautotrophy, and are equipped for living in dark environments.

Purple sulfur bacteria can be found in different ecosystems with enough sulfate and light, for example shallow lagoons polluted by sewage or deep waters of lakes, in which they could even bloom.

In the 1980s Thermochromatium tepidum, a thermophilic purple bacterium that can be found in North American hot springs, was isolated for the first time.

However the most important role is played by consuming hydrogen sulfide: a highly toxic substance for plants, animals and other bacteria.

The correlation between anoxygenic photosynthesis and the availability of solar radiation suggests that light is the main factor controlling all the activities of phototrophic sulfur bacteria.

The density of pelagic communities of phototrophic sulfur bacteria extends beyond a depth range of 10 cm (3.9 in),[30] while the less dense population (found in the Black Sea (0.068–0.94 μg BChle/dm3) is scattered over an interval of 30 m (98 ft).

Another example, Thermochromatium tepidum, has been found in several hot springs in western North America at temperatures above 58 °C (136 °F) and may represent the most thermophilic extant Pseudomonadota.

[30] Of the purple sulfur bacteria, many members of the Chromatiaceae family are often found in fresh water and marine environments.

Thus, a simultaneous growth of two bacterial partners takes place, which are fed by the oxidation of organic carbon and light substrates.