Purple Phototrophic Bacteria

November 27, 2019

These bacteria carry out an-oxygenic photosynthesis.
Thus, unlike the cyanobacteria oxygen is not released by these groups.
These bacteria are a morphologically diverse group and their classification has been established along phylogenetic, morphological and physiological lines.
Different genera fall within the Alpha-, Beta-, or Gammaproteobacteria.
Bacteriochlorophylls and carotenoid pigments are present on them.
Due to these two pigments, the bacteria have got its spectacular colors, usually purple, red or brown.
They also produce intra-cytoplasmic photosynthetic membrane systems into which their pigments are inserted.
These membranes may vary in their morphology but in all cases originate from invaginations of the cytoplasmic membrane.
These internal membranes allow purple bacteria to increase the amount of pigment they contain due to which they can utilize the available light.
When cells are grown at high light intensities, internal membranes are few and pigment contents will be low.
By contrast, at low light intensities, the cells are packed with membranes and photo-pigments.

Purple Sulfur Bacteria

Purple bacteria that utilize hydrogen sulfide (H₂S) as an electron donor for CO₂ reduction in photosynthesis are called purple sulfur bacteria.
The sulfide is oxidized to elemental sulfur (S⁰) that is stored in globules inside the cells.
The sulfur later disappears as it gets oxidized to sulfate (SO₄^2-).
Other reduced sulfur compounds are also used by many purple sulfur bacteria as photosynthetic electron donors.

Among them, thiosulfate (S₂O₃^2-) is a key one commonly used to grow laboratory cultures.
All purple sulfur bacteria are Gammaproteobacteria that has been discovered so far.
These are generally found in illuminated anoxic zones of lakes, and other aquatic habitats where there is accumulation of H₂S and also in sulfur springs.
In these habitats, geochemically or biologically produced H₂S can trigger the formation of blooms of purple sulfur bacteria.
Meromictic (permanently stratified) lakes are more favorable for development of these bacteria.
These lakes have denser (usually saline) water in the bottom and less dense (usually fresh water) nearer the surface.
The sulfide formed by the sulfate reduction in sufficient sulfate condition produced in the sediments diffuse upward into the anoxic bottom waters.
Here, the bacteria can form dense cell masses called blooms usually in association with green phototrophic bacteria.
Ectothiorhodospira and Halorhodospira are two genera of interest as they oxidize H₂S to produce S⁰ outside the cell rather than inside the cell unlike other purple sulfur bacteria.
These genera are also extremely halophilic (salt loving) or alkaliphilic and are among the most extreme in these characteristics of all known bacteria.
They are found typically in saline lakes, soda lakes and salterns.

Purple Non-sulfur Bacteria

Some purple bacteria were originally thought to be unable to use sulfide as an electron donor for the reduction of CO₂to cell material.
So, these were known as purple non-sulfur bacteria.
Infact, sulphide can be used by most species in this group although the levels of sulfide ideal for purple sulfur bacteria (1-3mM) are often toxic to most purple non-sulfur bacteria.
Some of them can also grow anaerobically in the dark using fermentative or anaerobic respiratory metabolism and most can even grow aerobically in darkness by respiration.
The synthesis of the photosynthetic machinery is repressed by oxygen under the latter conditions mentioned above and the electron donor can be an organic compound or in some species even an inorganic compound such as H₂S.

However, these group have capacity of photo-heterotrophy (a condition where light is the energy source and an organic compound is the carbon source) that accounts for their competitive success in nature.
These groups are typically nutritionally diverse, using fatty, organic, or amino acids; sugars; alcohols; or even aromatic compounds like benzoate or toluene as carbon sources.
Most species are also able to grow photo-autotrophically with CO₂ + H₂ or CO₂ + low levels of H₂S.
Mineral salts medium supplemented with an organic acid as carbon source can be used for enrichment and isolation of purple non-sulfur bacteria.
Such media, inoculated with a mud, lake water or sewage sample and incubated anoxically in the light, invariably select for these bacteria.
Omitting fixed nitrogen sources (for example: NH4⁺) or organic nitrogen sources (yeast extract or peptones) from the medium can increase the selectivity of enrichment cultures.
Along with it, a gaseous headspace of N₂ should be supplied where virtually all purple non-sulfur bacteria can fix N₂ and will thrive under such conditions typically out-competing other bacteria.