Iron- and sulphur-oxidising Proteobacteria

Iron- and sulphur-oxidising Proteobacteria

Two further groups of environmentally significant chemolithoautotrophs derive their energy through the oxidation of reduced iron and sulphur respectively.

Among the sulphur oxidisers, perhaps the best studied are members of the genus Acidithiobacillus*, which includes extreme acidophiles such as A. thiooxidans that arecapable of growth at a pH as low as 1.0! These may utilise sulphur in its elemental form, as H2S, metal sulphides, or other forms of reduced sulphur such as thiosulphate:

2S0+ 3O2+ 2H2O âˆ’−−−−−−→ 2H2SO4

S2−+ 4O2−−−−−−−→ 2SO42−

S2O32−+ 2O2+ H2O âˆ’−−−−−−→ 2SO42−+ 2H+

The result of all these reactions is the production of sulphuric acid and a lowering of the environmental pH. Bacteria such as these are responsible for the phenomenon of acidmine drainage. A particularly valuable organism in the latter context is A. ferrooxidans, due to its ability to use not only reduced sulphur compounds as energy sources, but also reduced iron .

A second group of sulphur oxidisers are bacteria that exist not as single cells, but join to form filaments, the best known of which is Beggiotoa. These are typically found in sulphur springs, marine sediments and hydrothermal vents at the bottom of the sea.

Acidithiobacillus ferrooxidans is also an example of an iron oxidiser. At normalphysiological pH values and in the presence of oxygen, reduced iron (iron II, Fe2+ ) is spontaneously oxidised to the oxidised form (iron III, Fe3+ ). Under very acidic con-ditions, the iron remains in its reduced form, unless acted on by certain bacteria. A.ferrooxidans; is an obligate aerobe able to use iron II as an energy source, converting itto iron III at an optimum pH range of around 2:

2Fe2++1/2O2+ 2H+−−−−−−−→ Fe3++ H2O

Gallionella ferruginea, on the other hand, grows around neutrality in oxygen-poorenvironments such as bogs and iron springs. Ferric hydroxide is excreted from the cell and deposited on a stalk-like structure projecting from, and much bigger than, the cell itself. This gives the macroscopic impression of a mass of red/brown twisted filaments.

Representative genera: Acidithiobacillus, Beggiotoa (sulphur oxidisers) , Leptospirillum, Gallionella (iron oxidisers)