In the absence of oxygen, conditions are
created in which toxic chemical species
are generated, reduced forms of iron (Fe2 + ) and manganese (Mn2+), for
example, being more soluble, and therefore more chemically reactive, than their
oxidised forms (Fe3+, Mn4+). Reduction of these metals, which will not occur until
oxygen and nitrates have been completely exhausted, is carried out by anaerobic
bacteria.
Sulphates (SO4), too, are reduced, in
the total absence of oxygen or nitrates, to
iron sulphide (FeS) or hydrogen sulphide (H2S), again by anaerobic bacteria. In the
absence of iron, H2S, which is highly toxic, will be produced as a gas; its release
when anaerobic sediments are physically disturbed gives off the distinctive
sulphurous smell characteristic of wetland soils.
Under extremely anaerobic conditions,
methanogenic bacteria reduce carbon
dioxide (CO2) to methane (CH4), another gas which is toxic at high concentrations.
