Some aspects of iron cycling in maritime antarctic lakes

Iron occurs in extremely high concentrations in certain maritime Antarctic freshwater lakes which seasonally develop an anoxic zone. In oligotrophic Sombre Lake the data show that Fe(II) precipitates as Fe(III) oxyhydroxides which bind phosphorus and return it to the sediments. In nutrient-enriched Amos lake, significant quantities of sulphide are also produced and this binds a proportion of the released Fe(II) so reducing the ratio of total iron to phosphorus at the redox boundary where the oxyhydroxides are formed. A proportion of the sediment-released phosphorus therefore reaches the upper waters of this lake (unlike in Sombre Lake) and provides the initial nutrient source for under-ice phytoplankton development in spring. Iron-reducing bacteria have been isolated, from Sombre Lake sediments, which apparently utilise the abundant Fe(III) oxyhydroxides. From thermodynamic considerations (assuming Fe(III) is not limiting) these should outcompete sulphate reducers and methanogens (both previously reported from Sombre and Amos Lakes) and could therefore constitute an important component of the anaerobic mineralisation of organic carbon in such lakes.