IS IRON AN ENERGY SOURCE FOR EARLY LIFE ON ANOXIC EARTH?

Water is necessary for life to emerge, and available oxygen appears to be necessary for multicellular organisms to evolve, but what was the source of energy in the early anaerobic world? The presence of oxygen was unnecessary for life at this time as prokaryotic bacteria are able to obtain energy in a remarkable diversity of ways. The chemiosmotic theory describes the change of chemical energy into an electrochemical potential across a membrane. In principle, any electron potential between an electron donor and acceptor such as oxidation or reduction of inorganic compounds and transitional elements can be utilized. So long as the necessary enzymatic machinery is available, the source of energy makes little difference to the organism, whether nitrate, manganese, uranium, iron, sulfate, or carbon dioxide: a sequence of decreasing redox potential and thermodynamic feasibility. In the natural environment iron is the most important of these sources since, although the energy it provides is limited, it is readily available and is widely distributed.

Laboratory experiments have shown that several different natural biofilm microbial consortia can rapidly extract energy from iron, either in solution or in minerals, by the removal of electrons from ferric iron through direct enzymatic dissimulatory iron reduction. Iron is also indirectly precipitated due to the biofilm environment. Both types of precipitate are enmeshed within the biofilm, although their final oxidation state is controlled by the nutrient supply. In experiments, these iron particles can form fine layers or granules where the biofilm has been rolled by oscillating waves, structures that are similar to those found in banded iron formations (BIF). Since the acquisition of energy from iron is inefficient, a vast quantity of microbially mediated iron is discharged back into the environment as waste, where it may have created many of the formidable iron deposits that we know today.