Iron Metabolism During The Early Proterozoic

The structure of the oxygen-evolving complex. This enzyme is involved in
the very first step of photosynthesis, its purpose being to obtain electrons
and hydrogen from water. The manganese ions are shown in purple

How photosynthesis evolved is an evolutionary question of importance. Particular focus has been given to the oxygen-evolving complex - an enzyme responsible for obtaining hydrogen and electrons from water.

The hydrogen and electrons are used in later stages of photosynthesis whilst oxygen from the water is released as a waste product. The oxygen-evolving complex contains manganese ions which act almost like a rechargeable battery by giving up their outer shell electrons so that they may be used in photosynthesis and then restoring those electrons by pilfering them from water. The manganese ions change their oxidation state in the process.

A study conducted a few years ago suggests that photosynthesis evolved from a more primitive metabolic system whereby bacteria harvested electrons from manganese atoms for use in their metabolic pathways. The evidence for this comes from extensive beds of oxidised manganese found in early Proterozoic rocks, their oxidised state showing that the electrons had been removed. In time manganese ions which continually lost and gained electrons became a rechargeable battery within the oxygen evolving complex. Water became the new source of electrons and so oxygenic photosynthesis evolved.

Now a study published this year shows that other bacteria may have used iron as a source of electrons for their metabolic pathways. It focused on a banded iron formation from the Karijini National Park in Western Australia. It was thought that the iron was precipitated from hydrothermal vents at a mid ocean ridge. This study has shown, however, that potentially half of the iron atoms in the formation were precipitated as a result of losing electrons to the metabolic pathways of bacteria. 'The process is really deep in the tree of life, but we've had little evidence from the rock record until now,' said Clark Johnson from University of Wisconsin-Madison . 'These ancient microbes were respiring iron just like we respire oxygen.'

2.5 billion year old banded iron in the Karijii
National Park in Western Australia

Laser pulses lasting for less than a trillionth of a second were used to vaporise samples of the banded iron taken from cores, but without actually causing the material to heat up. It has taken three yeas to perfect the laser and the analysis equipment.

The isotopic composition of the sample was then analysed using mass spectrometry, with a focus on iron and neodymium. The results showed that the isotopic composition was due to around half of the iron atoms having been metabolised by bacteria as a source of electrons.

'What vestiges of the iron-rich world remain in our metabolism?' asked Johnson. 'It's no accident that iron is an important part of life, that early biological molecules may have been iron-based.' The introduction of oxygen into the atmosphere, however, made the metal-based metabolism of Proterozoic bacteria impossible. Oxygen is chemically voracious and strips electrons from metals with great ease, forming oxides of those metals. In an oxygenated atmosphere virtually all metals spontaneously oxidise. Rusting of iron is a prime example. By incorporating ions of metals into proteins and using water as an electron source, bacteria were able to sidestep the metabolic obstruction presented by oxygen. Life continued with very little change for the next billion years.