It has long been known that iron is an essential element for the metabolism of many organisms, including humans. Iron is one of the most abundant elements in the Earth's crust, and the Fe(n)-Fe(in) redox couple provides for facile electron-transfer reactions:
As a result, iron-containing enzyme proteins are among the most common electron-transfer catalysts (Table 1).
Of particular importance to photosynthesizing algae are the photosystem proteins which are involved in the splitting of water to form O2 and which contain a number of Fe redox centers. Prokaryotic phytoplankton (microbes lacking a cell nucleus), which evolved early on in the evolution of the ocean during the Archean period at least 2.5 billion years ago, used iron as the basis of these redox catalysts because of the high abundance of iron in the early ocean. in the absence of free O2, iron forms the quite soluble form Fe(ii) and probably had concentrations on the order of 1 mM during Archean times.
ironically, photosynthesis began to win out over other biological strategies, and free O2 became increasingly available during Proterozoic times. This change in the oxygen status of the ocean had serious consequences for the photosynthesizing algae, for it led to the oxidation of Fe(ii) to the much less soluble form Fe(iii). During the latter stages of the Archean, this gave rise to immense deposits of Fe(iii) oxides on the seafloor known as the 'banded iron formations' (BiFs). The banded nature of BiFs resulted from periodic cycles of 'boom and bust' as the algae coped with the episodic delivery of Fe(ii) through upwelling from the anaerobic deep ocean and its subsequent oxidation by the O2 generated by the algae. This process resembles a giant titration of the Earth's Fe(ii), both in the ocean and on the land, by the algal waste product O2.
Table 1 Some iron-containing enzyme proteins and their functions
Photosynthetic and respiratory e transfer
Cytochrome oxidase Fe-superoxide dismutase Catalase Peroxidase Ferredoxin
Succinate dehydrogenase Nitrate reductase Nitrite reductase Nitrogenase
2H2O2 ! 2H2O + O2 R(OH)2 + H2O2 ! RO2 + 2H2O e~ to NADP+, NO3 , SO2, N2, thioredoxin FAD + succinate ! FADH2 + fumarate
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