Definition

Aluminum is the third most abundant element in the Earth’s crust and the most abundant metal (Exley 2003). Aluminum is neither required by biological systems nor is it known to participate in any essential biological processes. While today all living organisms contain some aluminum, there is no scientific evidence that any organism uses aluminum for any biological purpose. There is similarly no evidence from the proteome or genome that any organism has utilized aluminum at any time in the evolutionary record. Aluminum’s abundance and paradoxical lack of biological function remains a biochemical enigma.

It is argued that aluminum’s absence from biochemical processes can be best explained in terms of its “historical” absence from biochemical evolution (Exley 2009a). In spite of its abundance in the Earth’s crust, aluminum was not biologically available for the greater part of biochemical evolution. It was not available to participate in the natural selection of the elements of life. It is shown that silicon, as silicic acid, is a geochemical control of the biological availability of aluminum and has probably played a significant role in keeping aluminum out of biota (Exley 1998).

If aluminum had been biologically available during early biochemical evolution then this encyclopedia entry would have been listing and describing myriad proteins and biochemical systems which are either built around or require aluminum. This can be anticipated as it is now known that when aluminum is biologically available it is a serious ecotoxicant and can act so at to displace or nullify biologically essential metals and, in particular, magnesium, calcium, and iron. It is known that aluminum is the principal antagonist in fish death in acid waters, in forest decline in acidified catchments, in poor crop productivity on acid soils, and it is also known that aluminum is both acutely and chronically toxic in humans (Exley 2009b). The scientific evidence indicates that biologically available aluminum is biologically reactive and not always to the detriment of the biochemical processes concerned. For example, while some phosphoryl-transferring enzymes, such as hexokinase, are potently inhibited by aluminum, others are not and appear at least to use Al-ATP as effectively as Mg-ATP as a biochemical substrate (Furumo and Viola 1989; Korchazhkina et al. 1999). In addition, while aluminum is not known to play any role in any enzyme it has been shown that when aluminum is engineered into the active site of a purple acid phosphatase the activity of this enzyme was not hindered by the presence of aluminum. This was described by the authors as the first example of an active aluminum enzyme (Merkx and Averill 1999). The lack of biochemical essentiality of aluminum is best explained as being due to it being excluded from biochemical evolution (Exley 2009a). If biologically available aluminum had been present, then aluminum would either be an essential element in modern biochemistry (the phenotype of which would not necessarily be similar to what is known today) or there would still be clues in the evolutionary record pertaining to the selection of aluminum out of biochemical processes. It is argued that today aluminum is a silent visitor to contemporary biota including human beings and is only now participating in the evolutionary process. These “visits” are not inconsequential and are already being manifested as both acute and chronic toxicity (Exley 2009b). The latter may prove to be an unrecognized burden upon biological systems including human health unless steps are taken to protect the environment and the body from its burgeoning presence in modern life and its increasing participation in modern biochemistry (Exley 1998).