A fascinating discovery reported at the Science magazine website reports on a microorganism that not only tolerates arsenate, a classically recognized toxin, but actually uses it in its metabolism. The genetic code used by all life on Earth consists of “letters”, called nucleosides, which each contain a single sugar molecule (deoxyribose) attached to a single organic base (adenine, cytosine, guanine or thymine). These nucleosides are linked into very long chains by means of phosphate (HPO4=) ions, each of which is bonded to two sugars in two consecutive nucleosides: sugar-phosphate-sugar-phosphate-sugar- etc.
This microorganism freely substitutes arsenate (HAsO4=) for phosphate in the DNA molecule. The element arsenic is chemically similar to phosphorus, both of which belong to the family nitrogen-phophorus-arsenic-antimony-bismuth. Nitrates are strong oxidizing agents and react readily with organic matter to “burn” it. Phosphates are readily available in nature and quite stable. Arsenic is about 200 times less abundant than phosphorus in the universe, but can be concentrated by geochemical processes on Earth. Antimony is far less abundant than arsenic, but may possibly follow arsenic into DNA in tiny traces. Bismuth is less similar to phosphorus and even rarer.
But phosphates have another major role in biochemistry: the energy management of cells depends on adenosine diphosphate and adenosine triphosphate (ADP and ATP). So the big question is, are the ADP and ATP molecules in the mitochondria of this strange organism also tolerant of arsenate: do they actually use it the power-generating system of the cell?
Truly, one cell’s food is another cell’s poison.