Elemental abundance as a factor in the origins of mineral nutrient requirements.

No element is found to be commonly required if it has an abundance of less than about 2 nM in the ocean, 20 mumoles/kg in the earth's crust, or 200 mumoles/100 moles Si in the cosmos. More than 40 elements are above these limits, but only 18 of them are commonly required (6 of these being dispensed with by some organism). It is postulated that all of the required elements fall under one of four hypotheses: H-I--a unique requirement dating from the origin of life; H-II--a unique requirement, acquired later; H-III--a primordial requirement which was satisfied by a number of elements, evolutionary adaptation being made to the most abundant member; H-IV--same as III, but a later acquisition. It is suggested that H, K (vs. Na), Mg (vs. Ca), C, N, O, P, S and Fe fall under H-I. Special requirements such as for B, Se and I fall under H-II. In H-III are K vs. Rb, Mg vs. Be(?), S vs. Se, Clvs. Br, H vs F(?), and Zn and Mn vs. various metals. In H-IV probably fall Ca vs. Sr, Na vs. Li (?), Mo vs. V, and Si vs. Ge. The most abundant heavy metal in the ocean is Zn, which may account for its utilization; other required heavy metals have special utility as electron carriers.