Some aspects of element turnover in living organisms

Abstract

The net uptake and loss of any element by a living organism can be described as the quotient of the total amount of the element, present in the organism, and its residence time in the organism. Theoretically it can be derived that the residence time τ _i , equals V ^1−b _i /k, in which b, the morphometric coefficient, is related to size and shape of the organism (volume V _i ); k, the turnover coefficient, is related to its metabolic activity. The net uptake or excretion, Φ _i , of an element then follows from Φ = k C _i V ^b _i , C _i being the (average, whole animal) concentration of the element.

Residence times, derived from the quotient of body volume (weight) and daily food intake of various animal species of different sizes, show that, independent of animal or element species, the morphometric coefficient and turnover coefficient have values of b = 0.735 and k = 71.4 dm. year^-1 respectively. The turnover coefficient may show some variation, related to the metabolic activity of the organism. The value of the morphometric coefficient implies that residence times may vary with body size: in small animals the residence times of the elements may be in the order of several hours, whereas in the largest organisms residence times may be as long as several years.

The uptake of Na, Mg, Cl and P calculated from the above equation (Φ _i = k C _i V ^b _i ) corresponds very well with data in the literature on the mineral requirements of domestic animals; for Ca the calculated values were lower, for I higher, but they show the same weight-dependence as predicted from the equation. This suggests that the equation may be used tentatively to assess dietary requirements for elements for which up till now no such information exists.

Values derived for the uptake and excretion of various elements by the whole biosphere correspond roughly to those estimated for their oceanic fluxes (from data compiled by Brewer, 1975). This supports the hypothesis that the global turnover of (many) elements is closely related to biological (metabolic) processes.