Uptake and utilization of dissolved glycine by Aurelia aurita scyphistomae: temperature effects on the uptake process; nutritional role of dissolved amino acids.

TLDR: The investigation of the possibility of an analogous effect of temperature on the kinetics of amino acid uptake is clearly indicated and this approach has been taken in the present study of Aurelia aitrita polyps (scyphistomae) using dissolved glycine.

Abstract: During the last decade there has been a considerable renewal of interest in the uptake of dissolved organic matter (especially amino acids) by marine in vertebrates. Among the goals of these investigations is the elucidation of the importance of dissolved free amino acids to the nutrition of these animals (for recent reviews and bibliographies, see : Johannes, Coward and Webb, 1969; Stephens, 1972 ; Dixit, 1973 ; Schlichter, 1973) . Most experiments attempting to demonstrate a nutritive role of these substances have utilized well fed animals presumably having high levels of metabolic substrates, although a priori, it is in the starved animal that one might expect a significant supplemental nutritional contribution by dissolved compounds. There appears to be very little direct evidence supporting the hypothesis that dissolved amino acids (at environmentally realistic concentrations) are in fact an energy source for marine invertebrates. Stephens ( 1967) has pointed out that not all of the assimilated material is necessarily oxidized. but that it may also exert a sparing effect on a variety of metabolic pathways and on growth and re productive processes. This implies that there need not necessarily be a net uptake of these compounds for them to be nutritionally significant. In the absence of data regarding the total flux of amino acids, an alternative course of investigation is to make qualitative and quantitative comparisons of biochemical, physiological and developmental processes among fed, starved and starved/amino acid-exposed animals. It is this approach that has been taken in the present study of Aurelia aitrita polyps (scyphistomae) using dissolved glycine. The uptake of dissolved free amino acids generally follows Michaelis-Menten kinetics, and while this does not necessarily imply that the uptake process is enzyme-mediated, the calculation of the kinetic constant K@permits comparisons of the affinities of the amino acid uptake systems among different groups of ani mals. To date, the K@values reported for marine invertebrates have been dis cussed almost exclusively in terms of the animals' adaptations to the concentra tions of dissolved free amino acids in their respective habitats (Southward and Southward, 1972a, 1972b; Stephens, 1972), although an additional consideration has been demonstrated by Dixit (1973), who found ontogenetic differences in K@for glycine uptake by a sea urchin. In view of the known influence of tem perature on enzyme kinetics in poikilotherms (Hochachka and Somero, 1973), the investigation of the possibility of an analogous effect of temperature on the kinetics of amino acid uptake is clearly indicated.