Modelling protein synthesis, a step to an accurate estimate of net primary production: Phaeocystis pouchetii colonies in Belgian coastal waters

Abstract

Light and dark rates of net protein synthesis by Phaeocystis pouchetii colonies were measured in Belgian coastal waters during the spring bloom of 1984 by following I4C-bicarbonate and l5N03/13NH4: assimilation into cellular proteins during natural 1ight:dark cycles. A close agreement was found between the 2 tracer methods. Ambient inorganic nitrogen was shown to be the most important factor controlling protein synthesis in the light, which was considered to proceed at a constant rate in the whole euphotic layer. Protein synthesis in the dark, on the other hand, was found to be controlled by both inorganic N and 'previous light history' of the cells. The relation between specific protein synthesis and external inorganic nitrogen obeyed Michaelis-Menten kinetics for both light and dark processes. The Michaelis-Menten constant K, was 4 µmoles N 1-1 while the maximum specific protein synthesis rates p,, were respectively 3 and 1.8 % h-' for the light and dark processes. The relation between protein assimilation in the dark and 'previous light history' could be expressed by a linear relation between dark specific protein synthesis and the number of hours spent at 3 8 J cm-'h-', the energy threshold for the occurrence of dark protein synthesis. From the knowledge of these control mechanisms a simple mathematical model was established in order to calculate daily integrated net primary production from the hourly variations of light intensity in the water column and ambient inorganic N concentrations. These estimates have been compared with those calculated from classical methods for measurements of primary production.