Modified assay procedure for enhanced sensitivity of in vitro glutamine synthetase (GS) activity measurements in marine phytoplankton

Glutamine synthetase (GS) is a key enzyme in the assimilation of combined inorganic nitrogen in marine microalgae. Consequently, the development of a sensitive assay technique for GS measurements can have important applications for studies of nitrogen metabolism in both laboratory cultures and field populations of phytoplankton. Bressler & Ahmed (1984) have documented an in vitro assay technique for measurements of biosynthetic GS activity in a variety of marine phytoplankton species. To enhance the general applicability of this procedure, several modifications have been made to increase both the detection capability for the inorganic phosphate end product (Pᵢ) and the recovery of total enzyme activity from phytoplankton samples. These improvements have resulted in an approximate 50-fold increase in the overall detection capability for the enzyme, and should accelerate the interest in the use of this enzyme as a sensitive tool in physiological and ecological studies.