Relat~ve in wtro actlv~ty levels for the enzyme glutamate synthase (COGAT) with the cofactors NADH NADPH and reduced ferredoxln were determined wlth a I4C method in 6 specles of phytoplankton from 3 taxonomic classes The NADH form of the enzyme was the dominant isozyme in all of the marlne and estuanne specles, whde the ferredoxln form was dominant in the 1 fresh-water species tested Significant amounts of an NADPH specific GOCAT were not detected In any of the orgalusms A sensihve fluorometnc assay for m wtro measurements was subsequently u a z e d to determme the substrate lunetics, pH ophmum and temperature-dependent activation energy for NADH-GOGAT m the diatom Skeletonema costatum Good agreement was obtained with the optlmal m wtro assay condit~ons for achvlty measurements of NADH-GOGAT on a conlmon phytoplankton extract with the I4C and fluorometnc assay methods The sensitlvltles of the 2 assays (l e fluorometric and "C) make them suitable for apphcatlon in studies of nltrogen asslmilat~on and phytoplankton growth rate dynamcs in aquatic ecosystems

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 vltro 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 (P,) 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.