FIELD ASSAYS FOR MEASURING NITRATE REDUCTASE ACTIVITY IN ENTEROMORPHA SP. (CHLOROPHYCEAE), ULVA SP. (CHLOROPHYCEAE), AND GELIDIUM SP. (RHODOPHYCEAE)1

In situ and in vitro nitrate reductase (NR) activity assays designed for use in the field on Enteromorpha sp., Ulva sp., and Gelidium sp. are described. In optimizing each assay, a variety of compounds and assay conditions were tested for their ability to extract NR and preserve its activity. Enteromorpha sp. had similar levels of in vitro NR activity after exposure to the in situ assay buffer, demonstrating that neither NR induction nor activation likely occurs during the in situ assay. Storing freshly collected Enteromorpha sp. led to a reduction in NR activity over time. However, the use of liquid nitrogen to freeze algal tissue on site and subsequent storage at ?80° C preserved NR activity and allowed for later laboratory use of the optimized in vitro assay. Application of the in situ and in vitro assays to stands of Enteromorpha sp., Ulva sp., and Gelidium sp. in the field consistently found NR activity. In situ NR activity over 9 consecutive days in January demonstrated that Enteromorpha sp. responds to increases in nitrate availability. The influence of light on diel patterns of in vitro NR activity in the field was demonstrated for the first time as well. For the three species tested, these two assays provide a reliable tool for field investigation of the interaction between environmental signals (e.g. nutrient levels) and physiological signals (e.g. tissue metabolite levels) on nitrate reduction.