Temperature dependence of nitrate reductase activity in marine phytoplankton: Biochemical analysis and ecological, implications

The temperature dependence of NADH:NR activity was examined in several mari ne phytoplankton species and vascular plants. These species inhabit diverge nt thermal environments, including the chromophytes Skeletonema costatum (1 2-15 degrees C), Skeletonema tropicum (18-25 degrees C), Thalassiosira anta rctica (-2 to 4 degrees C), and Phaeocystis antarctica (-2 to 4 degrees C), the green alga Dunaliella tertiolecta (14-28 degrees C), and the vascular plants Cucurbita maxima (20-35 degrees C) and Zea mays (20-25 degrees C), D espite the difference in growth habitats, similar temperature response curv es were observed among the chromophytic phytoplankton, with temperatures op timal for NR activity being between 10-20 degrees C. In contrast, the chlor ophyll b-containing alga and vascular plants exhibited optimal temperatures for NR activity above 30 degrees C. Such dramatic differences in NR therma l characteristics from the two taxonomic groups reflect a divergence in NR structure that may be associated with the evolutionary diversification of c hromophytes and chlorophytes. Further, it suggests a potential contribution of the thermal performance of NR to the geographic distributions, seasonal abundance patterns, and species composition pf phytoplankton communities. NR partial activities, which assess the individual functions of Mo-pterin a nd FAD domains, were evaluated on NR purified from S. costatum to determine the possible causes for high temperature (>20 degrees C) inactivation of N R from chromophytes. It was found that the FAD domain and electron transpor t among redox centers were sensitive to elevated temperatures. S, costatum cells grown at 5, 15, and 25 degrees C exhibited an identical optimal tempe rature (15 degrees C) for NADH:NR activity, whereas the maximal NR activity and NR protein levels differed and were positively correlated with growth temperature and growth rate, These findings demonstrate that thermal acclim ation of NO3- reduction capacity is largely at the level of NR protein expr ession. The consequences of these features on NO3- utilization are discusse d.