Thermal stress responses of a sterile mutant of Ulva pertusa (Chlorophyta)

The thermal stress responses of a sterile mutant of the marine alga Ulva pe rtusa were investigated at 20 degreesC and 30 degreesC. The amounts of the photosynthetic pigments, beta -carotene, chlorophylls a and b, lutein, neox anthin, and violaxanthin, were 1.4-2.4 times higher in the 30 degreesC-cult ivated alga than in the 20 degreesC-cultivated alga. The free amino acids, asparagine, aspartic acid, glutamine, glutamic acid, glycine, and serine, w ere abundant in the 20 degreesC-cultivated alga, and increased 1.9-10.5-fol d in response to thermal stress (30 degreesC). Total carbon and nitrogen co ntents also increased in the 30 degreesC-cultivated alga. Sodium dodecylsul fate-polyacrylamide gel electrophoretic patterns of total proteins extracte d from both temperature-treated algae showed the increases of 20, 25, and 9 0 kDa proteins in the 30 degreesC-cultivated alga. Isozyme assays for 20 en zymes showed a different banding pattern only in the case of glutamate dehy drogenase (GDH). Although it was observed that both temperature-treated alg ae possessed NAD(+)- and NADP(+)-specific GDH, the 30 degreesC-cultivated a lga had an additional NADP(+)-specific GDH (NADP-GDH). These results sugges t that NADP-GDH compensates for the thermally induced decreases in nitrogen assimilation efficiency and thereby regulates nitrogen metabolism under co nditions of temperature stress.