Physiological and biochemical responses to thermal and salinity stresses in a sterile mutant of Ulva pertusa (Ulvales, Chlorophyta)

Like other organisms in the marine ecosystem, macroalgae are subjected to intense environmental stresses, particularly in the intertidal zone. The green seaweed Ulva inhabits rocky intertidal zones worldwide, suggesting that this alga may be a good model system for studying environmental stress responses in marine plants. Here, we review the physiological and biochemical responses to thermal and salinity stresses in a sterile mutant of Ulva pertusa. In response to high-temperature stress, the amount of photosynthetic pigments, major free amino acids (AA), and total carbon and nitrogen in U. pertusa increase. Changes in chemical components due to high-temperature stress are consistent with morphological changes in thalli subjected to high temperature and suggest that high-temperature stress mainly affects nitrogen metabolism. Isozyme assays show that the alga expresses a glutamate dehydrogenase isozyme in response to high-temperature stress, and that its expression was regulated at the mRNA transcription level. Chemical component changes due to salinity stress indicate a possibility that the low- and high-salinity conditions affect photosynthesis and carbon and nitrogen metabolism, respectively. In particular, it was observed that thalli exposed to hypersaline conditions rapidly accumulate the organic osmolyte proline, suggesting that free proline accumulation is an important tolerance mechanism in this alga for adapting to hypersaline conditions. Finally, we discuss future directions for the molecular analysis of environmental stress in U. pertusa.