Contrasting photoadaptive strategies of two morphologically distinct Dunaliella species under various salinities

Dunaliella successfully survives and photosynthesizes in hypersaline environments. To better understand the physiological and photosynthetic characteristics of Dunaliella exposed to long-term hypersaline conditions, we compared two morphologically distinct species, Dunaliella tertiolecta and Dunaliella salina. Despite similar glycerol accumulation patterns and maintenance of the maximum photosynthetic quantum yield, photosynthetic oxygen evolution was enhanced by increased salinity in D. tertiolecta but remained constant in D. salina. Total chlorophyll content was dramatically reduced in D. tertiolecta but did not change significantly in D. salina. In D. salina, beta-carotene content increased with increasing salinity and reached 9.2 +/- 0.15 fmol cell(-1) at 4.5 M NaCl; in contrast, that of D. tertiolecta was reduced. Expression of carbonic anhydrase and Rubisco activase, enzymes related to photosynthetic carbon assimilation, increased with increasing salinity in D. tertiolecta but not in D. salina. The expression of carotenoid biosynthesis-related protein, which is possibly involved in photoprotection, was increased by high salt. De-epoxidation of xanthophyll pigments in D. tertiolecta and the increased beta-carotene content in D. salina may play an important role as protective mechanisms to prevent photodamage in response to hypersaline conditions. We conclude that the two Dunaliella species likely employ different strategies to adapt to long-term hypersaline conditions.