Salinization in tropical estuarine environments is expected as a result of climate change. The physiological performance of mangrove-associated key macroalgae can negatively be affected by increased salinity in such habitats. Thus, we analyzed photobiological and biochemical responses of the closely related red algae Bostrychia calliptera and Bostrychia montagnei incubated under a range of salinities (5, 11, 18, 37, 47, and 57 S-A). Effective and maximum quantum yield, relative electron transport rate vs. photon fluence rate curves, photosynthetic parameters, and complementary energy dissipation pathways indicated that both species had lower photosynthetic performance under increased salinity, which was more strongly pronounced in B. calliptera. Both species increased their organic osmolyte contents with rising salinity stress. Dulcitol was the main organic osmolyte synthesized by B. calliptera, whereas B. montagnei synthesized dulcitol and sorbitol. Our results demonstrate that increased salinity in estuaries due to climate change will be detrimental to photosynthesis of both macroalgae, with B. calliptera more affected than B. montagnei. As B. montagnei synthesizes both dulcitol and sorbitol, it is more tolerant to salinity stress compared to B. calliptera. Our data document for the first time a new organic osmolyte distribution pattern in Bostrychia species, namely the occurrence of dulcitol only.
