Salinity affects high temperature tolerance in broadly halotolerant isolates of Nannochloris and Dunaliella

I studied the effect of growth salinity on short-term high temperature stress in new strains of broadly halotolerant Nannochloris and Dunaliella isolated from the Salt Plains National Wildlife Refuge in Oklahoma, USA. Photochemical yield (pulse modulated fluorescence) was unaffected by growth salinity in both algae. Dunaliella was progressively more heat tolerant (2 h at 41.5 degree C) with increasing salinity. Photochemical yield of cells at 100 and 50 g/L was inhibited by about 15 and 40%, respectively, and largely recovered within 30 min after return to 23 degree C. Thermal inhibition (2 h at 45 degree C) of photochemical yield in Nannochloris was about 45% at both 50 and 100 g/L, but recovery was slower at 100 g/L, and slower at both salinities compared to Dunaliella. At 20 g/L, both species were almost 90% inhibited by high temperature and required more than 24 h to recover. Salinity alone had little effect on in vivo absorption spectra, ambient and 77 K fluorescence excitation and emission spectra. However, 2 h heating increased the photosystem (PS) I:PS II emission ratio (714:690 nm) at all salinities. This ratio largely recovered within 24 h in Dunaliella at 50 and 100 g/L salinity, and partially recovered in Nannochloris at 100 g/L. Cells of both species heated at 20 g/L exhibited chlorosis the next day, but regreened after several days. These results are consistent with steady-state temperature-salinity experiments with Nannochloris.