INFLUENCE OF SALINITY AND TEMPERATURE ON GROWTH AND PHOTOSYNTHESIS IN THE EXTREMOPHILIC CHLOROPHYTE, NANNOCHLORIS SP.

Abstract

Major, K. M. & Henley, W. J.Department of Botany, Oklahoma State University, Stillwater, OK 74078‐3013 USAPreliminary data suggest Nannochloris sp., isolated from the Great Salt Plains National Wildlife Refuge, is a true extremophile. This alga is able to withstand salinities ranging from 0 to 150 ç and temperatures up to 45°C. To test the hypothesis that acclimation to high salinity confers tolerance to high temperature, experimental cultures were acclimated to salinities of 25 and 100 ç and/or temperatures of 23 and 38°C; irradiance (500 mol photons m‐2 s‐1) was saturating for both growth and photosynthesis. Cells acclimated to low salt and low temperature exhibited high photosynthetic performance in terms of both light‐saturated photosynthesis (Pmax; 45.0 fmol O2 cell‐1 h‐1) and light‐harvesting efficiency (0.103 fmol O2 cell‐1 h‐1/mol photons m‐2 s‐1). However, high‐salinity cells exhibited values for net Pmax (18.1 fmol O2 cell‐1 h‐1), (0.107 fmol O2 cell‐1 h‐1/mol photons m‐2 s‐1) and growth rates (ca. 0.4 d‐1) that were equal to, or higher than, those of low‐salinity cells when acclimated to high temperature. Both the amount of light required to achieve net photosynthesis (Ic) and that required to achieve light‐saturated photosynthesis (Ik) were lower in high‐salinity cells than those exhibited by low‐salinity cells grown at high temperature; reductions in Ic and Ik were primarily due to increases in light‐harvesting efficiency. We propose that an increase in growth temperature might release Nannochloris sp. from energy constraints associated with osmolyte production and low‐temperature effects on enzyme activity. These data are consistent with effects of short‐term temperature stress on Chl a fluorescence kinetics in this alga.