Does the lack of mannitol accumulation in an isolate ofrhodella maculata(rhodellophyceae, rhodophyta) from the brackish baltic sea indicate a stressed population at the distribution limit?

Abstract

The unicellular red alga Rhodella maculata Evans is reported for the first time from the central Baltic Sea (habitat salinity 5 psu). To evaluate whether this Baltic isolate is locally adapted to the brackish environment, its salt stress response was compared with a marine isolate (33 psu) by measuring growth rates, photosynthetic performance and the concentration of the osmolyte mannitol as a function of changing salinity between 1 and 60 psu. The brackish Baltic isolate grew between 5 and 40 psu, a narrower salinity range than in the marine isolate (1-60 psu). The marine isolate of R. maculata showed adaptations to an intertidal habitat with fluctuating salinities rather than to stable salinity conditions. The cells of the marine isolate performed a pronounced regulation of primary photosynthesis via non-photochemical quenching (NPQ), and accumulation of mannitol was involved in osmotic acclimation as intracellular concentrations increased considerably with rising salinities from 1 to 180 mu mol g(-1) dry weight. At its habitat salinity (5 psu), the brackish isolate achieved only 39% of the growth rate measured at 33 psu; when exposed to higher salinities it lacked the ability to carry out substantial NPQ regulation, and it did not accumulate mannitol. However, when cultured at 33 psu for more than three months, the Baltic isolate showed a considerably improved salinity response in the higher salinity range (33-60 psu). Thus, the Baltic Sea isolate of R. maculata does not seem to be locally adapted to its brackish environment, and should be regarded as a stressed population at its distributional limit.