Ecophysiological performance of the primitive red alga Dixoniella grisea (Rhodellophyceae) to irradiance, temperature and salinity stress: growth responses and the osmotic role of mannitol

A culture strain of the benthic unicellular red alga Dixoniella grisea was investigated under different stress conditions. The effects of salinity, temperature and irradiance on growth rates were examined in two-factorial experiments and the accumulation of mannitol in response to increasing salinity investigated. The strain grows in a broad salinity range, from brackish water to twice seawater (60 psu). At optimal salinity (10 psu) and optimal temperature conditions (25-30 degrees C), D. grisea grew best at moderate photon flux densities (PFDs; 50-100 mu mol photons m(-2) s(-1)). However, interactive effects between all factors were present. At suboptimal salinities and temperatures, maximal growth rates were shifted to lower PFD and growth was considerably reduced at 50 and 100 mu mol photons m(-2) s(-1). The polyol mannitol was the main low molecular weight carbohydrate in D. grisea. This was verified by C-13-nuclear NMR spectroscopy and HPLC analysis. Mannitol levels increased from 2 to 52 mu mol g(-1) dry weight (dw) with increasing salinities between 10 and 60 psu, indicating its role as an osmolyte for the first time in a unicellular red alga.