Physiological plasticity of symbiotic Desmodesmus (Chlorophyceae) isolated from taxonomically distant white sea invertibrates

The physiological heterogeneity of closely related symbiotic microalgae from taxonomically distant animal hosts was studied for the first time. Three strains of unicellular algae from the genus Desmodesmus (Chlorophyceae) isolated from White Sea benthic invertebrates were used as the object in this study. The effects of nitrogen starvation and high light intensity on the growth, changes in chlorophyll and total carotenoid contents and fatty acid content and composition of the microalgal cell lipids were followed. Nitrogen starvation declined the biomass accumulation rate as well as chlorophyll and carotenoid contents on the background of the enhanced fatty acid accumulation in all strains studied. The ultrastructural study revealed the reduction of photosynthetic apparatus and an increase in the proportion of cell volume occupied by oil bodies and starch grains as well as an increase in the cell wall thickness. A decline in the effective per cell irradiance in the cultures of higher cell density, as a rule, slowed down the changes in pigment and fatty acids composition characteristic of nitrogen starvation. The rates of biomass accumulation and cell biochemical composition under the nitrogen-starvation conditions were strain-specific. In most cases, the decline in chlorophylls proceeded at a higher rate in comparison with that of carotenoid decline. In two of the three strains studied, both these process occurred synchronously with the decline in the unsaturation of the cell lipid fatty acids. The possibilities of biotechnological application of the symbiotic microalgae are discussed with the peculiarities of their stress physiology in mind.