Pavel Scherbakov

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.

A new subarctic strain of Tetradesmus obliquus. Part II: comparative studies of CO2-stress tolerance

A huge interest in CO2-tolerant microalgae is fueled by development of CO2-biomitigation methods based on intensive cultivation of microalgae. Still, the mechanisms of CO2-tolerance are scarcely investigated. Previously, we described a symbiotic Desmodesmus sp. IPPAS S-2014 from a White Sea hydroid tolerant to extremely high (20–100%) CO2 levels. In the present work, we compared its ultrastructural and physiological responses to those of a novel free-living White Sea strain of Tetradesmus obliquus IPPAS S-2023 characterized in the companion paper. The strain S-2023 is closely related to Desmodesmus sp. IPPAS S-2014 but lacks its tolerance to extremely high CO2 (it is unable to survive at 100% CO2 and exhibits a reduced-growth phenotype when sparged with 20% CO2: air mixture). We compared the responses of the cell organization and photosynthetic activity to 20% CO2 in the tolerant and the intolerant White Sea chlorophytes using chlorophyll fluorescence measurements and ultrastructural analysis (transmission electron microscopy). The features peculiar to the CO2-intolerant chlorophyte include (i) inability to maintain pH homeostasis, (ii) a steady decline in the photosynthetic activity of the cells, (iii) a reduction of the photosynthetic membranes, and (iv) delayed accumulation of starch (starch grains) and its subsequent conversion to reserve lipids (oil bodies). Nitrogen starvation enhances the effects of high-CO2 stress in the CO2-intolerant microalga. The results of this work are discussed in the context of selection of tolerant algal strains for CO2 biomitigation applications.

Stress-induced changes in the ultrastructure of the photosynthetic apparatus of green microalgae

In photosynthetic organisms including unicellular algae, acclimation to and damage by environmental stresses are readily apparent at the level of the photosynthetic apparatus. Phenotypic manifestations of the stress responses include rapid and dramatic reduction of photosynthetic activity and pigment content aimed at mitigating the risk of photooxidative damage. Although the physiological and molecular mechanisms of these events are well known, the ultrastructural picture of the stress responses is often elusive and frequently controversial. We analyzed an extensive set of transmission electron microscopy images of the microalgal cells obtained across species of Chlorophyta and in a wide range of growth conditions. The results of the analysis allowed us to pinpoint distinct ultrastructural changes typical of normal functioning and emergency reduction of the chloroplast membrane system under high light exposure and/or mineral nutrient starvation. We demonstrate the patterns of the stress-related ultrastructural changes including peculiar thylakoid rearrangements and autophagy-like processes and provide an outlook on their significance for implementation of the stress responses.