ИСТИНА

We report on peculiar mechanisms of stress acclimation in the photosynthetic apparatus (PSA) of Haematococcus pluvialis (Chlorophyceae). Under favorable conditions, the microalga exists as green cells converted under stress to red haematocysts (HC) enriched with ketocarotenoid astaxanthin, an added-value nutraceutical much sought after on the world market. The extremely stress-tolerant HC were thought of as resting, metabolically quiescent cells. Recent evidence indicates that the HC formation involves deep changes in cell ultrastructure, metabolism and energy acquisition mode. We followed the engagement of photoprotective mechanisms (i) under favorable conditions, (ii) in the course of stress-induced haematocyst formation and (iii) during recovery from the stress. In the case of the stressed cells converting to HC (or recovering from the stress) their PSA was reduced and energy-dependent photoprotective mechanisms such as qT and NPQ, were down-regulated along with astaxanthin accumulation. On this background, a transient up-regulation of PSA was detected (after 20-50 h of the stress) tentatively related with the peak of metabolic activity found earlier in the forming haematocysts. The stress response in this system encompasses secondary carotenogenesis together with a reversible transition from ‘active’ (energy-dependent) to ‘passive’ photoprotective mechanisms. The processes confer H. pluvialis its unique ability to endure harsh habitats from deserts and mountains to Arctic seas. Funding by Russian Scientific Foundation (14-50-00029) is gratefully acknowledged.