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Reduction of photosynthetic apparatus plays a key role in survival of the microalga Haematococcus pluvialis (Chlorophyceae) at freezing temperatures

Chekanov, K., Vasilieva, S., Solovchenko, A., Lobakova, E.
Photosynthetica 2018 v.56 no.4 pp. 1268-1277
Haematococcus pluvialis, antioxidant activity, astaxanthin, autotrophs, cell viability, cell walls, chlorophyll, cold tolerance, cryoprotectants, enzyme activity, freeze-thaw cycles, freezing, frost, microalgae, models, photosynthesis, vegetative cells, water content
The microalga Haematococcus pluvialis is a biotechnologically important microorganism producing a ketocarotenoid astaxanthin. Haematococcus exists either as metabolically active vegetative cells with a high chlorophyll content or astaxanthin-rich haematocysts (aplanospores). This microalga featuring outstanding tolerance to a wide range of adverse conditions is a highly suitable model for studies of freezing tolerance in phototrophs. The retention of H. pluvialis cell viability after freezing–thawing is ascribed to elevated antioxidant enzyme activity and high ketocarotenoid content. However, we report that only haematocysts characterized by a lower photosynthetic activity were resistant to freezing–thawing even without cryoprotectant addition. The key factors of haematocyst freezing tolerance were assumed to be a low water content, rigid cell walls, reduction of the membranous structures, photosynthesis downregulation, and low chlorophyll content. Collectively, viability of Haematoccus after freezing–thawing can be improved by forcing the transition of vegetative cells to freeze-tolerant haematocysts before freezing.