Abstract
The microalga Haematococcus lacustris (formerly H. pluvialis) is able to accumulate high amounts of the carotenoid astaxanthin in the course of adaptation to stresses like salinity. Technologies aimed at production of natural astaxanthin for commercial purposes often involve salinity stress; however, after a switch to stressful conditions, H. lacustris experiences massive cell death which negatively influences astaxanthin yield. This study addressed the possibility to improve cell survival in H. lacustris subjected to salinity via manipulation of the levels of autophagy using AZD8055, a known inhibitor of TOR kinase previously shown to accelerate autophagy in several microalgae. Addition of NaCl in concentrations of 0.2% or 0.8% to the growth medium induced formation of autophagosomes in H. lacustris, while simultaneous addition of AZD8055 up to a final concentration of 0.2 µM further stimulated this process. AZD8055 significantly improved the yield of H. lacustris cells after 5 days of exposure to 0.2% NaCl. Strikingly, this occurred by acceleration of cell growth, and not by acceleration of aplanospore formation. The level of astaxanthin synthesis was not affected by AZD8055. However, cytological data suggested a role of autophagosomes, lysosomes and Golgi cisternae in cell remodeling during high salt stress.
Highlights
The xanthophyll astaxanthin has important applications as antioxidant in pharmacology as well as in fish and livestock farming
The results show that addition of 0.2 μM AZD to H. lacustris cells exposed to 0.2% NaCl enhanced cell yield while in cells exposed to 0.8% NaCl, it rather exerted surplus stress instead of causing cytoprotection
Haematococcus lacustris cells were cultured in Optimized Haematococcus Medium (OHM; [25]) under low light (25 μmol m−2 s−1; see Materials and Methods for details) and continuous aeration
Summary
The xanthophyll astaxanthin has important applications as antioxidant in pharmacology as well as in fish and livestock farming. Under optimal conditions of autotrophic growth, astaxanthin is mainly located in the eyespot apparatus of the cells. During stress response H. lacustris cells synthesize high amounts of astaxanthin because it protects the alga from light and oxidation. LDs derive from endoplasmic reticulum (ER); their stress-induced formation in H. lacustris cells occurs simultaneously with the degeneration of chloroplasts [2,3,4]. The biosynthetic precursor of astaxanthin, beta-carotene, is transported from chloroplasts to LDs via an as yet unknown mechanism. The biosynthesis of astaxanthin from beta-carotene requires several enzymatic reactions which are catalyzed by ketolase and hydroxylase. Astaxanthin resides in LDs in an esterified form. The astaxanthin-containing LDs shield nuclear DNA from oxidation and protect the whole cells from photooxidative damage, acting as a light screen (reviewed in [5])
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