Role of Autophagy in Haematococcus lacustris Cell Growth under Salinity

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.

Salinity tolerance and desalination properties of a Haematococcus lacustris strain from eastern Hungary

“…Three strains isolated in Australia by Gao et al (2015) were also found to be salt tolerant [there was a detectable growth even at a concentration of 0.17 M (~10,000 mg l −1 ) NaCl]. The salt tolerance of H. lacustris used in this study can be considered as "average": It showed growth even at Zharova et al, 2022), but 4,000 mg l −1 NaCl has already inhibited growth, similarly to other observations (Li et al, 2022).…”

“…For other Haematococcus isolates, there are relatively few data in the literature detailing the relationship between growth and salt concentration. Studies agree that salt concentrations of 0.8% (8,000 mg l −1 ) or higher cause complete inhibition of growth (Boussiba and Vonshak, 1991;Zharova et al, 2022). Chekanov et al (2014) isolated a strain on Kost'yan island from a habitat characterized with extreme light, temperature and salinity changes.…”

“…Most of the literature demonstrated that motile cells lose their flagella with increasing salinity, pigment accumulation occurs in the emerging non-motile cells ( Boussiba and Vonshak, 1991 ; Cordero et al, 1996 ; Gao et al, 2015 ; Liu et al, 2021 ; Zharova et al, 2022 ). The dynamics of the process, as well as the salt concentration at which the individual morphological changes occur, show a great variety between the different strains.…”

“…The loss of flagella was reported already after 24 h with 0.1% (~1,000 mg l −1 ) and 86 mM (~5,000 mg l −1 ) NaCl ( Cordero et al, 1996 ; Liu et al, 2021 ). The appearance of the red pigment content was observed after 1–2 days ( Gao et al, 2015 ), the majority of cells transformed into red aplanospores in 3–5 days ( Zharova et al, 2022 ). In the case of the strain investigated in the present study, a more significant difference from general observations is that completely red cysts appeared late (after 9 days at the earliest) and in a relatively small proportion.…”