Humidity and Light Modulate Oxygen-Induced Viability Loss in Dehydrated Haematococcus lacustris Cells

Haematoccocus lacustris (previously H. pluvialis) is a desiccation-tolerant unicellular freshwater green alga. During acclimation to desiccation, astaxanthin-rich lipid bodies and low-molecular-weight antioxidants (α-tocopherol, glutathione) accumulate, while the chloroplast area and chlorophyll contents decrease, which may facilitate desiccation tolerance by preventing damage mediated by reactive oxygen species (ROS). Here, we investigated the influence of moisture, light, oxygen, and temperature on redox homeostasis and cell longevity. Respiration and unbound freezable water were detectable in cells equilibrated to ≥90% relative humidity (RH), a threshold above which viability considerably shortened. At 92.5% RH and 21 °C, antioxidants depleted over days as cells lost viability, especially in an oxygen-rich atmosphere, supporting the role of ROS production in uncoupled respiration in viability loss. At 80% RH and 21 °C, redox homeostasis was maintained over weeks, and longevity was less influenced by oxygen. Light and oxygen was a lethal combination at 92.5% RH, under which pigments bleached, while in the dark only astaxanthin bleached. Viability positively correlated with glutathione concentrations across all treatments, while correlation with α-tocopherol was weaker, indicating limited viability loss from lipid peroxidation at 80% RH. In cells equilibrated to 50% RH, longevity and redox homeostasis showed strong temperature dependency, and viability was maintained at sub-zero temperatures for up to three years, revealing cryogenic storage to be an optimal strategy to store H. lacustris germplasm.