Terrestrial algal tolerance of a semi-hydrated state relates to species habitat

Aeroterrestrial algae often encounter a semi-hydrated state between full hydration and dehydration, in which the cytoplasm has fluidity but metabolism is impaired, potentially inducing stress. We hypothesised that not all desiccation tolerant (DT) algae tolerate semi-hydration equally, but that longevity in a semi-hydrated state aligns with species habitat. To test this, we monitored photochemical activity between 75–99% relative humidity (RH), as well as resistance to being held at ∼90% RH, 20 °C, under a diurnal light cycle, in six DT algae from a range of habitats. Compositions of photosynthetic pigments, tocopherols (lipid antioxidant) and lipid peroxidation-derived reactive carbonyl species were assessed as stress markers after 0 and 3 weeks of semi-hydration. Initial dehydration activated the photoprotective xanthophyll cycle in all species and on average elevated carbonyls 14-fold more in species less tolerant of semi-hydration, while tocopherols accumulated ∼4-fold in more tolerant species. The least DT species, Zygnema circumcarinatum, with primarily an aquatic habit, had barely detectable photochemistry at 95% RH and pigments broke down during desiccation. In contrast, Asterochloris glomerata (aeroterrestrial lichen photobiont) was photochemically active at 87.5% RH and least stressed by semi-hydration. However, even in aeroterrestrial species, cells aged at ∼90% RH, showing that cellular repair was impaired in a semi-hydrated state. Overall, semi-hydration is a vulnerable state, whereby certain algal species have adapted to tolerate it more than others.