Osmotic adjustments in a psychrophilic alga, Xanthonema sp. (Xanthophyceae)

Extremophile microalgae show a remarkable ability to cope with harsh environments, yet still relatively little is known about the molecular basis for such tolerance. In this work the susceptibility of a psychrophylic alga isolated from alpine snowbanks, Xanthonema sp., to either water or salt stress conditions was assessed, and mechanisms for osmotic adjustment were investigated. Cultures were treated with increasing concentrations of either salts or non-permeant solutes, as polyethylene glycol, and the resulting effect on growth rate was measured. Both the accumulation of compatible osmolytes and the activity of cation transporters were studied in response to the exposure to hyperosmotic conditions. Xanthonema showed a differential sensitivity to osmotic and ionic stress, with a noteworthy tolerance to NaCl. No evidence was found supporting an osmo-induced intracellular accumulation of the most common osmoprotectants. Salt tolerance seems to rely upon the inducible expression of an amiloride-resistant Na⁺/H⁺ antiporter. Since in snow fields osmotic unbalance due to freeze/thaw is more likely to occur than excess salts, results suggest an allochthonous origin of the strain.