DUNALIELLA—A MODEL EXTREMOPHILIC ALGA

ABSTRACT

The unicellular alga Dunaliella is unique in its ability to adapt to extreme environmental conditions. Adaptation to extreme salinity involves short-term and long-term responses. The former include osmotic adjustment by accumulation of large amounts of intracellular glycerol and efficient elimination of Na⁺ ions by plasma membrane transporters. The latter involves synthesis of two extrinsic plasma membrane proteins: a carbonic anhydrase and a novel type of a transferrin-like protein. These proteins are associated with acquisition of CO₂ and Fe, respectively, whose availability is diminished in high salinity, limiting algal growth. Both proteins are functional over a wide range of salt concentrations and differ in structure from their mesophilic counterparts in possessing additional internal repeats and in having higher ratios of acidic: basic amino acids. Dunaliella acidophila survives at pH 0–1 by overexpression of a potent plasma membrane H⁺-ATPase which provides effective capacity for elimination of protons. Sequence comparisons of the ATPase genes from halophilic and acidophilic species reveals variations in charged amino acid composition within a distinct extrinsic C-terminal domain of the protein. Dunaliella bardawil adapts to high light intensity by several strategies: it accumulates large amounts of β-carotene which screens the photosynthetic system against photoinhibition and it modifies the photosynthetic machinery by synthesis of a special light-harvesting protein which presumably functions in dissipation of excessive light energy. Both responses depend on synthesis of special proteins and enzymes. Signal transduction mechanisms mediating stress responses in Dunaliella are poorly understood. Sensing osmotic/salinity changes involves specific plasma membrane sterols and activation of a plasma membrane protein kinase. Induction of β-carotene accumulation can be mimicked by reactive oxygen species generators.