ABSTRACT
The halotolerant alga Dunaliella salina can adapt to practically the entire range of salt concentrations. This capacity is achieved primarily by massive accumulation of glycerol and by efficient elimination of sodium ions, which require major metabolic investment.
To clarify the molecular mechanisms of salinity tolerance in D. salina, we performed a proteomic differential analysis aimed to identify salt-regulated proteins in different sub-cellular fractions. Soluble proteins were identified by 2D IEF/SDS-PAGE combined with MALDI-TOF MS/MS, whereas plasma membrane proteins were biotin-tagged, separated by blue-native/SDS-PAGE, and identified by nano-LC/MS-MS.
High salt up-regulated plasma membrane carbonic anhydrases, which mediate bicarbonate acquisition, key enzymes in Calvin cycle, starch mobilization and redox energy production. These results indicated that D. salina enhances photosynthetic CO2 assimilation and diverts its principle carbon and energy resources for synthesis of glycerol.
In the plasma membrane we identified: (i) bacterial-type surface coat proteins (peptidoglycan-associated lipoprotein) and tubulin, which probably function in stabilizing the membrane against osmotic lysis; (ii) small GTP-binding proteins which may be involved in signal-transduction in response to salt/osmotic stress; (iii) lipid metabolizing enzymes, possibly associated with osmotic sensing; (iv) chaperones and proteolytic enzymes probably involved in enhancing the turnover and stabilization of membrane proteins at high salinity; (v) ion transporters for protons, iron, nitrate, ammonium and possibly sodium.
342Taken together, these results suggest that concerted changes in multiple pathways contribute to unique ability of D. salina to withstand high salinity.
A surprising observation was the finding that high salinity induced iron deficiency stress in D. salina. We discovered that D. salina evolved special strategies to cope with iron limitation. It utilizes a unique mechanism for iron acquisition, via membrane-associated transferrins, that bind and internalize ferric ions into acidic vacuoles. In the chloroplast, iron deprivation induced one major protein, identified as a PS-I chlorophyll a/b-binding protein, which largely increased the size of PS-I units. Interestingly, iron-deprived cyanobacteria, accumulate in PS-I a different type of chlorophyll-binding protein. This may represent a general strategy of photosynthetic organisms to adapt to iron deprivation.
