Phytoplankton under nutrient limitation respond with both general and specific physiological changes. The responses of the photosynthetic apparatus to changes in nitrate, phosphate, and iron availability were studied in North Atlantic phytoplankton populations (Graziano et al. 1996, Aquat Microb Ecol 11: 53). The responses of the chlorophyte Dunaliella tertiolecta to phosphate deficiency were studied in the laboratory under controlled degrees of P-limitation, using chemostat cultures (Graziano et al. 1996, J. Phycol. 32: 825). A protein observed uniquely under P-stress in D. tertiolecta was partially characterized. This protein is a potential diagnostic indicator of P-stress in D. tertiolecta.