Effects of Bicarbonate Limitation and Salinity Stress on Growth and Photosynthesis in the Green Alga Picochlorum Oklahomensis

Photosynthetic organisms rarely experience optimal growth conditions in their natural habitat, and at any given time, two or more physical and chemical variables are likely to be suboptimal. The extreme environment of the Great Salt Plains (GSP) results in low algal biomass, such that natural selection is likely driven by survival of multiple abiotic stresses rather than rapid growth and biotic interactions. The objective of this study is to determine the effect of combined salinity stress and carbon limitation on growth and photosynthesis in the green alga Picochlorum okla-homensis isolated from the GSP habitat. Algal cells were grown in batch cultures under bicar-bonate sufficiency (control) or low bicarbonate at salinities of 10, 50 and100 ppt. in artificial seawater (AS 100) medium. Cells were physiologically characterized by initial growth rates, cell yields, photosynthetic light-response curves (oxygen evolution), pigment composition, and the chlorophyll fluorescence parameters Fv/Fm, �PSII, qP and NPQ. Low bicarbonate and increasing salinity from 10 to 100 ppt, as well as the bicarbonate-salinity interaction resulted in significant lower cell yields and initial growth rates. In general, high salinity resulted in significant primary effects for the majority of variables but a significant effect of bicarbonate and bicarbonate-salinity interaction were less common among the physiological variables.