Light and Calcium Interactions in Chlorella Inhibited by Sodium Chloride

Abstract

Analysis of NaCl toxicity in Chlorella sorokiniana showed decreased growth rates, increased dry weight per cell, increased intracellular Na(+) and Cl(-), more total chlorophyll per cell, a decreased chlorophyll a to chlorophyll b ratio, increased rates of O(2) evolution, and decreased rates of CO(2) fixation when the extracellular concentration of NaCl was increased from zero to 0.3 m. Cultures did not grow at concentrations greater than 0.3 m NaCl unless 10 mm calcium salts were present. Inclusion of that concentration of Ca(2+) extended the tolerance to 0.5 m NaCl before growth stopped. Increasing the light intensity from 1.2 to 9.4 mw/cm(2) increased growth rates for cultures in 0.10 to 0.45 m NaCl. At 14 mw/cm(2) added Ca(2+) reduced growth rates of cultures in 0.3 m NaCl compared to controls without added Ca(2+). Maximal growth rates for cultures in NaCl media were achieved by addition of 10 mm CaSO(4) and maintenance of the light intensity at 9.4 mw/cm(2). The maximal growth rate of the organism was 9.6 doublings/day achieved at 2.7 mw/cm(2) for control cultures. In 0.3 m NaCl the growth rate was 4.3 doublings/day at 2.7 mw/cm(2) and 8.2 doublings/day at 9.4 mw/cm(2) with 10 mm CaSO(4) added.Increasing light intensities from 2.7 to 9.4 to 14 mw/cm(2) decreased intracellular Na(+) in cells cultured in 0.3 m NaCl medium without added Ca(2+) and increased Cl(-) uptake in cells cultured in 0.3 m NaCl medium with and without added Ca(2+). For cells cultured in 0.3 m NaCl medium at 14 mw/cm(2) intracellular Na(+) was 0.68 meq/g dry weight with Ca(2+) added and 0.81 meq/g dry weight without Ca(2+) added. Addition of Ca(2+) at 2.7 mw/cm(2) reduced intracellular Na(+) to similar values. It is postulated that energy requirements for active Na(+) exclusion were reduced by addition of Ca(2+) allowing more energy to be used for cell growth resulting in increased growth rates.O(2) evolution and CO(2) fixation studies indicated that increased photosynthetic energy, probably actuated by a high proton gradient accompanying Cl(-) influx and uncoupled from CO(2) fixation, was available for maintenance of cellular integrity and active control of intracellular ionic ratios. The O(2) evolving capacity was destroyed at 12 and 29 mw/cm(2) for cells cultured in 0.3 m NaCl medium respectively with and without the addition of Ca(2+). Control cultures continued producing O(2) at light intensities up to 115 mw/cm(2).