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 O2 evolution, and decreased rates of CO2 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 Ca2+ extended the tolerance to 0.5 m NaCl before growth stopped. Increasing the light intensity from 1.2 to 9.4 mw/cm2 increased growth rates for cultures in 0.10 to 0.45 m NaCl. At 14 mw/cm2 added Ca2+ reduced growth rates of cultures in 0.3 m NaCl compared to controls without added Ca2+. Maximal growth rates for cultures in NaCl media were achieved by addition of 10 mm CaSO4 and maintenance of the light intensity at 9.4 mw/cm2. The maximal growth rate of the organism was 9.6 doublings/day achieved at 2.7 mw/cm2 for control cultures. In 0.3 m NaCl the growth rate was 4.3 doublings/day at 2.7 mw/cm2 and 8.2 doublings/day at 9.4 mw/cm2 with 10 mm CaSO4 added.
Increasing light intensities from 2.7 to 9.4 to 14 mw/cm2 decreased intracellular Na+ in cells cultured in 0.3 m NaCl medium without added Ca2+ and increased Cl− uptake in cells cultured in 0.3 m NaCl medium with and without added Ca2+. For cells cultured in 0.3 m NaCl medium at 14 mw/cm2 intracellular Na+ was 0.68 meq/g dry weight with Ca2+ added and 0.81 meq/g dry weight without Ca2+ added. Addition of Ca2+ at 2.7 mw/cm2 reduced intracellular Na+ to similar values. It is postulated that energy requirements for active Na+ exclusion were reduced by addition of Ca2+ allowing more energy to be used for cell growth resulting in increased growth rates.
O2 evolution and CO2 fixation studies indicated that increased photosynthetic energy, probably actuated by a high proton gradient accompanying Cl− influx and uncoupled from CO2 fixation, was available for maintenance of cellular integrity and active control of intracellular ionic ratios. The O2 evolving capacity was destroyed at 12 and 29 mw/cm2 for cells cultured in 0.3 m NaCl medium respectively with and without the addition of Ca2+. Control cultures continued producing O2 at light intensities up to 115 mw/cm2.
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Selected References
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