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 02 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 Physiological mechanisms of salt tolerance have been primarily concerned with ionic compositions and fluxes. Regulation of ionic composition in marine algae consists of Na+ exclusion with accumulation of K+ and C1-and generally requires cellular energy and the presence of Ca2`(27). Studies of ionic fluxes in the red marine alga Gracilaria foliifera (12), in the fresh water algae Hydrodictyon africanum (23) and Chlorella pyrenoidosa (2), and on the internodal cells of Nitella translucens (18) have demonstrated light-dependent active transport for Na+ exclusion and K+ and Cl-accumulation. Work on the red marine alga Porphyra perforata showed a requirement for Ca2+ in the medium for intracellular retention of K+ (7).Two considerations indicate that light-dependent ion fluxes in green plants are controlled by photosynthetic rather than respiratory energy. First, light generally has little effect on ion fluxes in nongreen tissue, and second, the action spectrum of ion fluxes in green tissue matches that of chlorophyll absorption (23). The activation of K+ and Cl-fluxes in Nitella was separated respectively between photosynthetic systems II and I (18). Under conditions of either DCMU inhibition or far red light irradiation, where system I was functional, K+ absorption was not affected and Cl-absorption was reduced compared to conditions where both photosystems were operative (2, 18, 24). The selective participation of these systems was also reported in Chlorella pyrenoidosa (2) and Hydrodictyon africanum (24). System I was also implicated in Na+ efflux (24).Direct involvement of ATP from cyclic photophosphorylation for light-dependent Na+ and K+ fluxes was established by using uncouplers of phosphorylation. Addition of carbonylcyanide-m-chlorophenylhydrazone (2,17,25,28) and i...

The unicellular algae Chlorella sorokiniana was

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Exposure of Chlorela sorokiniwa (07-11-05) to ozone inhibits photosynthesis. In this study, the effects of ozone on 02 evolution and fluorescence yields are used to characterize this inhibition. At an ozone dose of about 3 micromnoles delivered to 2 x 10' cells, the photosynthetic rate of the cells is inhibited 50%, as indicated by a decrease in bicarbonate-stimulated 02 evolution (control rate, 1.4 ± 0.3 x 10"1 moles per ceil per minute). Plant productivity is decreased by ozone exposure in three major ways: (a) induction of stomatal closure in sensitive plants, which protects them from further injury but reduces the plant's photosynthetic capacity; (b) reduction in the chloroplast's ability to carry out efficient photosynthesis (8, 10); and (c) induction of necrosis and chlorosis which generally appears from 1 to 2 days after ozone exposure.Chlorella has been used previously to study ozone effects on ion transport pathways (1, 11). In the present study, we describe ozone alterations of photosynthetic competence in Chlorella as they are measured by 02 evolution and fluorescence yield kinetics. In addition, we assess the effects of an externally applied osmotic pressure used to vary the internal water potential. MATERIALS AND METHODSChlorella sorokiniana (07-11-05) were grown autotrophically as described previously (2). Either 02 or ozone was bubbled into 10 ml of a stirred buffered phosphate solution containing 2 x 108 cells/ml for varying times (11). A 1.0-ml cell sample was removed and, after addition of 20 mm NaHCO3, assayed for 02 evolution. 02 evolution was measured by a YSI Clark 02 electrode in a water-jacketed cuvette (total volume about 2 ml) maintained at 38°C. The cuvette was illuminated by red light with wavelengths greater than 600 nm and at an intensity of 50 mw/cm2 (about 80-90%o saturating). The cuvette was constructed so that the electrode just fit into the top opening, minimizing diffusion of 02 from the air but allowing additions by microsyringe. Light intensity was varied with Kodak neutral density filters and measured with a Licor radiometer (corrected to PAR).Ozone was generated in an 02 gas stream by UV irradiation and quantitated colorimetrically by reaction with a neutral solution of KI (3).Fluorometer measurements, using both actinic and measuring beams of light, utilized a noncommercial fluorometer (partially described in ref. A broad-band blue actinic beam of light (450 ± 50 nm; 11 kergs cm-2 s-1) was produced by tungsten bulb projector and blue cutoff filter (Coming CS-4-96), a Wratten blue filter (Type 48A), and a 500-nm cut-offfilter (Optics Technology, Palo Alto, CA). Actinic light passed into the sample along the same optical axis as the measuring beam, but at a 1800 angle to it.Fluorescence was measured at a 900 angle from the optical axis of the measuring beam by an S-20 photomultiplier (Type 9558B, E.M.I.). The photomultiplier was screened by a red cut-off filter (Corning CS-2-60), a red Wratten cellulose filter (Type 26) and a 680-nm interference filter (half-width, ...

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