Abstract
Treatment of Chlamydomonas reinhardtii cells, cultured at 5% CO2, with 1 to 1000 micrograms triacontanol (TRIA) per liter resulted in 21 to 35% increases in cell density, 7 to 31% increases in total chlorophyll, and 20 to 100% increases in photosynthetic CO2 assimilation. The increase in CO2 fixation with TRIA treatment occurred before, and was independent of, increases in total chlorophyll or cell number. Chlamydomonas cells responded to a broad range of TRIA concentrations that were at least one order of magnitude above the optimum concentration established for higher plants. The necessity for larger concentrations of TRIA may be due to destabilizing effects of Ca2+ and K+ present in the Chlamydomonas growth medium. These ions caused flocculation of the colloidally dispersed TRIA in apparent competition with binding of [14C]TRIA to Chlamydomonas cells. Octacosanol inhibited the effect of TRIA on photosynthetic CO2 assimilation. TRIA treatment did not alter the distribution of 14C-label among photosynthetic products. The effect of TRIA on photosynthetic CO2 assimilation increased with time after treatment up to 3 days. Chlamydomonas cells that had been grown at low-CO2 (air) did not respond to TRIA, and transfer of high-CO2 (5%) grown cells that had responded to TRIA to a low-CO2 atmosphere resulted in a loss of the effect of TRIA. The effect of pH on photosynthetic CO2 assimilation indicated that CO2 is probably the species of inorganic carbon utilized by control and TRIA-treated Chlamydomonas cells.
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