CHLORELLA-VULGARIS (CHLORELLACEAE) DOES NOT SECRETE AUTOINHIBITORS ATHIGH CELL DENSITIES

Filtrates (conditioned medium) from high-density Chlorella vulgaris cu ltures in photobioreactors were obtained and tested for autoinhibitory activity under different conditions. Exponentially growing cells were inoculated at low initial cell concentration (2 x 10(5) cells/ml) in 90% conditioned medium (CM) supplemented with 10% fresh medium (FM) at low (atmospheric) CO2 levels. The time sequence of DNA histograms of cells in CM cultures showed that there is an accumulation of cells wit h two and four DNA equivalents in the culture over a period of time, s ignifying a blockage of cells at the division stage of the cell cycle. Examination of the chemical composition of CM showed the presence of high concentrations (> 10 mM) of bicarbonate. Adding similar bicarbona te concentrations to FM were found to have similar effects as CM cultu res, causing blockage of cell division, though the intensity of the bl ocking effect was lower. The bicarbonate-free CM did not show any cell cycle modulating or inhibitory activity. The growth of cells cultivat ed at high (5%) CO2 levels in 90% CM supplemented with 10% FM was comp arable to 10% FM cultures, indicating nutrient limitation in 90% CM cu lture. When the 90% CM culture was supplemented with 100% nutrients, t he growth rate and final cell concentration was similar to 100% FM cul ture. Based on these results we conclude that C. vulgaris does not sec rete any autoinhibitor(s) or cell cycle modulating compound(s) under t he conditions from which the CM was obtained.