C hlorella vulgaris (C hlorellaceae ) does not secrete autoinhibitors at high cell densities

Filtrates (conditioned medium) from high‐density Chlorella vulgaris cultures in photobioreactors were obtained and tested for autoinhibitory activity under different conditions. Exponentially growing cells were inoculated at low initial cell concentration (2 × 10 5 cells/ml) in 90% conditioned medium (CM) supplemented with 10% fresh medium (FM) at low (atmospheric) CO 2 levels. The time sequence of DNA histograms of cells in CM cultures showed that there is an accumulation of cells with two and four DNA equivalents in the culture over a period of time, signifying 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 bicarbonate concentrations to FM were found to have similar effects as CM cultures, causing blockage of cell division, though the intensity of the blocking effect was lower. The bicarbonate‐free CM did not show any cell cycle modulating or inhibitory activity. The growth of cells cultivated at high (5%) CO 2 levels in 90% CM supplemented with 10% FM was comparable to 10% FM cultures, indicating nutrient limitation in 90% CM culture. When the 90% CM culture was supplemented with 100% nutrients, the growth rate and final cell concentration was similar to 100% FM culture. Based on these results we conclude that C. vulgaris does not secrete any autoinhibitor(s) or cell cycle modulating compound(s) under the conditions from which the CM was obtained.