Changes in the proportion of endogenous osmotic solutes accumulated by Chlorella emersonii in the light and dark

The type of endogenous osmotic solute accumulated by Chlorella emersonii grown at high external osmotic pressure (π ext ) depended on the light/dark conditions: proline accumulated to high concentrations in cells in the light, while sucrose accumulated to high concentrations in the dark. These findings were made during the alternating light dark cycles used to obtain synchronized cultures, i.e. cultures containing cells at only one stage of development at any one time. Similar decreases in proline and increases in sucrose in the dark were found for cells previously grown in continuous light to obtain non‐synchronized cultures, i.e. cultures containing cells at all stages of development. In cultures synchronized at 200 mol m −3 NaCl (π ext = 1.01 MPa), recently divided ‘daughter cells’ at the beginning of the light periods contained 60 mol m −3 proline and 100mol m −3 sucrose, while mature cells towards the end of light periods contained 130 mol m proline and 20 mol m −3 sucrose. The changes in proline and sucrose which occurred in synchronized cultures were due mainly to light/dark conditions and to a much lesser extent to different stages of cell development. The proportion of proline to sucrose in daughter cells collected from non‐synchronized cultures in continuous light was not different from the proportion in heterogeneous populations of cells. Results are discussed in relation to the accumulations of two, rather than one, endogenous osmotic solute and to growth reductions of C. emersonii exposed to high external osmotic pressures.