Maximum Growth Rates of Chlorella in Steady-state and in Synchronized Cultures.

Multicellular organisms and steady-state microbial populations consist of cells in all possible developmental stages. In such populations some cells are vigorously growing, others are preparing to divide, and stll others are actually in the process of cell division. Environmental conditions necessary for the optimum development of one stage in the life history of the cell may be totally inadequate or even inhibitory for growth during another stage. Two paramount processes in the growth and development of cells are the accumulation of cell material and cell division. In autotrophic algal cells the accumulation of cell material depends directly on photosynthesis and consequently on light. In Chlorella pyrenoidosa cell division does not depend on light in the sense that after accumulation of cell material a cell can divide in complete darkness. In synchronized algal suspension the accumulation of cell material can be separated in time from cell division. The studies reported here were designed to determine the maximum rates of growth attainable during the period of synthesis and accumulation of cell material, and to explore some of the factors which might be stimulatory to this phase of life history but inhibitory to cell division. Experimental.-The work was done with the green alga, Chlorella pyrenoidosa, high-temperature strain 7-11-05.1 Details of the experimental technique have been described in previous publications.1-4 Two series of experiments were conducted. In one series cells were grown in a continuous culture apparatus5 under constant conditions of temperature, illuminance, and nutrient supply. As the cells grew during the experiment the optical density of the algal suspensionwas photometrically maintained at a constant level by automatic dilution with fresh medium. Thus, the illuminance experienced by individual cells was kept at a constant level. Incandescent lamps placed in four banks around the growth chamber served as a source of light. The light intensity at the surface of the growth chamber measured with model 756 Weston Illumination Meter provided with a quartz filter was 1,000 ft-c. In the second series of the experiments algal suspensions were grown in test tubes containing a constant volume of medium. As a source of light day-light, coolwhite fluorescent lamps were placed in two banks at the opposite sides of a thermostatically controlled plexiglass water-bath. The light intensity measured at the level of the culture tubes was 2,000 ft-c. As the cells grew the optical density of the algal suspensions increased and the illuminance, as experienced by the individual cells, declined. However, it has been established previously that the growth rate remained exponential for the duration of the experiments. This observation was taken as an indication that the decrease in the illuminance as the result of cell growth did not become limiting to growth under these conditions. It should be emphasized that the light intensities measured in the two series of experiments are not directly comparable because of the difference in the light source, the geometry of illumina-