Correlation between photosynthesis and light-independent metabolism in the growth of Chlorella

Abstract

1. 1. Observations on the culture of Chlorella have revealed that the algal cells assume two distinct forms in the course of their growth. One form, which we called “dark cells”, is smaller in size, richer in chlorophyll content, and stronger in photosynthetic activity than the other, which we referred to as “light cells”. When illuminated, dark cells grow and, with a substantial increase in mass, turn into light cells; the latter, when ripened, bear autospores in themselves (on the average 6–7 per cell) and eventually burst setting free the autospores which then become individual dark cells. The transformation of light cells into dark cells involves no increase of cell mass and occurs only under aerobic conditions, irrespective of whether the cells are in the light or in the dark. The dark cells freshly born from light cells are somewhat smaller in size and contain less chlorophyll than the “active” dark cells, into which the former turn rapidly under the influence of light. 2. 2. The processes of transformation between these types of cells were investigated separately under various experimental conditions, and it was concluded that (i) the main event occurring in the transformation of dark cells into light cells is photosynthesis, although it is accompanied by some other metabolic processes, which are to be distinguished from the photosynthetic process in the ordinary sense; and that (ii) the transformation of light cells into dark cells involves a light-dependent and accompanied also by some photosynthetic processes. 3. 3. The steady state of growth, as it is affected by light intensity and temperature, was investigated in detail, and its rate was compared with the photosynthetic rate using a common unit of measurement, i.e., in terms of weight of organic matter synthesized per unit time and per unit weight of cells. It was found that under weak light the rate of growth is exclusively determined by the photosynthetic process, whereas under strong light the light-independent metabolic processes become more or less significant in determining the rate of over-all growth. It was observed that the relative abundance of dark and light cells in the cultures varies considerably according to the culture condition, and indeed that the proportion of dark cells becomes larger in weaker light and at higher temperatures. 4. 4. Based on the experimental evidence mentioned above, the following formulae were set forth to describe symbolically the course of events occurring in the growth of algae. ▪, ▪ where D and L represent dark and light cells, respectively, k p the rate constant of photosynthesis, by which dark cells are changed into light cells, k D the rate constant of increase in cell number in the dark process, and n the number of dark cells arising from one light cell. Steady state kinetics based on this scheme lead to the formulae representing the over-all growth rate and the relative abundance of dark and light cells in the culture as functions of (i) light intensity, (ii) the rate of dark process, and (iii) the light-saturated and light-limited rates of photosynthesis. Correspondence between the theory and the observations was found to be, by and large, satisfactory. 5. 5. It was revealed by this analytical study that the dark process involved in the algal growth has a remarkably large temperature coefficient, especially in the range of lower temperatures.