THE GROWTH OF CHLORELLA PYRENOIDOSA UNDER VARIOUS CULTURE CONDITIONS

Abstract

The unicellular algae are being used with increasing frequency in studies on the nature of the photosynthetic mechanism. These algae are excellent experimental material. Their measurable gas exchange is not subject to certain of the complexities of higher plants, e.g., stomatal behavior. They can be used in thick suspensions to obtain almost total light absorption or in such thin suspensions that the variation in light intensity within the sample becomes negligible. They are adapted to the comparatively simple and accurate manometric measurement of gas exchange. Furthermore, effects of individual variation among samples from any one batch of cells are minimized since any one sample usually contains billions of individuals. The procedure of growing algae introduced by Warburg (16) and described in detail by Gaffron (4) has been commonly followed. Certain strains of the alga Chlorella have been most widely used. In one sense the choice of this form is not particularly fortunate since it reproduces by formation of a variable number of autospores within the parent cell. This means that Chlorella cells may be subject to wider variation in size than might be expected of a form such as Stichococcus which multiplies by binary fission. Cultures are commonly grown in a mineral salt solution in 250to S00-ml. flasks, with a current of air fortified with carbon dioxide (^ 3 to 5 per cent.) bubbled through, at a temperature in the range of 20 to 25? C, and illuminated by a tungsten filament lamp giving an incident intensity in the range of 50 to 500 foot candles. This technique has proved so convenient and reliable that no comprehensive study of growth conditions has been undertaken. A few workers have chosen other algae or followed other procedures of culture, but the study of growth conditions or comparison between various algae has been limited. In one sense the standardization of procedure in culture is fortunate since comparisons between data of different laboratories are facilitated. But by the same token the photosynthetic mechanism has been given an appearance of stability which may not be warranted. It would seem that a point has been reached at which the relations between culture conditions and the development of the photosynthetic mechanism can profitably be explored. It is the thesis of this and succeeding papers of the series that one approach to the photosynthesis problem lies in a study of photosynthetic behavior in relation to conditions of culture. A step in this direction has been taken by Sargent (12) in studying certain effects of light intensity on photosynthetic behavior.