THE MEASUREMENT OF AVAILABLE LIGHT BY THE USE OF CHLORELLA

Abstract

During a study of seasonal variations in tropical high forest and savanna woodland measurements of various microclimatic factors including light intensity were required. The measurement of light intensity for ecological purposes has always been unsatisfactory. This is partly due to the lack of a suitable instrument for measuring the 'photosynthetic efficiency' (or some similar value) of light and partly to the constantly changing light intensity found in natural conditions. It was decided to try to use the unicellular alga Chlorella vulgaris Beijerinck as a phytometer. There were three reasons for this choice. First, it can be grown in pure culture under controlled and repeatable conditions so far as the chemical composition of the medium is concerned. Secondly, it can be grown in flasks which protect it from predators and parasites. Thirdly, intensive laboratory studies have been carried out on the effect of external factors on the growth rate of Chlorella; in particuar Myers (I946) has shown that the growth-rate graph is a 'typical light saturated curve'. (Temperature also influences the growth rate of Chlorella but its effects are relatively small under conditions of light limitation.) The investigation was successful and as the method has been used satisfactorily for almost 2 years it was felt that it may be of use to other workers. The Chlorella was grown in 250 ml conical flasks each containing ioo ml of a standard liquid medium. The choice of both strains of Chlorella and medium are relatively unimportant provided that the same ones are used for a complete series of observations. For these observations a high temperature strain (designated Chlorella Tx7II5) was grown in a modified Knops solution, a medium in which it has been found to grow successfully (Myers, personal communication). A stock of Chlorella growing on agar slopes and in liquid medium in flasks was always kept over a cold light in the laboratory. These flasks were shaken twice daily to prevent the clumping of divided daughter cells. The number of cells per unit volume in a flask from which the inoculum was to be taken was determined by haemocytometer count. From this was calculated the volume of inoculum to be added to each flask in order to give these flasks an initial population of about 8oo,ooo cells - a suitable value found from preliminary experiments. (High cell densities are to be avoided as they limit the supply of both light and carbon dioxide to much of the culture.) Inoculation was done by pipette in a sterile chamber and the inoculated flasks were closed by a sterile rubber bung. The flasks were then transported in the dark in specially made boxes to the experimental site where the rubber bungs were removed and replaced by cotton-wool plugs and beaker covers. During this process a spirit lamp was used for sterilization. At a specified time the flasks were removed from the dark and placed in the positions