An evaluation of methods for measuring algal growth

Algal growth was expressed as milligrams dry weight per litre with use of the electronic particle counter for determining cell number and cell volume. A gravimetric constant in milligrams dry weight per cubic micrometre was determined. This constant was applicable throughout the growth curve since there was a linear relationship between total cell weight and total cell volume. Cell volume must be considered in measuring growth because there is a change in average cell volume in addition to an increase in cell numbers throughout the growth curve. Chlorella vulgaris and Selenastrum capricornutum exhibited similar growth curves when growth was expressed as cells per litre or as milligrams dry weight per litre. Chlamydomonas reinhardtii appeared to have a lower algal growth potential when measured in cells per litre but had the same growth potential as C. vulgaris and S. capricornutum when expressed as milligrams dry weight per litre. The growth rate for Scenedesmus quadricauda was less than the other three, regardless of how the results were expressed. The image analysis system was used to count different types of algae in a mixed culture. C. reinhardtii, C. vulgaris, and Anacystis nidulans were not grossly distorted by the sample preparation method and could be distinguished from one another by variations in morphology. The counts could be separated by computing the ratio of area to length for each particle and by allowing the computer to sort and count each of the three types. Typical ratios ranged from 5 to 7 for G. reinhardtii, 2 to 4 C. vulgaris, and 0.2 to 1.3 for A. nidulans. The number of types to be counted in a mixture was limited only by the ability to apply shape factors without having an overlap between each type. Both the image analysis and electronic particle counter produced equal count and size measurements. Fluorescence was a sensitive measure of growth on a laboratory prepared medium since there was a logarithmic relationship between fluorescence and cells per millilitre for both C. vulgaris and S. capricornutum. Fluorescence measurements were not satisfactory for determining growth in samples of varying chemical composition (which occurs in natural samples) because the chlorophyll-to-cell ratio varied with media composition. This was especially pronounced with C. vulgaris.