A SIMPLE IN-VITRO FLUORESCENCE METHOD FOR BIOMASS MEASUREMENTS IN ALGAL GROWTH-INHIBITION TESTS

The estimation of biomass concentrations in algal growth inhibition te sts from measurements of pigment fluorescence in extracts of 20% sampl e (final v/v) prepared by direct addition to dimethylsulfoxide/acetone solvent offers several advantages compared to currently used direct o r indirect methods. The extraction stops the electron transfer and oth er processes which interact with chlorophyll fluorescence when measure d in vivo. As a result the response is stabilized and the sensitivity improved. The injection method is very fast, has a high potential for automation, allows storage of samples and is suitable for small sample volumes (e.g. 0.2 ml). The typical initial cell density in standard t oxicity tests of 10(4) cells ml(-1) of Selenastrum capricornutum was m easured precisely with a standard fluorimeter set-up, and 10(3) cells ml(-1) of S. capricornutum was measured reliably with a sensitive fluo rimeter. At low levels of toxicity by the model test compound potassiu m dichromate, the proposed fluorescence method resulted in very simila r inhibition figures as obtained with electronic particle counting. At high levels of toxicity, on the other hand, biomass determinations fr om pigment fluorescence readings were markedly affected by toxicant-in duced changes of the algal physiology. The low effect part of a dose r esponse curve is normally that one of major interest, and biomass esti mation errors associated with fluorescence measurements on extracts ar e thus considered acceptable in most situations. When the entire datas et was applied for endpoint estimation by the Weibull model, EC-1 esti mates were markedly affected by the curve fitting to data in the high inhibition range, while EC-10 and EC-20 were less and EC-50 almost una ffected. The method is expected to be less suitable for toxicity testi ng of herbicides specifically inhibiting photosynthesis. (C) 1997 Else vier Science Ltd.