A fast colorimetric assay to control growth of algal strains:: Application to numeration of marine diatom cells

The suitability of the oxidation-reduction indicator MTT [3-(4,5-dimethyl-thiazol-2yl)-2,5diphenyl-tetrazolium bromide] to measure viability of marine diatoms is evaluated. This tetrazolium salt is reduced to form bluish violet formazan derivative in living cells while it is not reduced in dead cells. Using a contact time of three hours the formazan is spectrophotometrically quantified. Culture conditions were standardised in 24-multiwell microculture plates using four marine diatom species, some of which are currently grown in aquaculture, to determine the minimal cell concentration for detecting of formazan metabolisation. Our results demonstrate that the absorbance due to the reduction of MTT is strictly related to the diatom cellular activity. This simple colorimetric assay permits rapid cell numeration of a great number of samples in comparison to the classic haematimetric method, notably for in vitro bioassays in ecological and physiological diatom investigations. This method is only applicable to unicellular algal strains (monospecific and axenic culture). For that reason, this method is suitable for algal bioassay (e.g. water fertility tests, allelopathic studies).