A method for assaying for loss of viability of a microorganism comprised in water after the microorganism has been treated by exposure to a stressor, said stressor being an agent, condition or stimulus that causes stress to the microorganism, the method aiming to quantify both damage to photosystems and their repair, the method comprising the steps of: a) measuring the variable fluorescence (F v ) of an untreated sample of the microorganism; b) measuring the variable fluorescence (F v ) of a treated sample of the microorganism; c) calculating a photosynthesis repair process index using the measurements obtained in Steps (a) and (b), the photorepair index involving protein synthesis; and d) correlating the photosynthesis repair process index calculated in Step (c) to a normalized viability for the microorganism, wherein Step (a) comprises one or more of the following: (i) measuring the variable fluorescence (F v ) of the untreated sample of the microorganism to obtain a first value (F v Untreated ) ; (ii) incubating the untreated sample in the presence of radiation at a first intensity to induce a prescribed reduction in variable fluorescence ( F v ) from said first value (F v Untreated ) ; (iii) measuring the variable fluorescence (F v ) of the untreated sample after Step (ii) to obtain a second value (F v Untreated ); (iv) incubating the untreated sample in presence of radiation at a second intensity to induce a photosynthesis repair process involving protein synthesis and an increase in variable fluorescence ( F v ) from said second value ( F v Untreated ) ; and (v) measuring the variable fluorescence of the untreated sample after Step (iv) to obtain a third value (F v Untreated ). a) measuring the variable fluorescence (F v ) of an untreated sample of the microorganism; b) measuring the variable fluorescence (F v ) of a treated sample of the microorganism; c) calculating a photosynthesis repair process index using the measurements obtained in Steps (a) and (b), the photorepair index involving protein synthesis; and d) correlating the photosynthesis repair process index calculated in Step (c) to a normalized viability for the microorganism, wherein Step (a) comprises one or more of the following: (i) measuring the variable fluorescence (F v ) of the untreated sample of the microorganism to obtain a first value (F v Untreated ) ; (ii) incubating the untreated sample in the presence of radiation at a first intensity to induce a prescribed reduction in variable fluorescence ( F v ) from said first value (F v Untreated ) ; (iii) measuring the variable fluorescence (F v ) of the untreated sample after Step (ii) to obtain a second value (F v Untreated ); (iv) incubating the untreated sample in presence of radiation at a second intensity to induce a photosynthesis repair process involving protein synthesis and an increase in variable fluorescence ( F v ) from said second value ( F v Untreated ) ; and (v) measuring the variable fluorescence of the untreated sample after Step (iv) to obtain a third value (F v Untreated ). (i) measuring the variable fluorescence (F v ) of the untreated sample of the microorganism to obtain a first value (F v Untreated ) ; (ii) incubating the untreated sample in the presence of radiation at a first intensity to induce a prescribed reduction in variable fluorescence ( F v ) from said first value (F v Untreated ) ; (iii) measuring the variable fluorescence (F v ) of the untreated sample after Step (ii) to obtain a second value (F v Untreated ); (iv) incubating the untreated sample in presence of radiation at a second intensity to induce a photosynthesis repair process involving protein synthesis and an increase in variable fluorescence ( F v ) from said second value ( F v Untreated ) ; and (v) measuring the variable fluorescence of the untreated sample after Step (iv) to obtain a third value (F v Untreated ). <EOS>