Rapid evaluation of three biocide treatments against the cyanobacterium Nostoc sp. PCC 9104 by color changes

Repeated short–term exposures to: (1) a commercial isothiazoline biocide (Biotin T®), (2) constant temperature (37 °C) and (3) UV-C germicidal irradiation (254 nm) on planktonic culture of an aeroterrestrial cyanobacterium of the genus Nostoc was carried out to assess (1) the effectiveness of the three trial treatments, referred to here as biocide strategies, and (2) the usefulness of CIELAB color coordinates (L*, a*, b*, C*ab, and hab) in monitoring their effectiviness. After each exposure, spectrophotometric measurements of chlorophyll-a, phycobiliproteins (phycocyanin PC, allophycocyanin APC, and phycoerythrin PE), and total carotenoids were conducted together with CIELAB color measurements and the adenosine triphosphate (ATP) assay. In terms of effectiveness of biocide treatments, Biotin T® was the most effective, followed by UV-C irradiation. Constant 37 °C temperature showed no biocidal effect, as ATP, chlorophyll-a, phycocyanin, and allophycocyanin values increased. Results showed that L*, a*, and hab CIELAB parameters could be usefully used to monitor the effectiveness of these three biocide strategies being all the CIELAB color coordinates significantly correlated with chlorophyll-a, phycocyanin, allophycocyanin, and ATP contents. In particular, for the first time, linear regression equations were calculated to predict chlorophyll-a and ATP from parameters L* and a*, and phycocyanin and allophycocyanin from parameters L* and hab, obtaining values of adjusted R 2 close to 0.9. Future considerations will include the application of this technique to cyanobacterial biofilms present on stone surfaces, since the evaluation presented in this study is limited to planktonic cultures.