The rapid counting method for 2-MIB-producing cyanobacteria (Pseudanabaena sp.) using fluorescence detection of phycocyanin pigments in algal cells

The sudden occurrence of odor in drinking water is one of the significant challenges for drinking water utilities. The concentrations of 2-methylisoborneol (2-MIB)-producing cyanobacteria, Pseudanabaena sp., can be a surrogate indicator for alarming the potential of odor occurrence. This study is aimed to develop a rapid and automatic method to accurately measure Pseudanabaena sp. concentrations using the fluorescence detection of auto-fluorescent chemical in cyanobacteria, phycocyanin. This study first identified that increase in 2-MIB concentrations in a lake almost simultaneously occurred according to the rise in the counts of Pseudanabaena sp., which was only <0.5% of the entire algae population. The developed method using a phycocyanin filter and threshold settings in fluorescence microscopy (algal cell width and length, fluorescence intensity, and shape) confirmed that almost exclusively cyanobacteria appeared in fluorescent photos, unlike another commonly-used method using a chlorophyll filter that captured all algae. As a result, a high correlation (R-2 = 0.998) was observed between manually-counted and automatically-counted concentrations of Pseudanabaena sp. The detection limit of this method was determined at 11 units per mL. Furthermore, the accuracy of the new method using a phycocyanin filter was 88%, while the previously developed method using a chlorophyll filter showed approximately 62%. These results suggest that the developed method with a phycocyanin filter can provide a more reliable warning of potential 2-MIB occurrence in drinking water sources.