Phycocyanin

Phycocyanin is found only in cyanobacteria (blue-green algae), the group responsible for most toxin-producing freshwater blooms, and also the more likely to produce tast and odor compounds like MIB and geosmin. Because chlorophyll-a alone cannot distinguish harmless green algae from toxin and T&O producers, phycocyanin fluorescence is the parameter that converts a generic "algae is up" reading into an actionable cyanobacteria alert. It is excited in the green or orange range (~575-590 nm) and emits in the deep red (~650 nm in a wider range), so it can be measured optically and continuously alongside chlorophyll-a.