If you have purified phycocyanin in solution, you can measure its concentration using its absorbance spectrum and Beer's Law.

https://en.wikipedia.org/wiki/Phycocyanin#:~:text=Spectral%20characteristics,-C%2Dphycocyanin%20has&text=Its%20emission%20maximum%20is%20~642,at%20533%20and%20544%20nm.

Adam B Shapiro Thanks for your answer it's very helpful.

I agree with Adam. In our last study we quantified phycocyanin from Lyngbya purpurem (cyanobacteria) using sprectroscopy.

Here is our paper, I hope you find it useful: Article Influence of Low Salt Concentration on Growth Behavior and G...

Susana Fuentes-Tristan Thanks for sharing the article. It's beneficial.