Phycocyanin-specific absorption coefficient: Eliminating the effect of chlorophylls absorption

Abstract

The applicability of algorithms for estimation of phycocyanin (PC) concentration based on light spectral reflectance heavily depends on the specific absorption of the pigment. But the determination of PC-specific absorption coefficient is not a straightforward task, as PC optical activity is overlapped by absorption of chlorophylls. The aim of our study was to determine aPC(625)—the specific absorption coefficient of PC at 625 nm, in samples with PC concentrations ranging from 0.5 mg m−3 to 126.4 mg m−3 and varying proportions of chlorophylls a, b, and c in the samples. The effect of chlorophylls was subtracted from total absorption at 625 nm using Chl a absorption at 675 nm as a reference;the contribution of the chlorophylls was computed on the basis of their relative absorption at 625 nm and concentrations. The major effect on the precision of aPC(625) determination was imposed by Chl a absorption, but the effect of the accessory chlorophylls ought to be accounted for in order to arrive at reliable PC-specific absorption values. aPC(625) varied widely, from 0.002 mg m−2 to 0.027 mg m−2, and decreased with increase of PC concentration. As PC concentration exceeded 10 mg m−3, aPC(625) was almost invariable, slightly oscillating around 0.007 mg m−2 and similar to PC specific absorption coefficient estimated by alternative methods. It is clear that a universal value is unfeasible, but aPC(625) of 0.007-0.008 mg m−2 seems an appropriate value for use in algorithms destined for estimation of phycocyanin in typical mesotrophic and eutrophic inland waters dominated by cyanobacteria.