In vivo mass-specific absorption spectra of phycobilipigments through selective bleaching

We describe how bleaching with beta-cyclocitral can be used to isolate the in vivo absorption of phycobilipigments in phytoplankton, particularly cyanobacteria. Exposure to an emulsion of beta-cyclocitral bleaches chlorophylls, carotenoids, and xanthophylls within minutes while phycobilipigments bleach at a slower rate. Serial spectral absorption measurements are used to isolate the absorption spectrum of the phycobilipigments after other pigments have already bleached. Subsequently, in vivo absorption spectra are reconstructed for both pigment groups and expressed as the pigment mass-specific absorption coefficients. These coefficients are used widely in optical modeling and remote sensing studies but have thus far not been provided for the group of phycobilipigments. In vivo mass-specific absorption spectra for phycoyanin (with allophycocyanin) and chlorophyll a (with carotenoids, xanthophylls) were highly consistent between different species of cyanobacteria that were rich in phycocyanin. Fitting these pigment-specific absorption spectra to the in vivo absorption of cultures that also contain phycoerythrin, we were able to derive specific absorption spectra for phycoerythrin. All tested cyanobacteria and rhodophytes responded to the treatment, with varying pigment bleaching rates. A diatom also bleached rapidly whereas two chlorophytes were highly resistant. The method is therefore not yet suitable for (natural) mixed phytoplankton samples.