Seaweeds have remarkably rich absorption spectra.
Chlorophyll is responsible for the basic photosynthetic processes but, as the depth in the sea increases, the energy distribution of the incident light changes and auxiliary pigments are required to transfer the photon energy to the chlorphyll absorption band in the red part of the spectrum. The figure above shows the spectra of brown and purple seaweeds in transmission. The energy transfer from the phycoerythrin to the chlorophyll is by fluorescence. This can be seen by illuminating the root nodules with UV light from a mercury vapour lamp and observing the yellowish fluorescence spectrum. Since the nodules are very small, this experiment was performed on a microscope stage.