Spectral properties and photosynthetic action in red-tide populations of Prorocentrum micans and Gonyaulax polyedra

Spectral attenuance and photosynthetic performance over the range 300 to 700 nm were examined in red-tide populations of Prorocentrum micans and Gonyaulax polyedra collected off La Jolla, California, USA, in May 1969 and May-June 1985; and in spring 1969, November 1981 and April-May 1982, respectively. In the near ultraviolet (UV), high attenuance and diminished photosynthetic effectiveness were observed in both dinoflagellates. This decline in relative quantum yield is ascribed to masking absorption by unidentified and spectrally different UV-absorbing compounds which were soluble in methanol. In the visible region, photosynthetic action spectra displayed the characteristic shapes expected from efficient Photosystem II light harvesting by peridinin, chlorophylls a and c₂, in conformity with previous studies on cultured dinoflagellates. In the case of P. micans, a high content of diadinoxanthin was found and the possible role of this yellow xanthophyll as a photoprotective pigment is discussed. However, photosynthetic enhancement spectra suggest that some portion of the diadinoxanthin pool functions as a Photosystem I antenna in this species. Our data on P. micans and G. polyedra support the hypothesis that UV-absorbing compounds and diadinoxanthin play protective roles in screening harmful radiation in red-tide dinoflagellates exposed to high-light conditions in nature.