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    Received 16 October 1990. Accepted 12 February 1991.

  • We thank D. Gleason for the use of the spectrophotometer, H. Howard for sharing photography and microscopy skills, W. R. Sistrom for the use of a French Press, and D. Wimber for lending his mirrospectrophotometer. D. Carlson (Oregon Slate University) kindly made his spectrofluorometer available to us. Valuable criticism was provided by S. Neuer (O.S.U.) and W. R. Sistrom (U. of O.). F.G-P. was supported by a Fulbright/La Caixa Scholarship during part of this work, and part of his field collecting in Mexico was supported by a U.C.L.A. grant. We thank the U.S. National Science Foundation (Grant BSR-9007633) for support during part of this work. R.W.C. also thanks the Antarctic Division, D.S.I.R., New Zealand, for support during Antarctic collections.

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Scytonemin, the yellow-brown pigment of cyanobacterial (blue-green algal) extracellular sheaths, was found in species thriving in habitats exposed to intense solar radiation. Scytonemin occurred predominantly in sheaths of the outermost parts or top layers of cyanobacterial mats, crusts, or colonies. Scytonemin appears to be a single compound identified in more than 30 species of cyanobacteria from cultures and natural populations. It is lipid soluble and has a prominent absorption maximum in the near-ultraviolet region of the spectrum (384 nm in acetone; ca. 370 nm in vivo) with a long tail extending to the infrared region. Microspectrophotometric measurements of the transmittance of pigmented sheaths and the quenching of ultraviolet excitation of phycocyanin fluorescence demonstrate that the pigment was effective in shielding the cells from incoming near-ultraviolet-blue radiation, but not from green or red light. High light intensity (between 99 and 250 μmol photon · m−2· S−1, depending on species) promoted the synthesis of scytonemin in cultures of cyanobacteria. In cultures, high light intensity caused reduction in the specific content of Chl a and phycobilins, increase in the ratio of total carotenoids to Chl a, and scytonemin increase. UV-A (320–400 nm) radiation was very effective in eliciting scytonemin synthesis. Scytonemin production was physiological and not due to a mere photochemical conversion. These results strongly suggest that scytonemin production constitutes an adaptive strategy of photoprotection against short-wavelength solar irradiance.