CHARACTERIZATION AND BIOLOGICAL IMPLICATIONS OF SCYTONEMIN,A CYANOBACTERIAL SHEATH PIGMENT1

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.     

WAVELENGTH DEPENDENCY OF THE MAXIMUM QUANTUM YIELD OF CARBON FIXATION FOR TWO RED TIDE DINOFLAGELLATES,HETEROCAPSA PYGMAEA AND PROROCENTRUM MINIMUM (PYRROPHYTA): IMPLICATIONS FOR MEASURING PHOTOSYNTHETIC RATES1

The influence of photoadaptive state on the spectral dependency of the maximum quantum yield for carbon fixation was determined for two red tide dinoflagellates, Heterocapsa pygmaea Loeblich, Schmidt, et Sherley and Prorocentrum minimum Pavillard. Cultures were acclimated to green, blue, red, and white light. The spectral dependency in the light-limited slope of the photosynthesis–irradiance curves (α) was measured with carbon action spectra that, when divided by the spectrally weighted absorption coefficient, provided estimates of the maximum quantum yield (φ_max) for carbon fixation. Values of φ_max varied with wavelength within each culture condition as well as between different culture conditions. The degree to which the spectral dependency in φ_max was influenced by the presence of photoprotective carotenoids and/or energy imbalances between photosystems I and II was assessed for both dinoflagellates. The impact of photoprotective pigmentation on the spectral dependency of φ_max was most significant for cells grown under high light conditions reflecting the enrichment of diadinoxanthin. Energy imbalances between the photosystems was assessed by quantifying enhancement effects on spectral φ_max in the presence of background illumination. Under our experimental conditions, enhancement effects on carbon action spectra were evident for H. pygmaea under nearly all growth conditions but were not detectable for P. minimum under any growth condition. We hypothesize that sensitivity to enhancement effects reflected differences in the structure of the photosynthetic machinery of these two peridinin-containing dinoflagellates. While measurements of φ_max are sensitive to the color of the light within an incubator, the relative impact on the spectral dependency of a was less than the wavelength dependency associated with the cellular absorption properties. Finally we used our data to validate an approach proposed by others to aid in the correction of photosynthetic measurements where the in situ spectral light field cannot be easily mimicked. The average error using this approach was 8%, which was significantly less than the error associated with ignoring the spectral dependency in α.