Ultraviolet radiation and the snow alga Chlamydomonas nivalis (Bauer) Wille[para]

Photochemistry and Photobiology, Jun 2003 by Gorton, Holly L, Vogelmann, Thomas C

The central location of the chloroplast and nucleus is ideal to screen them from UV damage, as it is for reducing photodamage by VIS light. In algae, screening protection against UV is generally provided by compounds localized either in the cell wall or in the cytoplasm. Walls may contain sporopollenin, a group of structurally related compounds that are resistant to acetolysis and that confer protection from chemical and microbial attack. Sporopollenin also absorbs UV radiation and is often found in the walls of UV-tolerant cells, and its presence can be correlated with UV tolerance (45). Cytoplasmic compounds may also provide screening. Whereas higher plants contain flavonoids, usually localized in the vacuoles of epidermal cells, algae often synthesize UV-absorbing mycosporine-like amino acids (MAA), water-soluble compounds with absorption maxima between 310 and 359 nm (44,46,47).

We investigated whether C. nivalis aplanospores could use screening by both wall and cytoplasmic components. A paste of isolated walls showed little transmittance in the UV. However, transmittance measurements of materials such as this cell wall preparation might be affected by scattering both because scattered light might be missed by the detector, as in a traditional spectrophotometer, and because there may be a wavelength-dependent effect on path length, such that shorter wavelengths are scattered more and thus transmitted less. Our measurements were made with an integrating sphere to capture all transmitted light-scattered or direct-obviating the first problem. It was conceivable, however, that path length within the cell wall paste changed with wavelength so that the absorbance curve of the paste really represented absorbance plus a path length effect that would register as false absorption; this is unlikely to be a significant effect.

The effect of scattering depends on the size of the scattering particles. Scattering by particles larger than the wavelength of light, Mie scattering, would be independent of wavelength for non-uniform particles such as the broken cell walls and would thus not cause spectral changes. On the other hand, scattering by molecules or particles smaller than the wavelength of light, Rayleigh scattering, would be greatest for shorter wavelengths and would lead to increased light absorption at these wavelengths. However, the presence of significant Rayleigh scattering would be evident from a blue cast to the light reflected from the sample relative to the light transmitted through the sample. Our samples were semitransparent and clearly pigmented with a light-brown color, and there was no difference obvious in spectral balance between reflected and transmitted light that would indicate Rayleigh scattering. Moreover, significant Rayleigh scattering in the biological world is usually associated with structures such as small air vesicles, protein particles or lipid droplets (48); this phenomenon is common in animals but rare in plants and not obvious in the structure of C. nivalis cell walls (1). Other small particles would have been removed in the preparation of the sample, and Rayleigh scattering by molecules rather than particulate materials (as in clear solutions) is relatively weak. Thus, we believe that the spectral characteristics of the sample were consistent with light absorption rather than scattering, justifying the conversion of transmittance to absorbance. Although the absorbance spectrum may have some distortion from scatter, it is clear that scatter is not the predominant cause of light extinction in the sample and that absorption increases with decreasing wavelength.