ABSTRACT
This chapter reviews the general literature on the physiological and developmental effects of solar UV and simulated solar UV radiation on Dunaliella and related microalgae. The depletion of stratospheric ozone allows increased UV-B (280–320nm) in the biosphere without affecting the quantity or quality of UV-A (320–400nm). These studies have provided impetus for new UV-B research, but risk diverting attention from the UV-A photobiology of Dunaliella and related species.
The physiological and metabolic UV effects on photosynthetic organisms are diverse. The growth, morphological, photomovement and metabolic responses of Dunaliella species to UV exposure are often, but not always, adverse. UV-A radiation exerts major developmental effects in algae acting through blue-light photoreceptors (cryptochromes and phototropins). Recent studies with algae closely related to Dunaliella, (i.e. Chlamydomonas spp.) have demonstrated UV-A acting through these photoreceptors will initiate biosynthesis of chlorophyll, carotenoids, light harvesting proteins, heat-shock proteins and chemotactic sensitivity. While UV-B appears to initiate important defensive signals for microalgae, its photoreceptor(s) remain unknown. UV-B radiation will directly damage DNA, and UV-A can activate DNA repair mechanisms. UV-A, but not UV-B causes some Dunaliella species to over-produce β-carotene. This increased β-carotene screens UV-A and this correlates with UV-B screening to reduce the damaging effects of those radiation bands.
The photosynthetic process is very sensitive to UV radiation. Some Dunaliella species appear to be more tolerant of UV-B than others. Photosystem II is a major site of UV-B induced damage in Dunaliella species as in other algae and higher plants. Nevertheless, some reports with Dunaliella have found minimal or no inhibition of PSII when UV-B similar to solar was used. The rate of repair of damage to PSII plays an important role in mitigating the loss of photosynthetic activity during and subsequent to UV-B exposure. The carbon fixing reactions of photosynthesis (Rubisco) are also damaged by UV-B radiation. Studies with higher plants and microalgae suggest that the carbon-fixing reactions are inhibited before damage to the thylakoid reactions becomes apparent. One report indicates that the carbon concentrating mechanism in Dunaliella can be affected by UV-B. The relative UV-B sensitivities of the light and dark reactions of photosynthesis in Dunaliella species needs to be clarified. That UV-A radiation also damages photosynthesis in Dunaliella is well established. Both UV-A and UV-B radiation lead to oxidative stress in Dunaliella as measured in changes in the level of diverse antioxidant enzymes and substrates. Our understanding of the UV effects on the physiology, development and biochemistry of Dunaliella in the natural environment are inadequate, in part because experimental conditions that are often used differ greatly from sunlight both quantitatively and qualitatively. The ratios of UV-B to both UV-A and to visible radiation often differ drastically from solar, and UV-B lamps emit excess short wavelength radiation that makes it more difficult to extrapolate findings to ecological settings.
