Ecology of UV
UV introduction and basic optics
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Photoinhibition
prepared by Pat
Neale
Molecular Levels and Photosynthesis
The primary mechanism by which UV inhibits photosynthesis in algae is uncertain but there are many possible targets (Vincent & Neale 2000). Several laboratory studies using high UV-B exposures have demonstrated damage to a primary molecular complex that all plants use for photosynthesis, photosystem II (PSII). However, field studies with algae grown under natural exposure conditions (solar irradiance) have found little or no effects on the perfomance of PSII in algae (Vincent & Neale 2000). An integral part of algal defense against UV is rapid repair of damaged photosynthetic complexes. This can be inferred from the time course of photosynthesis under UV exposure: after an initial period of decline in photosynthesis, a steady-state rate is attained and typically sustained over many hours of exposure (Cullen & Lesser 1991, Lesser et al 1994, Neale et al 1998a, Heraud & Beadall 2000). This steady state is interpreted as an equilibrium between damage and ongoing repair processes (Lesser et al 1994, Neale 2000). Because of repair, UV inhibition of phytoplankton photosynthesis does not typically obey reciprocity, i.e. damage is not solely a function of cumulative exposure. Under some conditions repair is slowed, increasing sensitivity to UV. This occurs when protein synthesis is inhibited (Cullen & Lesser 1991, Lesser et al 1994, Neale et al. 1998a) or under low nitrogen availability (Cullen & Lesser 1991, Litchman et al. 2002). For phytoplankton in deeply-mixed layers of the Southern Ocean, repair is so slow that equilibrium is not attained under high UV-B exposure conditions (Neale et al 1998b). In this case, photosynthesis drops off exponentially as exposure continues.
References:
Cullen, J.J., and Lesser, M.P. 1991. Inhibition of photosynthesis by ultraviolet radiation as a function of dose and dosage rate: Results for a marine diatom. Mar. Biol. 111:183-190.
Heraud, P., and Beardall, J. 2000. Changes in chlorophyll fluorescence during exposure of Dunaliella tertiolecta to UV radiation indicate a dynamic interaction between damage and repair processes. Photosynthesis Res. 63:123-134.
Lesser, M.P., Cullen, J.J., and Neale, P.J. 1994. Carbon uptake in a marine diatim during acute exposure to ultraviolet B radiation: Relative importance of damage and repair. J. Phycol. 30: 183-192.
Litchman, E., Neale, P.J., and Banaszak, A.T. 2002. Increased sensitivity to ultraviolet radiation in nitrogen-limited dinoflagellates: photoprotection and repair. Limnol. Oceanogr. 47: 86-94.
Neale, P.J., Banaszak, A.T., and Jarriel, C.R. 1998a. Ultraviolet sunscreens in dinoflagellates: Mycosporine-like amino acids protect against inhibition of photosynthesis. J. Phycology 34: 928-938.
Neale, P.J., Davis, R.F., Cullen, J.J. 1998b. Interactive effects of ozone depletion and vertical mixing on photosynthesis of Antarctic phytoplankton. Nature 392: 585-589.
Vincent, W.F., and Neale, P.J. 2000. Mechanisms of UV damage to aquatic organisms.
In: de Mora SJ, Demers S, Vernet M (eds) The effects of UV radiation on marine
ecosystems. Cambridge Univ. Press, Cambridge, p 149-176.