CARBON UPTAKE IN A MARINE DIATOM DURING ACUTE EXPOSURE TO ULTRAVIOLET-B RADIATION - RELATIVE IMPORTANCE OF DAMAGE AND REPAIR

Experiments on a marine diatom, Thalassiosira pseudonana (Hustedt) clo ne 3H, demonstrate that under moderate photon flux densities (75 mumol quanta.m-2.s-1) Of visible light the inhibition of photosynthesis by supplemental ultraviolet (UV) radiation (UV-B: 280-320 nm) is well des cribed as a hyperbolic function of UV-B irradiance for time scales of 0.5-4 h. Results are consistent with predictions of a recently develop ed model of photosynthesis under the influence of UV and visible irrad iance. Although net destruction of chlorophyll occurs during a 4-h exp osure to UV-B, and the effect is a function of exposure, the principal effect of UV-B is a decrease in chlorophyll-specific photosynthetic r ate. The dependence of photoinhibition on dosage rate, rather than cum ulative dose, and the hyperbolic shape of the relationship are consist ent with net photoinhibition being an equilibrium between damage and r epair. The ratio of damage to repair is estimated by a mathematical an alysis of the inhibition of photosynthesis during exposures to UV-B. A nitrate-limited culture was much more sensitive to UV-B than were the nutrient-replete cultures, but the kinetics of photoinhibition were s imilar. The analysis suggests that the nutrient-limited culture was mo re sensitive than the nutrient-replete cultures because repair or turn over of critical proteins associated with photosynthesis is inhibited. An inhibitor of chloroplast protein synthesis was used to suppress re pair processes. Photoinhibition by UV-B was enhanced, and inhibition w as a function of cumulative dose, as would be expected if damage were not countered by repair. The fundamental importance of repair processe s should be considered in the design of field experiments and models o f UV-B effects in the environment, especially in the context of vertic al mixing. Repair processes must also be considered whenever biologica l weighting functions are developed.