Effects of ultraviolet-B radiation on the productivity and pigmentation of Gracilaria tikvahiae (Rhodophyta)

The effects of ultraviolet-B radiation (280 to 320 nm) on the photosynthetic capacity and light-harvesting pigment levels of Gracilaria tikvahiae, an important primary producer in many eastern North American coastal ecosystems, were determined under carefully defined conditions. Effects on productivity and chlorophyll and phycoerythrin concentrations of G. tikvahiae were measured on UV-B- and non-UV-Birradiated algae over 96 h. Three UV-B dose rate treatments were conducted: low = < 0.18 W m^-2, medium = < 0.43 W m^-2 and high = 0.43 - 0.79 W m^-2.

During the first 12 h of the medium UV-B dose rate treatment, UV-B exposure stimulated productivity by 66% from the mean control productivity of 2.16 mg C (g dry wt)^-1 h^-1. At the same time, light-harvesting pigment concentrations were increased. During the first 24 h, UV-B exposure increased chlorophyll concentrations by 38% from the mean control concentration of 1.85 mg (g dry wt)^-1 and increased phycoerythrin concentrations by 50% from the mean control concentration of 29.78 mg (g dry wt)^-1 '. There were no effects on productivity or pigment concentrations at low doses (dose rate x exposure time) in the low or high UV-B dose rate treatments which may indicate the existence of a threshold dose rate for beneficial effects of UV-B.

UV-B radiation decreased primary production in all treatments, but only after a 12-h lag period in the low and high UV-B dose rate treatments and after the 12-h period of increased productivity in the medium UV-B dose rate treatment. Since increasing the UV-B dose rate did not accelerate the occurrence of decreased productivity, it appeared that UV-B damage had to accumulate for 12 h before producing damaging effects. High doses of UV-B also decreased chlorophyll and phycoerythrin concentrations and increasing the UV-B dose rate accelerated the occurrence of the decreased pigment concentrations. In the low UV-B dose rate treatment, there were no effects on pigment concentrations by 72 h, while in the high UV-B dose rote treatment, UVB exposure decreased chlorophyll concentrations by 72 h and phycoerythrin concentrations by 48 h (p < 0.05). Since the pigment concentration decreases occurred more gradually than the observed productivity decreases, initial productivity decreases were probably the result of UV-B effects on components of the photosynthetic system other than the light-harvesting pigments.

Statistically significant decreases in phycoerythrin concentration in UV-B-irradiated G. tikvahiae coincided with color change in the algae from purple to yellow-green. The yellow-green color, decreased pigment concentrations and decreased productivity in the UV-B-irradiated algae were maintained throughout the experiments and were not reversed by removal from the UV-B radiation and exposure to 48 h of visible light. The results indicate the potential for damage to these components of coastal ecosystems by elevated UV-B radiation and the need for on accurate assessment of ultraviolet radiation effects on nearshore systems.