Influence of UV-B radiation on photosynthesis near the sea-surface

Middle ultraviolet radiation (UV-B) is harmful to marine phytoplankton. This has been demonstrated repeatedly in the laboratory and in the field. In a typical field experiment, photosynthesis (uptake of [sup 14]C-biocarbonate) is reduced in samples exposed to the full range of environmental radiation as compared to samples from which UV-B has been screened. The nature of this inhibition of photosynthesis is not well documented. Specifically, it is not known how much of the effect is due to reduction of photosynthetic capacity and how much is due to destruction of photosynthetic pigment or disruption of sensitive cells. Experiments were performed to help resolve unanswered questions about the effects of UV-B on photosynthesis of phytoplankton near the sea-surface. Samples from the sea-surface microlayer (SSM), upper 10cm, and greater depths were exposed to different irradiance regimes for several hours and analyzed for changes in in vivo fluorescence, chlorophyll a concentration, and photosynthesis vs irradiance. Consistent with previous studies, exposure to environmental UV-B resulted in reduced rates of photosynthesis in confined samples. Samples from the SSM were less susceptible to solar irradiance that were those from the upper 10cm. Deeper samples were the most sensitive. The principal reason for reduced photosynthesis was destruction ofmore » chlorophyll, not inhibition of photosynthesis per unit chlorophyll. These results suggest that the increased resistance of near-surface populations to UV-B might be in part due to the fact that at the time of sampling, sensitive cells have already been damaged and their photosynthetic pigment destroyed. If this is true, simple measurements of chlorophyll concentration might be useful in assessing the effects of UV-B on phytoplankton.« less