Physiological Studies On The Responses Of The Diatom Skeletonema Costatum To Solar UV Radiation

Skeletonema costatum is not only one of major causative organisms of red tide blooms but an economic species as a food organism in marine aquaculture, It is very significative to investigate the physicological responses of S. costatum to solar ultraviolet radiation. However, little is known about the effects of UVR on carbon fixation of diatom and the co-effects of zinc and UVR. Therefore, S. costatum is studied to answer the questions and the main results are as follows:CO₂ supply for photosynthesis is facilitated by extracellular (periplasmic) carbonic anhydrase (CAext) in marine phytoplankton species with it. Whether CAext activity can be influenced by UVR remains to be asertain. We have now shown that the CAext activity of S. costatum was stimulated when treated with UV-A (320-400 run) or UV-A+UV-B (280-320 nm) in addition to visible light. The presence of UV-A and UV-B, respectively, enhanced the activity by 28% and 24% at the low irradiance (PAR 175, UV-A 40, UV-B 1.3 W m^-2) and by 21% and 19% at the high irradiance (PAR 356, UV-A 82, UV-B 2.6 W m^-2) level for 1h. Higher UV dose for 2h exposure at the high irradiance level reversed the stimulation to inhibition of the CAext activity. Both UV-A and UV-B enhanced the redox activity at the plasma membrane, which coincided with the stimulated CAext activity. In addition, low UVR stimulated the synthesis of CAext.Previous studies have suggested that Zn availability in seawater may limit oceanic primary production by affecting CO₂ acquisition processes for photosynthesis and therefore influencing the global carbon cycle, It remains to be ascertained whether Zn availability and UVR exposure act synergistically, antagonistically or independently on oceanic primary production. With the intention of finding an answer to this question, we cultured the ecologically relevant diatom S. costatum under different radiation treatments with or without UVR, at three Zn concentrations. Specific growth rate (μ), photosynthetic carbon assimilation and extracellular carbonic anhydrase activity increased with increasing concentrations of Zn²⁺ from 0 to 10 pmol L^-1, irrespective of the radiation treatment. An approximately linear relationship betweenμ, and daily dose of PAR was observed at all Zn concentrations. However, when UVR was supplied to the cells in addition to PAR, the linearity in the relation between growth and daily dose disappeared. UV-A stimulated the growth on cloudy days and became inhibitory on sunny days. However, UV-B almost invariably exerted an inhibitory effect on both growth and photosynthesis. The inhibitory effect of UVR was inversely related to Zn²⁺ concentrations. Zinc limitation, at least in S. costatum, thus exacerbates sensitivity to UV radiation.