Effects of ultraviolet radiation at elevated temperature and light levels on the physiology and biogenic sulfur in the sea-ice diatom, Fragilariopsis cylindrus
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Stratospheric ozone depletion over Antarctica is expected to continue for approximately the next 50 years, resulting in higher fluxes of ultraviolet B (UVB) radiation (290-320 nm). UVB exerts oxidative and photochemical stress on photosynthetic organisms such as diatoms. The sea-ice diatom, <i>Fragilariopsis cylindrus</i>, a dominant member of the Antarctic marine ecosystem, plays an important role in the global carbon and sulfur cycles due to its production of dimethylsulfoniopropionte (DMSP). DMSP is believed to help mitigate cellular damage by its proposed function as an antioxidant within the cell. The goal of this study was to demonstrate the physiological effects that high UVB (HUVB) has on <i>F. cylindrus</i> under elevated light and temperature levels. Interactive effects of temperature and light level were explored as <i>F. cylindrus</i> was exposed to temperatures of 0°C or 4°C and light levels of 15 µE m<sup>-2</sup> s<sup>-1</sup> or 100 µE m<sup>-2</sup> s<sup>-1</sup>. Relative to control conditions of no UVB, growth and photosynthetic efficiency declined 40-145% and 50-90%, respectively, following T96 of HUVB exposure. Intracellular DMSP at HUVB was dependent on temperature only: at 0°C DMSP accumulated 105-150% relative to its control and at 4°C intracellular DMSP was 86-93% less than its control. Exposure to HUVB under low light and high temperature resulted in the least amount of photodamage and intracellular DMSP. Conversely, high light level had the greatest decrease in growth and photosynthetic efficiency accompanied with accumulation of intracellular DMSP at low temperature. Elevated temperature at both light levels alleviated the damage caused by HUVB exposure such that high light-low temperature induced the greatest amount of photoprotective mechanisms, highest amount of intracellular DMSP, and greatest degree of photodamage. The findings in this study elucidate the influences that light level, HUVB, and temperature have on <i>F. cylindrus</i> and mechanisms such as accumulation of intracellular DMSP that may be utilized by <i>F. cylindrus</i> under future ocean stratification conditions.