Oligotrophic (sub-)tropical oceans receive intense incident ultraviolet radiation (UV, 280–400 nm) and their water columns are highly transparent due to their nutrient-deficient state. This combination suggests a high potential for adverse effects on organisms, yet only few reports describe the UV exposures received in these waters and the associated impacts on marine biota. Here, we aimed to investigate the UV bio-optics of various open ocean locations and, using the Red Sea as a representative oligotrophic environment, we investigated the pattern of UV attenuation over a wide latitudinal range, quantified UV exposures in the water column, and determined impacts of UVB (280–320 nm) on indigenous phytoplankton and scleractinian corals. Globally, the lowest average downwelling diffuse attenuation coefficients (Kd) in the UV spectrum were recorded in the ultra-oligotrophic Indian Ocean Subtropical Gyre (Kd(313nm): 0.110 m-1) and South Pacific Gyre (Kd(313nm): 0.098 m-1), while aCDOM(λ) was ~1–2 orders of magnitude higher than ap(λ), In the Red Sea, UV attenuation mirrored the prevailing latitudinal gradient in nutrients, with the lowest and highest Kd(313) of 0.130 m-1 and 0.357 m-1 measured in the far north and in the south of the basin, respectively. Central Red Sea waters were most transparent to UV in late summer, i.e., a few weeks after incident irradiances and SSTs reach their annual maximum. Although, the projected increase of SST due to climate change means that extreme UV exposure and temperatures could coincide in the near future. This finding is of particular relevance since we found that Red Sea diatom species such as C. closterium are highly sensitive to UVB-induced photoinhibition and cell decay (LRD50: 11.4 kJ). Water temperature also governed the UVB sensitivity of Synechococcus sp., although this group exhibited a high resistance overall (LRD50: 57 kJ to non-detectable). For corals, we found that UVB-removal generally had little impact on the oxidative stress levels and photophysiology of S. pistillata and P. verrucosa from shallow waters, but considerably accelerated the acclimation of upward transplanted corals, which highlights that UVB is a crucial stressor that governs the photoacclimation capacity of Red Sea corals.
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