PhD Dissertation Defense - Sebastian Overmans

Nov 10 2020 10:00 AM - Nov 10 2020 11:00 AM

TITLE: Variability and Biological Effects of UV Exposure in the Red Sea and Oligotrophic Marine Ecosystems

PRESENTER: Sebastian Overmans

ADVISOR: Professor Susana Agusti

DATE: Tuesday, November 10, 2020

TIME: 10:00 am - 11:00 am


ABSTRACT: 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 a 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.

BIO: Originally from Germany, Sebastian trained as a chemical lab technician at Bayer and studied chemical engineering in his hometown before moving to the UK, where he graduated with a BSc in Biology from Bath University and an MSc in Marine Biology from the University of Essex. During his postgraduate studies in the UK, he investigated thermal stress responses of Indo-Pacific octocorals. Since 2016, Sebastian has been a PhD candidate at the Red Sea Research Center’s Biological Oceanography Lab under the supervision of Susana Agusti. His current research focuses predominantly on the role of ultraviolet radiation in marine systems, and the impacts of UV-B radiation on Red Sea biota, in particular.