AbstractsBiology & Animal Science

Biological dinitrogen (N2) fixation exerts important control on the oceanâ s nitrogen (N) inventory and primary productivity. Among marine ecosystems, coral reefs are notable for their very high productivity in extremely oligotrophic waters, where N2 fixation has long been thought to satisfy much of the demand for new N. Furthermore, N2 fixation in coral reef ecosystems may importantly contribute to global marine N inputs. However, because of large uncertainty in (i) the distribution of N2 fixation in coral reefs and (ii) the tightness of the coupling between N2 fixation and primary production, the ecological significance of N2 fixation in coral reef ecosystems is still not resolved. The core of this thesis describes and discusses three different aspects of N2 fixation in coral reefs. The first part of the thesis, through a series of interconnected studies, focuses on the relationship between N2 fixation and primary productivity in the water column and in key benthic organisms at a fringing coral reef in the northern Red Sea (Gulf of Aqaba). This site harbours some of the northernmost warm water coral reefs on Earth. The characteristic seasonality of the sampling location allowed an investigation of the contribution of N2 fixing prokaryotes (diazotrophs) to changing N requirements of the primary producers in response to pronounced seasonal environmental variations. Findings reveal that the reef water and its planktonic community contributed only marginally to the input of fixed N into the reef ecosystem. Concurrently, benthic N2 fixation was conspicuous, and the rule rather than the exception in all benthic substrates investigated. Rates of N2 fixation varied greatly between different substrates. Microbial mats, turf algae, â bareâ hard substrates, and â bareâ sediment contributed most fixed N to the reef, while soft corals showed the lowest N2 fixation activity. N2 fixation rates were significantly higher in summer, when nutrient availability was lowest and water temperature and light intensity highest. This indicates an important role of diazotrophy in sustaining the N demand of reef primary producers during the nutrient-depleted season. Furthermore, the susceptibility of diazotrophs to the varying environmental conditions suggests that disturbances resulting from human activities will also likely determine changes in the extent of N2 fixation on reefs. In this context, in the second part of this thesis, results of two different manipulative experiments are presented that showed that ocean warming and ocean acidification have a high potential to impact N2 fixation in scleractinian corals. This part of the thesis focused on scleractinian coral holobionts (the cnidarian-dinoflagellate-diazotroph symbiosis) because of their role as primary reef ecosystem engineers. Results demonstrate that thermal stress impacted all holobiont members and suggest that physiological plasticity of coral-associated diazotrophs, by fixing additional N under high temperature, may help to maintain constant chlorophyll a levels and prevent…