AbstractsBiology & Animal Science

Tropical coral reefs are among the most productive ecosystems on this planet, despite being surrounded by very oligotrophic waters. Effective recycling processes of the limiting nutrients, particularly nitrogen (N), and input of new bioavailable N via dinitrogen (N2) fixation are essential to sustain such high gross primary production. In fact, several benthic reef organisms and substrates are associated with diverse communities of N2 fixing microbes (diazotrophs), but the respective contribution of the different benthic groups to total benthic N2 fixation and the effect of changing environmental conditions on N2 fixation have not been investigated yet. Therefore, this thesis, through a series of interconnected studies carried out in a seasonally dynamic coral reef system, the Northern Red Sea, and in a number of manipulative experiments, contributes to the understanding of benthic N2 fixation in coral reefs by answering the following key questions: 1) How much N2 is fixed by the dominant benthic groups in the Northern Red Sea? 2) What is the relative contribution of the benthic groups to total benthic N2 fixation within the reef? 3) What is the effect of seasonally changing environmental conditions and of single environmental factors on key metabolic processes, particularly N2 fixation, associated with the different benthic groups? Findings revealed that all investigated benthic groups showed N2 fixation activity, whereof bare coral rock, turf algae, carbonate sands and living hard corals were the main N2 fixing components contributing ~ 90% to benthic N2 fixation in investigated Red Sea coral reefs. Soft corals revealed the lowest N2 fixation activity among all investigated groups and released significantly less organic matter to the surrounding water compared to hard corals indicating that N2 fixation may also influence their role as allogenic ecosystem engineers. N2 fixation by most benthic groups was usually highest when nutrient availability was lowest, and water temperature as well as light intensity highest suggesting that the N2 fixation products fuel the metabolic N requirements of reef organisms, particularly during nutrient-depleted conditions (i.e. summer). The manipulation of single environmental factors revealed a stimulation of N2 fixation activities under global warming conditions and a reduction under ocean acidification scenarios indicating that global climate change will affect the nutrient status of reef organisms. In summary, this thesis underlines the ubiquity of N2 fixation associated with different benthic coral reef organisms and substrates, and highlights its importance in sustaining coral reef productivity. Both, the benthic community structure and the prevailing environmental conditions appear to be important in controlling the amount of N2 fixation in coral reefs. Finally, as coral reefs are increasingly and simultaneously exposed to global and local stressors, it is necessary to understand how important metabolic processes on coral reefs (i.e. organic matter fluxes and N2 fixation…