Nitrogen cycling associated with corals and other reef organisms under environmental change

Abstract

Nitrogen (N) is a limiting nutrient in highly productive tropical coral reefs, despite its key role for primary production. This requires efficient (re)cycling of N by the dwelling organisms, including the key reef ecosystem engineers, the hard corals. As such, corals evolved symbiotic relationships with eukaryotic and prokaryotic microbes, together called a holobiont, which aid in nutrient acquisition and recycling. The nutrient exchange symbiosis between the coral host and the eukaryotic photosynthetic dinoflagellates of the family Symbiodiniaceae has given corals an ecological advantage over other functional groups such as algae. The Symbiodiniaceae provide the coral host with carbon (C) rich photosynthates, while in return, the Symbiodiniaceae receive N and phosphorus (P). Additionally, diazotrophs, microbes capable of fixing atmospheric dinitrogen (N2), can provide the coral holobiont with bioavailable N. Coral holobionts benefit from low internal availability of N as N-limitation may maintain steady translocation of the photosynthates on which the corals rely. Thus, coral holobionts may be particularly susceptible to increases in (environmental) dissolved inorganic N (DIN) due to e.g. anthropogenic input, or stimulated activity of diazotrophs. As such, corals likely have mechanisms in place for the alleviation of excess N, i.e. denitrification, which may ultimately aid coral functioning. This thesis aims at extending the current knowledge on biogeochemical cycling of N associated with coral holobionts. Specifically, in addition to N2 fixation, we tested whether the antagonistic N-cycling pathway to N2 fixation, i.e. denitrification, is an active pathway in coral holobionts and whether it is affected by environmental change. In addition, we measured N-cycling pathways associated with other coral reef organisms and substrates under environmental change. This allowed us to make inferences for coral reef functioning when exposed to global and local stressors. We applied a combination of physiological and molecular analyses and used the strong seasonality of the northern and central Red Sea as a natural laboratory. Our findings reveal that denitrification was actively associated with all investigated coral species. Similar to diazotrophy, denitrification may thus be ubiquitously associated with coral holobionts. Under stable environmental conditions, denitrification and N2 fixation aligned and both N-cycling pathways correlated with Symbiodiniaceae cell densities. Thus, the relationship between denitrification and N2 fixation may be the result of a shared organic C limitation (by translocated photosynthates from the Symbiodiniaceae) within the holobiont. Higher seasonal availability of DIN (leading to higher DIN:dissolved inorganic P [DIP] ratios) dynamically shifted the ratio of denitrifiers and diazotrophs, in favour of the denitrifiers. The proliferation of Symbiodiniaceae suggests incomplete alleviation of excess N by denitrification. Indeed, Symbiodiniaceae cell densities also correlated with environmental DIN availability. In response to moderate in situ eutrophication of DIN and DIP, both N-cycling pathways more than doubled in activity. Surprisingly, the Symbiodiniaceae populations remained stable. In addition, there was no significant incorporation of N originating from the eutrophication event in the Symbiodiniaceae. This suggests that N-limitation was maintained, likely assisted by denitrification. These findings suggest that the dynamic interplay of denitrification and N2 fixation may regulate Symbiodiniaceae populations, but the extent to which they maintain N-limitation may depend on the environmental availability of DIN and DIP. By comparing coral holobiont associated N-cycling to other functional groups on coral reefs, we postulate that under local and global stress scenarios, coral holobionts may lose the competition for space to algae as they 1) can strongly capitalize on (anthropogenic) nutrient inputs, 2) have high associated N2 fixation rates that increase in response to ocean warming and moderate N/P eutrophication, and/or 3) have low associated denitrification. Turf algae and coral rubble exhibited ∼100-fold higher N2 fixation rates compared to hard corals. Contrastingly, denitrification rates were as low as those associated with hard corals. Therefore, coral reefs in the process of shifting towards algae dominance may get caught in a positive feedback loop where dead coral (coral rubble) is rapidly overgrown by algae which in return naturally provide the reef with bioavailable N. This may facilitate higher growth rates of reef algae. Collectively, the results described in this thesis suggest that the interplay of N2 fixation and denitrification associated with coral holobionts may indeed aid in coral functioning by maintaining healthy populations of Symbiodiniaceae. Increased activity of diazotrophs induced by thermal stress, both associated with the coral holobiont and other dwelling organisms, as well as eutrophication of N may ultimately shift the coral holobionts’ internal N:P ratios towards P limitation as denitrifiers may be unable to alleviate excess N. Thus, future management efforts should focus strongly on the local prevention of N eutrophication and the mitigation of global warming.