Nitrogen fixation in the mucus of Red Sea corals

Abstract

Scleractinian corals are essential constituents of tropical reef ecological diversity. They live in close association with diazotrophs [dinitrogen (N 2 )-fixing microbes], which can fix high rates of N 2 . Whether corals benefit from this extrinsic nitrogen source is still under debate. Until now, N 2 fixation rates have been indirectly estimated using the acetylene reduction assay, which does not permit assessment of the amount of nitrogen incorporated into the different compartments of the coral holobiont. In the present study, the 15 N 2 technique was applied for the first time on three Red Sea coral species. Significant 15 N enrichment was measured in particles released by corals to the surrounding seawater. N 2 fixation rates were species specific and as high as 1.6-2 ng N day. However, no significant enrichment was measured in the symbiotic dinoflagellates or the coral host tissues, suggesting that corals do not benefit from diazotrophic N 2 fixation.

Diazotrophs: Overlooked Key Players within the Coral Symbiosis and Tropical Reef Ecosystems?

“…This suggests that a combination of phosphorus and carbon-rich photosynthates are necessary to enhance N 2 fixation. In the same line, measurements performed with or without coral colonies also showed that corals can enhance N 2 fixation (Grover et al, 2014). This is due to the fact that corals form specific associations with diazotrophs (see Section Diversity of Diazotrophs in Tropical Corals), and they also release large amounts of mucus that constitutes an energetic substrate enhancing the growth of diazotrophs and N 2 fixation rates.…”

“…Function-diversity approaches are needed to elucidate which diazotrophs are actively performing N 2 fixation in corals and in which condition this newly fixed nitrogen provides profits to the corals. Indeed, while a recent study using 15 N 2 showed that DDN is transferred to the zooxanthellae of corals in New Caledonia, another study (Grover et al, 2014) did not find any DDN incorporation in the symbionts or in the tissue of corals in the Red Sea, despite N 2 fixation took place in the surrounding seawater.…”

“…So far only few studies have provided evidence of the assimilation of DDN by corals either using the natural δ 15 N signature of corals or the 15 N 2 labeling technique (Lesser et al, 2007;Grover et al, 2014;Benavides et al, 2016;Bednarz et al, 2017). Grover et al (2014) did not detect DDN assimilation by the coral, while the other studies did, underlining that DDN assimilation by corals is not a straightforward process but can be very variable depending on the current environmental conditions and the associated diazotrophic community.…”

“…On the other hand, corals like Montastrea cavernosa can be observed with or without some diazotroph (cyanobacteria) symbionts (Lesser et al, 2004), which raises the question whether diazotrophs are only acquired by corals upon nitrogen stress situations. In addition, corals do not always profit from DDN, especially shallow corals which receive sufficient dissolved inorganic nitrogen from the ambient seawater (Grover et al, 2014;Bednarz et al, 2017). In summary, these observations raise the question whether diazotrophs are real coral symbionts or if they are only acquired upon nitrogen stress situations.…”

“…This miscalculation has been shown to result in a severe underestimation of pelagic N 2 fixation rates (Großkopf et al, 2012), and can be also a problem when estimating coral-associated N 2 fixation (Grover et al, 2014;Benavides et al, 2016). To avoid this, Mohr et al (2010) proposed pre-dissolving 15 N 2 into seawater and hence adding the tracer in its liquid form to samples prior to their incubation, with the aim of maintaining the 15 N enrichment of the source N 2 pool constant overtime.…”