Abstract
Corals are associated with dinitrogen (N2)-fixing bacteria that potentially represent an additional nitrogen (N) source for the coral holobiont in oligotrophic reef environments. Nevertheless, the few studies investigating the assimilation of diazotrophically derived nitrogen (DDN) by tropical corals are limited to a single scleractinian species (i.e., Stylophora pistillata). The present study quantified DDN assimilation rates in four scleractinian and three soft coral species from the shallow waters of the oligotrophic Northern Red Sea using the 15N2 tracer technique. All scleractinian species significantly stimulated N2 fixation in the coral-surrounding seawater (and mucus) and assimilated DDN into their tissue. Interestingly, N2 fixation was not detected in the tissue and surrounding seawater of soft corals, despite the fact that soft corals were able to take up DDN from a culture of free-living diazotrophs. Soft coral mucus likely represents an unfavorable habitat for the colonization and activity of diazotrophs as it contains a low amount of particulate organic matter, with a relatively high N content, compared to the mucus of scleractinian corals. In addition, it is known to present antimicrobial properties. Overall, this study suggests that DDN assimilation into coral tissues depends on the presence of active diazotrophs in the coral’s mucus layer and/or surrounding seawater. Since N is often a limiting nutrient for primary productivity in oligotrophic reef waters, the divergent capacity of scleractinian and soft corals to promote N2 fixation may have implications for N availability and reef biogeochemistry in scleractinian versus soft coral-dominated reefs.
Highlights
Coral reefs are highly productive ecosystems despite thriving in oligotrophic waters, which contain low levels of essential dissolved inorganic nutrients such as nitrogen (N) (de Goeij et al, 2013)
The use of labeled 15N2 gas measures derived nitrogen (DDN) assimilation and distinguishes between the different compartments, but it is still poorly known to what extent and under which conditions corals profit from DDN
Four nubbins of each coral species per C. watsonii concentration were individually placed in 320-ml gas-tight bottles completely filled with 280 ml of 0.22 μm-filtered seawater amended with the adequate concentration of diazotrophs and 40 ml of 15N2 enriched seawater
Summary
Coral reefs are highly productive ecosystems despite thriving in oligotrophic waters, which contain low levels of essential dissolved inorganic nutrients such as nitrogen (N) (de Goeij et al, 2013). Corals, which are the main reef builders, can achieve high productivity thanks to their association with endosymbiotic dinoflagellates, which recycle the animal waste products and are efficient in Dinitrogen Fixation in Corals scavenging inorganic N dissolved in seawater (Wang and Douglas, 1999; Grover et al, 2003). Corals profit from high rates of N recycling in the water column and sediments via microbial processes (Carpenter and Capone, 2008), and from biological dinitrogen (N2) fixation of diazotrophic symbionts (Bednarz et al, 2017), which are heterotrophic bacteria and cyanobacteria that convert atmospheric N2 into bioavailable N. Many studies using the acetylene reduction assay (ARA) method have recorded high rates of N2 fixation either in reef waters or in the presence of corals (i.e., Bednarz et al, 2015; Rädecker et al, 2015; Cardini et al, 2016). The use of labeled 15N2 gas measures DDN assimilation (net N2 fixation) and distinguishes between the different compartments (i.e., seawater particles, tissue, dinoflagellate symbionts), but it is still poorly known to what extent and under which conditions corals profit from DDN
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