Nitrogen metabolism
Ammonia assimilation genes decreased in relative abundance after heat stress exposure, indicating that less ammonia is being used in the biosynthesis of bacterial compounds by the microbiome (Fig. 3B). Coral-associated bacteria compete with Symbiodiniceae for host-generated ammonia, which is the preferred form of nitrogen of the symbiotic algae (Rädecker et al. 2015; Bourne et al. 2016). Lower ammonia assimilation by the coral SML microbiome under heat stress is potentially beneficial to the holobiont because more ammonia may be available to the algal symbiont to support photosynthesis and prevent coral bleaching.
Nitrosative stress genes allow bacteria to detoxify nitric oxide (NO) and reactive nitrogen species (RNS) involved in the denitrification process (Poole 2005). Nitrosative stress genes decreased in relative abundance under heat stress and denitrification and NO synthase genes did not change in proportion (Fig. 3B). Therefore, lower relative abundance of nitrosative stress genes could be a result of stability in the levels of NO and RNS in the coral SML microbiome.
Nitrogen incorporation genes, via allantoin utilization and nitrogen fixation, significantly increased in the coral SML post-heat stress (Fig. 3B). Allantoin is a urea-related compound produced by plants that can be a nitrogen source to bacteria and a form to transport fixed nitrogen to plants when nitrogen is limiting (Cobo-Díaz et al. 2015; Minami et al. 2016). Bacteria can provide about 11% of the nitrogen required by Symbiodiniaceae via nitrogen fixation in the coral holobiont (Bourne et al. 2016; Cardini et al. 2016). The significant increase in the proportions of allantoin utilization and nitrogen fixation genes could be a key beneficial service offered by the coral SML microbiome to supply nitrogen to the holobiont to withstand heat stress.