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.