Sulfur metabolism
The relative abundances of bacterial genes related to glutathione
utilization, sulfur oxidation and taurine oxidation, increased after
heat stress in the coral SML microbiome (Fig. 3C). Glutathione is a key
sulfur-based compound used by bacteria for protection against oxidative
stress and by the coral holobiont as a source of organic sulfur (Masip
et al. 2006; Wegley et al. 2007; Bourne et al. 2016). Taurine oxidation
could be coupled to sulfur oxidation to increase sulfur availability in
the coral holobiont. The degradation of the amino acid taurine can
produce thiosulfate, which is converted to sulfate via sulfur oxidation
(Robbins et al. 2019, Lima et al. 2022). Sulfur oxidation genes increase
in relative abundance under stress and bleaching (Wegley et al. 2007;
Littman et al. 2011). Taurine oxidation by Ruegeria was coupled
to high primary productivity by planktonic dinoflagellates and plays a
key role in the organic sulfur turnover in pelagic environments (Landa
et al. 2019). Here, Ruegeria could be playing a similar role by
increasing sulfur availability via taurine oxidation to the
dinoflagellates Symbiodiniaceae in the coral holobiont under
stress.