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.