Sulfur assimilation in corals with aposymbiotic and symbiotic zooxanthellae

Although sulfate ions are the main form of sulfur in the ocean, there is limited knowledge on their use by living organisms. Stable isotope labelling and NanoSIMS analysis were used in this study to clarify how sulfate, in seawater, is assimilated by corals and zooxanthellae at the cellular level. Aposymbiotic and symbiotic coral juveniles from the genus Acropora were incubated for 2 days in filtered seawater with ³⁴ S-labelled sulfate. Further, the labelled corals were incubated for additional 2 days in natural seawater. Mapping of sulfur isotopes (³⁴ S/³² S) showed that the 'hotspots' were enriched in ³⁴ S on a sub-micro level and were heterogeneously distributed in the coral soft tissues. Specifically, ³⁴ S hotspots were found in both the symbiotic zooxanthellae and coral host tissues. In aposymbiotic corals, ³⁴ S was detected in the tissues, indicating that the host corals directly assimilated the sulfate ions without any aid from the zooxanthellae. Even after 2 days in normal seawater, the ³⁴ S label was clearly seen in both symbiotic and aposymbiotic corals, indicating that the assimilated sulfur was retained for at least 2 days.