Steroid markers for species-specific Symbiodiniaceae: insights from molecular and δ13C measurements on four scleractinian corals

Endosymbiotic dinoflagellates in the family Symbiodiniaceae are physiologically and ecologically important for reef-building corals. Certain symbiotic lineages may provide adaptive functions that aid corals in coping with and surviving rapid environmental and climate changes; and thus, there is a growing interest in exploring Symbiodiniaceae communities and their associated roles. Steroid biomarkers have been proposed as a means of identifying species-specific symbionts, but their reliability has yet to be examined through comparison of different coral species that harbor known Symbiodiniaceae. Here, steroid molecular and δ¹³C analyses are applied to four stony corals that host identified (Porites lobata and Galaxea fascicularis) or undescribed (Goniopora tenuidens and Goniastrea minuta) Symbiodiniaceae to address this issue. The 23-Me C₂₈Δ^5,22, 23,24-Me C₂₉Δ^5,22, and dinosterol are characteristic of P. lobata; while, C₂₉Δ⁵, gorgosterol, and 4α-methylgorgostanol are diagnostic for G. fascicularis, tracking the marked differences in their Symbiodiniaceae assemblages. These two steroid groups also exhibit largely similar (− 20.6 to − 17.0‰) and visibly variable (− 24.1 to − 17.3‰) δ¹³C signatures, following the low and high diversity of symbionts in P. lobata and G. fascicularis, respectively. Our present results, together with previous reports, clearly demonstrate the effectiveness of steroid molecular and δ¹³C features in depicting Symbiodiniaceae population, providing a useful complement to Symbiodiniaceae genetic analysis. A further comparison of steroid characteristics between unclear and known Symbiodiniaceae in our four corals reveals the almost identical symbiotic population between G. minuta and G. fascicularis and a restricted range of symbiotic composition in G. tenuidens that is shared with G. fascicularis (and G. minuta). Additionally, the occurrence of 5α-stanols suggestive of anaerobic conversion on Δ⁵-sterols implies complex steroid interactions within coral–algae–microbe holobionts.