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 delta C-13 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 C-28 Delta(5,22), 23,24-Me C-29 Delta(5,22), and dinosterol are characteristic of P. lobata; while, C-29 Delta(5), gorgosterol, and 4 alpha-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 parts per thousand) and visibly variable (- 24.1 to - 17.3 parts per thousand) delta C-13 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 delta C-13 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 5 alpha-stanols suggestive of anaerobic conversion on Delta(5)-sterols implies complex steroid interactions within coral-algae-microbe holobionts.