Background/Aims
Elasmobranchs inhabit a vast range of aquatic environments and play important roles in structuring food webs, cycling nutrients, and regulating prey populations. Nevertheless, elasmobranchs are currently facing a global extinction crisis, with a growing number of species threatened with extinction. In the face of this crisis, North-western Australia represents a global hotspot for elasmobranch species richness and an important refuge for a number of threatened elasmobranch species. For these species, there remains a great deal of uncertainty regarding their trophic ecology and, in particular, their gastrointestinal microbiomes.
Marine elasmobranchs maintain osmotic balance with seawater by accumulating high concentrations of urea and, consequently, trimethylamine N-oxide (TMAO) to stabilise proteins in the high-urea environment. This reliance on osmolytes imposes an elevated nitrogen demand, resulting in marine elasmobranchs becoming nitrogen-limited. Emerging research suggests the gut microbiome plays a direct role in this nitrogen handling, recycling urea secreted into the chyme during feeding into ammonia, improving host nitrogen reabsorption and retention.
Methods
By obtaining cloacal swabs from threatened species, including the critically endangered green sawfish (Pristis zijsron) and freshwater sawfish (P. pristis), genetic metabarcoding was used to characterise host diet and assess the diversity and inferred functional capacity of their gastrointestinal microbiomes across marine and freshwater habitats.
Results
The microbiomes of green sawfish and freshwater sawfish exhibited largely similar functional profiles despite their contrasting habitats. However, compared to other marine elasmobranchs (e.g., Rhynchobatus australiae, Carcharhinus cautus, Negaprion acutidens), the microbiome of the freshwater sawfish displayed significantly lower capacity for TMAO breakdown, arginase deiminase, and urease. Ongoing analyses of dietary composition will further reveal how prey assemblages structure these microbial communities and influence nitrogen cycling in elasmobranch hosts.
Conclusion
The conserved functional microbiome of the green sawfish and the freshwater sawfish highlight the joint importance of host-specific and habitat-specific drivers in shaping microbiome diversity and function.