Testing the relationship between microbiome composition and flux of carbon and nutrients in Caribbean coral reef sponges.

Background: Sponges are important suspension-feeding members of reef communities, with the collective capacity to overturn the entire water column on shallow Caribbean reefs every day. The sponge-loop hypothesis suggests that sponges take up dissolved organic carbon (DOC) and, via assimilation and shedding of cells, return carbon to the reef ecosystem as particulate organic carbon (POC). Sponges host complex microbial communities within their tissues that may play a role in carbon and nutrient cycling within the sponge holobiont. To investigate this relationship, we paired microbial community characterization (16S rRNA analysis, Illumina Mi-Seq platform) with carbon (DOC, POC) and nutrient (PO, NO, NH) flux data (specific filtration rate) for 10 common Caribbean sponge species at two distant sites (Florida Keys vs. Belize, ~ 1203 km apart).

Results: Distance-based linear modeling revealed weak relationships overall between symbiont structure and carbon and nutrient flux, suggesting that the observed differences in POC, DOC, PO, and NO flux among sponges are not caused by variations in the composition of symbiont communities. In contrast, significant correlations between symbiont structure and NH flux occurred consistently across the dataset. Further, several individual symbiont taxa (OTUs) exhibited relative abundances that correlated with NH flux, including one OTU affiliated with the ammonia-oxidizing genus Cenarchaeum.

Conclusions: Combined, these results indicate that microbiome structure is uncoupled from sponge carbon cycling and does not explain variation in DOC uptake among Caribbean coral reef sponges. Accordingly, differential DOC assimilation by sponge cells or stable microbiome components may ultimately drive carbon flux in the sponge holobiont.