How is Dissolved Organic Matter from Different Sources Partitioned in Tropical Sponge Holobionts? Phospholipid Derived Fatty Acid Analysis of Sponge Tissue and Associated Bacterial Communities
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Tropical reef ecosystems worldwide are experiencing phase shifts, where macroalgae and sponges are overtaking coral populations. Increasing ocean temperatures, acidity and nutrient loads are in part responsible for this disruption, but there is a prevalent lack of knowledge of the mechanisms behind the phase shifts, particularly on sponge population growth. To further increase our understanding of the role of sponges in tropical reef ecosystems, we performed a stable isotope tracer experiment to observe the path of 13C labelled dissolved organic matter (DOM) through sponge holobionts. Using 3 high- and 3 low microbial abundance (HMA & LMA) sponge species, we attempted to determine the relative quantities of carbon isotopes being partitioned into bacterial versus sponge phospholipid-derived fatty acids (PLFAs) as a measure of nutrient flow and assimilation. We tested for preferential assimilation of DOM from macroalgae-, coral- and diatom-derived exudates to infer how current reef phase shift trajectories would influence future reef community structures. Our results show that LMA sponges appear to assimilate DOM into sponge-specific PLFAs at a faster rate than HMA sponges, but that an average of 46% of all carbon assimilation took place in bacteria compared to only 2.7% in sponge-specific tissue. There is an overall preferential assimilation of macroalgal-derived DOM between all 6 species, but LMA sponges show a higher rate of diatom DOM assimilation into sponge-specific PLFAs, while coral DOM was incorporated at the lowest rate overall. These findings support previous research on macroalgal population increases fuelling the proliferation of sponges, and that bacteria are the primary processors of DOM in a typical sponge holobiont. In the future, the continued preferential assimilation of macroalgal-derived DOM will see a growth increase of sponges and algae in tandem, whilst exacerbating coral population declines.