Dissolved organic matter uptake balances carbon budgets of Red Sea bivalves and ascidians

Benthic suspension feeders have long been thought to depend solely on particulate organic matter (POM) for their nutritional needs. Our research, conducted in the Gulf of Aqaba (Red Sea), shows that the available concentration of planktonic carbon in the reef water is too low to meet the metabolic demands (measured through in situ oxygen uptake) of common bivalves (Spondylus sp. and Chama pacifica) and ascidians (Polycarpa mytiligera and Halocynthia spinosa). In situ measurements confirmed a concentration-dependent uptake of bulk dissolved organic carbon (DOC) and protein-like dissolved organic compounds (fDOM, analyzed via spectrofluorometry) by Spondylus sp. and P. mytiligera, suggesting that DOC uptake may balance their carbon budgets. Stable isotope (δ¹³C) mixing models indicate that reef-derived DOC (mean δ¹³C = -18.4 ± 0.7‰) emerges as the primary carbon source for bivalves (57% of their body composition), far surpassing contributions from particulate sources (26% from POC > 100 µm and 14% from POC < 100 µm). While ascidians still rely primarily on POM (60 to 71%), DOC plays an essential role as a supplementary food source (23 to 29%). These results challenge the prevailing assumption that POM is the exclusive food source for these organisms. Given the widespread presence of bivalves and ascidians in benthic habitats, our findings highlight their ability to bypass the microbial loop and directly utilize dissolved organic compounds, prompting a reevaluation of current benthic carbon budgets.