Nitrate release by Caribbean reef sponges1,2

Two coral reef sponges were examined in situ off Puerto Rico for fluxes of dissolved inorganic nitrogen. Chondrilla nucula , a ubiquitous Caribbean sponge with cyanobacterial symbionts, releases large amounts of nitrate [600 nmol N g ‒1 (dry wt) h ‒1 ; 4,000 µ mol N m ‒2 h ‒1 . Since C. nucula covers a mean of 12% of the substratum, it potentially contributes between 50 and 120% of the nitrogen required to sustain reef productivity. Anthosigrnella varians , a common sponge with zooxanthellae symbionts releases lesser amounts of nitrate [19 nmol N g ‒1 (dry wt) h ‒1 ; 20 µ mol N m ‒2 h Although A. varians may cover between 1.2 and 11% of the reef substratum, the potential contribution is <1% of the nitrogen required for reef productivity. These release rates for C. nucula are comparable to those reported for nitrogen fixation in reef environments and in excess of rates reported for nitrate production by other reef substrata. Large populations of non‐photosynthetic symbiotic bacteria are considered to nitrify amino nitrogen to nitrate and the differences are attributed to the size of bacterial populations and the nature of photosynthetic symbionts.