Sponge-mediated nitrification in tropical benthic communities

We examined changes In the levels of dissolved inorganic nitrogen (DIN) during incubation experiments with 4 conspicuous sponge species from Caribbean coral reefs, mangroves, or seagrass beds (Chondrilla nucula, ?Pseudaxinella zeai, Oligoceras violacea, Plakortis halichondroides). DIN accumulation in the incubation water was detected for all the species, but no significant DIN concentration changes were detected in the control experiments (seawater alone). NO< accumulated during all the experiments with 0. rfiolacea (170 to 580 nmol g.' h-'), while NO-, accumulated during most experiments with ?P. zeal (0 to 1033 nmol g.' h-'), C. nucula (360 to 2650 nmol g.' h-'), and P. hallchondroides (0 to 320 nmol g.' h' ) . These are the highest reported weight-spec~fic production rates of oxidized nitrogen fl-om benthic communitics The highest values are associated with the 3 species that possess cyanobacterlal endosymbionts. Potential NO1 efflux rates by 2 of the species, assuming 10Of"> area1 coverage, yielded values (21 1 to 396 mm01 m ' d.' for ?P. zeal and 242 to 413 mm01 m-' d ' for C. nucula) 2 to 4 orders of magnitude hlgher than the most active benthic nitnfication rates yct reported from the tropics. Extrapolating from incubation data (550 to 1030 nmol g' h-' and biomass estimates (440 g m-2) , the environmental NO.: efflux rate of ?P zeai on the Fore Reef at the Barrier Reef off Carrie Bow Cay. Bel17t. 15.8 to 10.9 mm01 m-2 d-!) surpasses considerably the highest benthic nitrification rates reported previously (unconsolidated reef sediments: 1.68 mm01 m-' d l ) . These results strongly suggest that sponge-mediated nitrification is not uncommon in tropical marine benthic communities, and might constitute a large input of oxidized nitrogen into those habitats in which sponges abound. Our results reinforce the notion that sponges harbor and nourish microbial organisms with metabolisms that are important to the productivity and nutrlent cycling in shallow benthic tropical communities. K E Y WORDS Nitrification Sponge associations . DIN fluxes Benthic processes