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 zeal, 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(2)(-)accumulated during all the experiments with O. violacea (170 to 580 nmol g(-1) h(-1)), while NO(3)(-)accumulated during most experiments with ?P. zeal (0 to 1033 nmol g(-1) h(-1)), C. nucula (360 to 2650 nmol g(-1) h(-1)), and P. halichondroides (0 to 320 nmol g(-1) h(-1)). These are the highest reported weight-specific production rates of oxidized nitrogen from benthic communities. The highest values are associated with the 3 species that possess cyanobacterial endosymbionts. Potential NO3- efflux rates by 2 of the species, assuming 100% areal coverage, yielded values (211 to 396 mmol m(-2) d(-1) for ?P. zeal and 242 to 413 mmol m(-2) d(-1) for C. nucula) 2 to 4 orders of magnitude higher than the most active benthic nitrification rates yet reported from the tropics. Extrapolating from incubation data (550 to 1030 nmol g(-1) h(-1) and biomass estimates (440 g m(-2)), the environmental NO(3)(-)efflux rate of ?P. zeal on the Fore Reef at the Barrier Reef off Carrie Bow Gay, Belize (5.8 to 10.9 mmol m(-2) d(-1)) surpasses considerably the highest benthic nitrification rates reported previously (unconsolidated reef sediments: 1.68 mmol m(-2) d(-1)). 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 nutrient cycling in shallow benthic tropical communities.