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 sp ecies, but no significant DIN concentration changes were detected in t he control experiments (seawater alone). NO(2)(-)accumulated during al l the experiments with O. violacea (170 to 580 nmol g(-1) h(-1)), whil e 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. h alichondroides (0 to 320 nmol g(-1) h(-1)). These are the highest repo rted weight-specific production rates of oxidized nitrogen from benthi c communities. The highest values are associated with the 3 species th at possess cyanobacterial endosymbionts. Potential NO3- efflux rates b y 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) f or C. nucula) 2 to 4 orders of magnitude higher than the most active b enthic nitrification rates yet reported from the tropics. Extrapolatin g from incubation data (550 to 1030 nmol g(-1) h(-1) and biomass estim ates (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 t o 10.9 mmol m(-2) d(-1)) surpasses considerably the highest benthic ni trification rates reported previously (unconsolidated reef sediments: 1.68 mmol m(-2) d(-1)). These results strongly suggest that sponge-med iated nitrification is not uncommon in tropical marine benthic communi ties, and might constitute a large input of oxidized nitrogen into tho se habitats in which sponges abound. Our results reinforce the notion that sponges harbor and nourish microbial organisms with metabolisms t hat are important to the productivity and nutrient cycling in shallow benthic tropical communities.