The vast majority of organic matter in the world ocean is found in the dissolved pool. However, no evidence has been demonstrated for direct uptake of bulk dissolved organic matter (DOM) by organisms other than bacteria and some invertebrate larvae. The total organic carbon (TOC) is 10-30% higher in coral reefs than in adjacent open waters. The dissolved organic carbon (DOC) accounts for Ͼ90% of the TOC. Using a new in situ technique for clean sampling of the seawater inhaled and exhaled by benthic suspension feeders, we measured directly the removal of DOC in the symbiont-bearing reef sponge Theonella swinhoei. The sponge removed up to 26% (mean Ϯ SD: 12% Ϯ 8%) of the TOC (dissolved and particulate) from the water it filtered during a single passage through its filtration system. Most of the carbon gained by the sponge was from the dissolved pool (10 Ϯ 7 mol C L Ϫ1 ), an order of magnitude greater than the carbon gained from the total living cells (phytoplankton and bacteria) the sponge removed (2 Ϯ 1 mol C L Ϫ1). In T. swinhoei, over two-thirds of the sponge biomass consists of symbiotic bacteria, which likely play an important role in DOC uptake. Our findings indicate that the role of DOC in metazoan nutrition and the role of metazoans in DOC cycling may have been grossly underestimated.The total organic carbon (TOC) in the ocean is divided into two major compartments: particulate (POC) and dis-1 Corresponding author (gitai.yahel@huji.ac.il).

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This study focuses on the seasonal changes in the Gulf of Aquaba, Red Sea, in nitrite concentration and their relationship with phytoplankton activity, which is mainly controlled by an alternation of water-column stratification with vertical mixing. Within the euphotic zone, during thermal summer stratification, nutrient depletion was severe, and no nitrite could be detected in the upper 70 m. However, during stratification, nitrite was always associated with the nutriclines and formed a deep maximum at the bottom of the euphotic zone. In contrast, nitrite accumulated in the mixed water column during winter, closely paralleling the development of phytoplankton biomass. In the Gulf of Aqaba, maximum nitrite accumulation occurred when winter mixing reached its greatest depth, which in turn was coincident with the height of the phytoplankton spring bloom. Thus, our field data suggest that accumulation of nitrite is associated with nutrient-stimulated phytoplankton growth. This hypothesis was supported by nutrient-enrichment bioassays performed concomitantly: only when phytoplankton growth was stimulated by nutrient additions, did nitrite accumulate in the water. In the bioassays, the time-course of nitrite accumulation closely paralleled the development of phytoplankton biomass during the incubation period. We therefore suggest that the accumulation of nitrite in the mixed water column during winter is due to excretion by algal cells. Our field and experimental data show that between 10 and 15% of the total amount of nitrogen entering the mixed-water column is released as nitrite by phytoplankton. Further, our field and experimental data support the hypothesis that nitrite excretion by phytoplankton has a significant role in the formation of the deep nitrite maximum (DNM) during stratification in summer. In the bioassays, phytoplankton cells excreted nitrite even when ammonia was the nitrogen source. This indicates a so far unrecognised physiological pathway involved in nitrite excretion by phytoplankton cells. KEY WORDS: Nutrients · Nitrogen species · Nitrite · PhytoplanktonResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 239: 233-239, 2002 In the well-oxygenated water column of the Gulf of Aqaba, the dissimilatory reduction of nitrate to nitrite by denitrifying bacteria is likely to be negligible. Thus, we consider the following processes as potentially responsible for the generation and consumption of ambient nitrite in the Gulf of Aqaba: (1) Excretion of nitrite during nitrate reduction, i.e. incomplete assimilatory reduction of nitrate by phytoplankton and bacteria (Vaccaro & Ruyther 1960, Wada & Hattori, 1971, Miyazzaki et al. 1973, Miyazzaki et al. 1975, Kiefer et al. 1976, Herbland & Voituriez 1979, Olson 1981a, Dore & Karl, 1996b, Collos 1998. (2) Ammonium oxidation to nitrite by autotrophic nitrifying bacteria (Brandhorst 1959, Miyazzaki et al. 1975, Olson 1981a,b, Ward 1986, Dore & Karl 1996b, Enoksson et al. 1996. (3) Nitrite assimilation by p...