Ammonia waste removal from recirculating aquaculture systems is typically accomplished via the action of nitrifying bacteria in specially designed biofilters that oxidize ammonia to produce nitrate. In the majority of these systems nitrate is discharged to the environment through frequent water exchanges. As environmental considerations have made it necessary to eliminate nitrate release, new strategies for nitrate consumption are being developed. Recently, it was shown that ammonia removal from wastewater could take place by a novel anaerobic activity carried out by bacterial Planctomycete sp. Referred to as 'anammox', this ammonia oxidation process occurs in the absence of an organic source and in the presence of nitrite (or nitrate) as an electron acceptor as follows: NH3 + HNO2 - N2 + 2H2O. Anammox has been estimated to result in savings of up to 90% of the costs associated with wastewater treatment plants. The ability to convert ammonia to nitrogen gas by a one-step process is an attractive approach for nitrogen removal in recirculating aquaculture systems. In this application we propose studies aimed at demonstrating the applicability of the anammox process in recirculating aquaculture systems. We hypothesize that incorporating anammox activity in aquaculture systems will replace half of the aerobic treatment and require no water replacement for nitrate control thereby reducing costs for both oxygen and water. Preliminary studies by our U.S. counterpart have demonstrated the presence and viability of Planctomycetes spp. in biofilter units used in recirculating mariculture systems. These studies, funded by BARD through a postdoctoral fellowship, demonstrated that biological transformation of ammonia to nitrogen gas in the presence of nitrite was detectable in these biofilters. Our objective is to study the applicability of the anammox process in a variety of recirculated aquaculture systems to determine optimal conditions necessary for efficient ammonia waste removal. We will test both seawater and freshwater systems operated with conventional aerobic treatment of ammonia to nitrate (U.S.) and similar systems that also include denitrifying biofilters as well as anaerobic sludge digestors (Israel). Molecular tools will be used to screen and monitor different treatment compartments for the presence of Planctomycetes. Optimal conditions for the enrichment of the anammox bacteria will be tested using laboratory scale biofilters. Our results will allow us to develop treatment schemes for optimizing and applying the anammox process to semi-commercial recirculating systems.