Evaluation of nitrifying bacteria product to improve nitrification efficacy in recirculating aquaculture systems

Recirculating aquaculture systems (RASs) rely on nitrification to convert toxic ammonia and nitrite to less toxic nitrate. Nitrification is accomplished using biofilters with nitrifying bacteria and can be inefficient in biofilters that are new or that have been compromised due to stressors. Failure in a biofilter can result in very high levels of ammonia or nitrite, both of which are toxic to aquatic animals and can result in health issues, suppressed growth, and mortalities. For these reasons, a commercially available nitrifying bacterial product (Pond Protect-L®, Novozymes Biologicals Inc., Salem, VA, US) was tested in both a controlled pilot-scale experiment and within production-scale RAS that were experiencing elevated levels of nitrite. Juvenile Pacific white shrimp (Litopenaeus vannamei) were used as the aquatic species in both the pilot- and production-scale systems. In the pilot-scale experiment, three aquaria received no product (control) and three systems received bacterial product using dosing specification as recommended by the manufacturer. Control systems experienced significantly (P < 0.05) higher levels of ammonia and nitrite compared to the tanks that received bacterial product. Quantitative PCR was used as tool to verify the presence of nitrifying bacteria (Nitrobacter winogradskyi strain in the bacterial product) in the systems that received bacterial product. Meanwhile, none of these nitrifiers were detected in the control systems. Similar nitrification benefits were noted in the production systems that received bacterial product. More specifically, improved nitrite-oxidation was observed. Within 2 days of the addition of bacterial product, nitrite levels began to decrease in the production systems and after a steady decrease remained within safe limits for the entire shrimp culture period.