Composition and role of microorganisms on nitrogen cycle in aquaculture pond

Abstract

This study investigated nitrogen treatment processes by natural microorganisms in sediment from white shrimp (Litopenaeus vanamei) pond. The first part of this study was conducted under laboratory condition. Sediment from shrimp pond in Pathum Thani Province was packed in sediment chamber, filled with seawater, and aerated using air stone. Concentrations of ammonia, nitrite, and nitrate in water column and in sediment were monitored. Changes of bacterial diversity in the sediment were examined by PCR-DGGE analysis of 16S rRNA gene. It was found that rapid changing of salinity from 5 to 20 PSU had no effect on ammonia oxidation process while denitrification was somewhat inhibited. The dominant bacterial species such as Marinobacter and Psuedomonas disappeared after salinity changed. Low DO concentration (2.5 mg/L) significantly inhibited nitrification process and also slightly decreased the diversity of bacteria in the sediment. Addition of NaHCO3 in the sediment chambers accelerated the nitrogen conversion by ammonia oxidation, nitrite oxidation and denitrification processes. Methanol addition had a negative effect on nitrification process in the sediment chamber. Results from DGGE analysis suggested that denitrifying bacteria Vibrio sp., Psuedomonas sp., Planococcus sp., Streptomyces sp. and Thioalcalovibrio sp. were stimulated after methanol addition. When the sediment chamber was illuminated, ammonia removal was accomplished by a combination of ammonia oxidizing by bacteria and ammonia uptake by phytoplankton. However, the major role of nitrogen removal in the sediment chambers was through bacterial nitrification and denitrification processes rather than photosynthetic microorganisms. Soil desiccation by sun drying strongly inhibited nitrification process in both ammonia oxidation and nitrite oxidation steps in the sediment chambers. The second part of this study was carried out in the outdoor artificial shrimp pond. Artificial shrimp pond was 500 L plastic tank packed with sediment from shrimp pond and filled with 450 L of 20 PSU seawater. The tanks were continuously aerated. The experiment consisted of control tank packed with wet untreated sediment and treatment tank packed with sun-dried sediment. It was found that sun drying inhibited nitrification process in the tank and peaks of ammonia and nitrite was found in treatment tank. However, peaks of ammonia and nitrite were lower than that found in the sediment chamber because of phytoplankton uptake. DGGE analysis illustrated that ammonia oxidizing bacteria (Nitrosomonas sp.) was disappeared from treatment tank but it was recovered after acclimation for three weeks. After pond acclimation, simulation of shrimp excretion by repeat addition of ammonium chloride into the artificial shrimp pond was performed and ammonia removal rate was examined. It was found that at the ammonia loading rate of 0.2 mg-N/L/day, the pond had a capability to remove all ammonia within one day. This was equal to the ammonia removal rate of 0.21-0.28 mg-N/m2/day and this ammonia loading was equal to the cultivation of 10 g shrimp at a density of 26 shrimp/m2.