Effect of hydrogen peroxide on the microbial community present in biofloc production systems of the shrimp Litopenaeus vannamei

The biofloc technology (BFT) allows the production of aquatic organisms in high stocking densities with low effluent production. BFT is a system compounded of flocs highly colonized by aerobic heterotrophic and chemoautotrophic biota, which guarantee good water quality and represent a supplementary food source for the produced organisms. However, the large abundance of bacteria, protozoan, metazoan and raised organisms, may lead to a rapid consumption of dissolved oxygen in case of aeration systems malfunction or energy outbreaks. In BFT systems, one extreme alternative to keep oxygen in high levels is the addition of hydrogen peroxide (H2O2). However, H(2)O(2 )is a neutral molecule that can easily cross the cell membranes by diffusion, producing lipid and protein peroxidation and affecting the cells integrity. In a 10 days experiment, we have evaluated the effect of H(2)O(2 )on the microorganisms present in bioflocs during the production of the white shrimp Litopenaeus vannamei in a BFT system. Twelve 45-l containers with saltwater and mature bioflocs were stocked with 15 (5.51 +/- 1.05 g) shrimp (333 shrimp m(-3)). Tanks were distributed into four treatments: control, with aeration and without application of H2O2, and the treatments 12 h, 24 h and 48 h with aeration suppression and the addition of hydrogen peroxide (10 mu L H2O2 L-1) during these periods, whenever necessary to maintain dissolved oxygen levels above 6 mg L-1. The mean levels of dissolved oxygen were similar among treatments. Total suspended solids were significantly higher in the 12-h treatment. The addition of H(2)O(2 )did not affect the shrimp survival, but negatively influenced the nitrification process with the increment of ammonia and nitrite concentration. However, ammonia levels decreased few days after stopping H2O2 addition, whereas nitrite was still present in high concentration at the end of the experiment, indicating that ammonia-oxidizing bacteria recovered faster than nitrite-oxidizing bacteria. The Chlorophycea Planctonema sp. was mostly affected by hydrogen peroxide, with higher abundance in control than in 24 h and 48 h treatments. The abundance of these microalgae probably decreased due to light limitation caused by high total suspended solids in tanks. Protozoans, especially flagellates, were also negatively affected by the H2O2. Considering the results presented here, we must say that H(2)O(2 )should be used only in extreme cases, as an emergency tool.