Importance of biofilm for water quality and nourishment in intensive shrimp culture

Experiments were conducted to test the usefulness of biofilms-a microbial consortium associated with extracellular polymeric substances attached to submersed surfaces-in reducing the levels of ammonium and phosphate of rearing system water, and as a food source for the shrimp Farfantepenaeus paulensis. A mature biofilm, which is able to keep ammonium and phosphate at low levels, occurred 10-15 days after tank cleanup, and was characterized by chlorophyll-a concentration around 5 mug/cm(2). It was mainly composed of pennate diatoms (Amphora, Campylopyxis, Navicula, Sinedra, Hantschia and Cylindrotheca; ca. 9 x 10(4) cells/mg of biofilm) and filamentous cyanobacteria (Oscillatoria and Spirulina; ca. 2 X 10(5) cells/mg), though bacteria (max. 1.48 x 10(7)/mg), flagellates (max. 1.08 x 10(3)/mg) and ciliates (max. 3.51 x 10(2)/mg) were also present. Pennate diatoms and filamentous cyanobacteria were responsible for the largest uptake of ammonium from the water, but nitrifying bacteria also played an important role. The presence of a biofilm lead to reduced exportation of phosphorus (33% less phosphate) and to a higher output of nitrate + nitrite, instead of ammonium. Biofilm was also an important complementary food source for the shrimp, increasing their growth. (C) 2002 Elsevier Science B.V. All rights reserved.