Analysis of the nutritional quality of biofilm
Biofilm is defined as a community of microorganisms associated with an organic matrix, which naturally forms on any solid surface in contact with water. The organic matrix contains polysaccharides, proteins, nucleic acids, among other polymers. In aquaculture, biofilm has been used to increase the system's productivity and survival rate. These improvements in the culture system can be attributed to the presence of microorganisms in the biofilm, whose biochemical composition provides macro and micronutrients that favor the growth of cultured organisms. In order to verify these hypotheses, this study aimed to analyze the nutritional quality of biofilm in three different strata of the water column, at different stages of its formation,and compare this information with the composition of the microorganism community. The experiment was carried out in the Saco do Justino cove, in the estuarine region of Lagoa dos Patos, RS, Brazil, in the summer of 2005. In order to provide the fixation of the biofilm, three fences were used with six polyethylene screens (musketeer type screen), having each 1.2 m², with a mesh opening of 1 mm, arranged vertically and fixed in bamboo (floating material). Biofilm samples were collected every five days in the three strata, surface (40 cm from the bottom), medium (20 cm from the bottom) and bottom (5 cm from the bottom), to check the chlorophyll a concentration, dry weight, characterization of the microorganisms, protein and lipid concentration. In parallel, water quality monitoring was carried out. The chlorophyll concentration ranged from 0.14 to 3,49 & # 956; g.cm-² in 30 days of the experiment, with no significant differences between strata. The dry weight value varied from 4 to 24 mg.cm-², and there were also no significant differences between strata. Our study suggests the verification of the biofilm's maturity through the parallel analysis of chlorophyll a and dry weight, as these measures cover all microorganisms that make up the biofilm, photosynthetic or not. The microorganisms of the biofilm reached average densities that varied between: 91,106 to 302,106 cels.cm-² of bacteria, 22,106 to 89,106 cels.cm-² of cyanobacteria, 4,104 to 76,104 cels.cm-² of centric diatoms, 1,104 to 26,104 cels. cm-² of streaked diatoms, 3,106 to 15,106 cels.cm-² of flagellates and 13 to 2468 cels.cm-² of nematodes. The protein values ranged from 0.3 to 2.7 mg.cm-²,and lipid between 8.2 to 105 mg.cm-². When converting the protein values to percentage, we recorded a maximum protein content of approximately 26% on a dry basis. When analyzing the correlations between microorganisms and biochemical composition, we found that the lipid input present in the biofilm was mainly due to nematodes and centric diatoms. While the variation in protein concentration was correlated with filamentous bacteria, adherent bacteria and mainly unicellular centric diatoms. This information will allow better handling of the biofilm in order to increase the supply of lipid and protein to cultured organisms.