Especificidade em associações alga/bactéria vinculadas aos carbidratos excretados por três espécies fitoplanctônicas de um reservatório tropical eutrófico do Estado de São Paulo (Barra Bonita).

Abstract

The release of carbohydrates by phytoplankton has been extensively studied because of its ecological significance. Bacterioplankton is a group of organisms that is able to use extracellular phytoplanktonic carbohydrates, as many authors have demonstrated. The aim of this thesis was to identify specific associations algae/bacteria in the planktonic environmental of a eutrophicated reservoir (Tiete river, Barra Bonita, SP), based on the carbohydrates released by the phytoplankton. The utilization pattern of such compounds was also studied. For this purpose, three phytoplanktonic species from Barra Bonita reservoir were used: Cryptomonas tetrapyrenoidosa (Cryptophyceae), Staurastrum orbiculare (Zygnematophyceae) e Thalassiosira sp. (Bacillariophyceae). The growth characteristics of these species in axenic conditions were determined and the extracellular carbohydrates produced by each one were characterized. Algae/bacteria cultivation experiments were performed inoculating the microbial community from Barra Bonita in axenic cultures of each algae. The phytoplanktonic growth pattern and the carbohydrate release were compared in axenic and contaminated cultures. From these algae/bacteria cultures, the specific bacteria community from each algae were isolated and they were re-inoculated in two kinds of culture media: composed by polysaccharides and low molecular weight released material. The bacterial diversity from the Barra Bonita reservoir was compared to the one observed in the algae/bacteria cultures and also in the two kinds of culture media, employing the DGGE method (Denaturing Gradient Gel Electrophoresis). The extracelullar carbohydrate production was exponentially increased at the stationary growth phase in the axenic cultures. The results showed clearly that the extracellular carbohydrates released by the phytoplanktonic species were used as an important carbon source to the bacterial populations, even when they occurred in very low concentrations (less than 1 mg/L). The polysaccharide utilization was remarkably distinct among the species under study. The results also showed that the polysaccharide degradations were not homogenous, but specific regions of them were degraded firstly. The results proved that the bacterial diversity patterns found in the Barra Bonita reservoir and in the different substrates were different. Specific bacterial populations were found as well as populations adapted to different classis of substrates.