Polymeric and free sugars released by three phytoplanktonic species from a freshwater tropical eutrophic reservoir

Abstract

Several studies have focused on the release of carbohydrates by phytoplankton because of the ecological significance of such compounds. This process increases the supply of carbon to the heterotrophic community, enhancing the phytoplankton/bacteria associations. In this article, we report investigations on the carbohydrate release, both polymeric and free sugars, in axenic batch cultures of three tropical freshwater phytoplanktonic species from different taxonomic positions: Cryptomonas tetrapyrenoidosa (Cryptophyceae), Staurastrum orbiculare (Zygnematophyceae) and Thalassiosira sp. (Bacillariophyceae). The total carbohydrate release rate was increased in the stationary growth phase in all the species under study. Most of the carbohydrates released by the three species were present in the polymeric form, although both polymeric and free carbohydrates could supply carbon enough to support bacterioplanktonic populations, according to rates of consumption found on literature. The composition of the carbohydrates differed significantly from one species to another, indicating that carbohydrate release might be a speciesspecific process. We also observed that the contributions of some components from polymeric sugars, such as fucose, rhamnose and arabinose increased with the advancing age of the cultures.