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

Several studies have focused on the release of carbohydratesby phytoplankton because of the ecological significance of suchcompounds. This process increases the supply of carbon to theheterotrophic community, enhancing the phytoplankton/bacteriaassociations. In this article, we report investigations on thecarbohydrate release, both polymeric and free sugars, in axenicbatch cultures of three tropical freshwater phytoplanktonicspecies from different taxonomic positions: Cryptomonas tetrapyrenoidosa(Cryptophyceae), Staurastrum orbiculare (Zygnematophyceae) andThalassiosira sp. (Bacillariophyceae). The total carbohydraterelease rate was increased in the stationary growth phase inall the species under study. Most of the carbohydrates releasedby the three species were present in the polymeric form, althoughboth polymeric and free carbohydrates could supply carbon enoughto support bacterioplanktonic populations, according to ratesof consumption found on literature. The composition of the carbohydratesdiffered significantly from one species to another, indicatingthat carbohydrate release might be a species-specific process.We also observed that the contributions of some components frompolymeric sugars, such as fucose, rhamnose and arabinose increasedwith the advancing age of the cultures.