Role of protozoans on the microbial ectoenzymatic activity during the degradation of macrophytes

A succession of microbial populations and ectoenzymatic activities was observed during the decomposition of the dissolved organic matter (DOM) leached from the macrophyte Potamogeton pectinatus. In an initial phase, free bacterial populations dominated. These bacteria produced ectoenzymes to hydrolyze dissolved carbohydrates (maltose, cellobiose and starch). When bacterivorous Protozoa entered the sample, free bacteria almost disappeared and most of the bacteria were then found attached, colonizing the macrophyte particles. Protozoan populations also showed a succession where heterotrophic nanoflagellates (HNF) grew first and then larger Protozoa, ciliates and amoeba, developed. During this second phase, high activities of degradation of structural polysaccharides such as cellulose and hemicellulose were detected. HNF seemed to play an important role during the degradation of macrophytes because they grazed mainly on free bacteria and left the attached bacteria to grow. Thus, they promoted a shift from the hydrolysis of non-structural to structural polysaccharides.