Contribution of fungi and bacteria to the formation of dissolved organic carbon from decaying common reed (Phragmites australis)

We examined release and subsequent utilization of DOC from leaves of common reed (Phragmites australis), a macrophyte which often dominates in shallow lakes and constitutes an important source of dissolved organic carbon (DOC). Leaves were incubated submersed in organic-free water for up to 63 days with natural and manipulated microbial communities. By this, we aimed to demonstrate differential effects of bacteria and fungi on the composition and amount of DOC originating from the leaves. DOC was analyzed by its total amount, spectral properties at wavelengths of 250-500 nm and its composition determined by size exclusion chromatography followed by organic carbon detection. Leaching of DOC was fast and the maximum DOC concentration was reached after 48 h. Mean molecule size increased during the first 14 days of incubation. Later on, humic-like substances accumulated, whereas low- and high-molecular-weight DOC were depleted. The formation of DOC from leaf detritus was strongly influenced by the composition of the microbial community present. Bacteria effectively removed low-molecular-weight DOC and accumulated high-molecular-weight DOC during a 7 day incubation. Leaf-degrading fungi promoted the accumulation of high amounts of intermediate-molecular-weight DOC, but were suppressed by the presence of bacteria. The presence of bacteria and/or fungi thus resulted in contrasting patterns of DOC composition, suggesting functional differences and strong interactions between those two major microbial groups during natural decomposition of leaves. The activity and interactions of both groups may therefore be significant for DOC composition in aquatic systems.