Effects of algae on microbial carbon cycling in freshwaters : with focus on the utilization of leaf carbon by heterotrophic bacteria and fungi Microbial processing of organic matter (OM) in the freshwater biosphere is a key component of global biogeochemical cycles. Freshwaters receive and process valuable amounts of leaf OM from their terrestrial landscape. These terrestrial subsidies provide an essential source of energy and nutrients to the aquatic environment as a function of heterotrophic processing by fungi and bacteria. Particularly in freshwaters with low in situ primary production from algae (microalgae cyanobacteria) microbial turnover of leaf OM significantly contributes to the productivity and functioning of freshwater ecosystems and not least their contribution to global carbon cycling. Based on differences in their chemical composition it is believed that leaf OM is less bioavailable to microbial heterotrophs than OM photosynthetically produced by algae. Especially particulate leaf OM consisting predominantly of structurally complex and aromatic polymers is assumed highly resistant to enzymatic breakdown by microbial heterotrophs. However recent research has demonstrated that OM produced by algae promotes the heterotrophic breakdown of leaf OM in aquatic ecosystems with profound consequences for the metabolism of leaf carbon (C) within microbial food webs. [...]