Towards a better understanding of phagotrophic protists in leaf processing of freshwaters: Studies on micro-environment and food web structure

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Forested lowland streams, ponds and ditches characterized by low flow velocities receive considerable input of leaf litter from riparian vegetation. Leaf-associated microbial activity and negligible advective transports lead to hypoxia hostile for shredding invertebrates. However, phagotrophic protists that can tolerate low oxygen concentrations are suggested to promote leaf litter processing. The proposed project focuses on the significance of phagotrophic protists for carbon transfer and the regulatory mechanisms involved in leaf litter processing in hypoxic environments. The following key hypotheses will be tested: (1) Leafassociated phagotrophic protists accelerate carbon flow during leaf processing by grazing leaf-associated bacteria and fungi. (2) Micro-currents generated by movement and filtration of phagotrophic protists enhance nutrient and oxygen transfer towards leaf-associated microbial communities and consequently microbial activity. Carbon transfer from leaves through the leaf-associated microbial food web will be analyzed with stable isotopes in combination with phospholipid fatty acids that are biomarker for specific bacterial and protist groups. The micro-environment at leaf surfaces and inside leaves will be observed with tracer particles microscopically and with micro-sensors.