Evaluation and quantification of microbial degradation of allochthonous organic matter by "priming" (MicroPrime)

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The main goal of this project is to characterize future changes in aquatic C-cycling via microbial mechanisms, in particular by focusing on priming effects, which have been little studied in aquatic ecosystems. Depending on environmental conditions, priming can either stimulate or inhibit microbial C-cycling. Therefore, we aim to unravel future changes in the role of priming via compound specific stable isotope analysis (e.g. 13C/12C ratio in microbial phospholipid fatty acids (PLFA)) both in field and laboratory experiments. We intend to developing suitable protocols to combine PLFA analyses with molecular methods, e.g. stable isotope probing (DNA-SIP; RNA-SIP), in particular for bacteria and fungi. This study aims to determine the impact of terrigenous OC input on priming effects, and hence on the aquatic-terrestrial coupling as well as their role for C-cycling in aquatic systems affected by global change. To our knowledge, this will be the first study testing for the metabolic transfer of 13C-labled OC from both, labile and recalcitrant OC-pools in one approach and hence investigating the underlying mechanisms of priming effects in aquatic ecosystems. Both C-pools will be supplied with different pulsing-regimes and nutrient-conditions to unravel effects of labile-OM-availability on recalcitrant organic matter (OM) degradation processes. In the first phase, laboratory experiments consisting of defined microorganisms with well characterized abilities to degrade defined substrates will be carried out and thus enable studies on basic mechanisms, interactions between organisms and their role for OM degradation. Special emphasis will be set on the role of fungi in aquatic mineralization processes. We first want to elucidate basic mechanisms concerning priming, synergistic effects of bacteria and fungi in these processes and quantify which parts of the OM-pool (labile vs. recalcitrant) are respired or incorporated into microbial biomass. In a second phase, the role of characterized basic mechanisms and organism-interactions for OM cycling will be studied using natural communities, different time courses of C-additions (pulsing to simulate more frequent inputs of C in aquatic systems) and different nutrient conditions in the medium (elemental stoichiometry). Later in a third phase, these studies will be extended to much more complex environmental conditions by using mesocosm-experiments (http://www.lake-lab.de/). Here we mainly want to challenge our gained knowledge from phase one and two under environmental conditions, in particular addressing particle fluxes under natural conditions. Our modular approach will enable us to elucidate the role of priming in C-cycling in the water column as well as at aquatic-terrestrial interfaces and hence to improve C-budgets in aquatic systems.