Zyklische Redoxreaktionen organischer Kohlenstoffverbindungen in Binnengewässern
Zusammenfassung der Projektergebnisse
In aquatic ecosystems, anoxic conditions develop where the renewal of oxygen (O2) is slower than its consumption, for example in pore waters of sediments, or the stagnant deep waters of lakes. Currently, some of the most pressing global trends - river damming, warming, eutrophication - are known to promote an unidirectional response of aquatic ecosystems: their deoxygenation. The transit of organic matter (OM) through these aquatic systems has been intensively studied, but traditionally with a focus on OM processing and degradation, and because this is so intimately related to oxidation, studies rarely expanded to anoxic waters. Instead, the notion tenaciously persists that where O2 is absent, OM mineralization (i.e., its oxidation to CO2) is greatly reduced, and strictly controlled by the availability of alternative, inorganic oxidants (i.e., electron acceptors). However, where OM is in high abundance, there is a considerable potential of OM to (auto-)support its own microbial mineralization through the use of OM-inherent quinones as organic electron acceptors. The resulting dual role of OM in microbial respiration, namely as both the substrate (and electron donor) on the one hand, and electron acceptor on the other hand, has, to date, received little attention in lake ecosystems. The funded proposal built on the hypothesis that the use of DOM as electron acceptor is widespread in the anoxic hypolimnetic waters of temperate and boreal lakes, and that this function of DOM has the potential to contribute to the OM mineralization happening within these systems. The study aimed specifically at quantifying the ecosystem-scale mineralization that is dependent on this microbial respiration pathway. We found that DOM reduction is prevalent in the anoxic waters of the studied lakes of the north-temperate zone (Quebec), as rationalized from the increasingly reduced OM redox state, the longer OM has remained under anoxic conditions. In line with expectation, methane production (methanogenesis) balanced the largest share of hypolimnetic OM mineralization. Interestingly though, the traditional inorganic electron accepting species (nitrate, iron, sulfate) contributed similar proportions to microbial CO2 production, as did the organic OM-based electron acceptor system (3-10% vs. 2-5%). We further observed that anoxic conditions do not only change the molecular-level composition by promoting microbial reduction of OM quinones, but also triggered the release of large amounts of additional dissolved OM to the anoxic water column. Following the release to anoxic waters, the traditional view suggests that the prevailing anoxic condition favors ambient OM stability and preservation. Opposite to expectation we found the released OM to be of high biological reactivity, so that the anoxia-triggered OM release is partly offsetting the lakes’ function as landscape C sinks. Taken together, these results suggest an unforeseen role of ecosystem deoxygenation in the landscape: the role of creating reaction sites that unlock, transform and mineralize OM of aquatic and terrestrial sources. There is clearly still much to be learned concerning the dynamics of O2 and OM in aquatic landscape compartments, but our results suggest that anoxic waters must be viewed as an integral part of the landscape C decomposition system, because key reactions in the C cycle may only occur here. Our understanding of the aquatic-terrestrial C cycle and regional OC sources and sinks is, thus, incomplete without the inclusion of anoxic processes, particularly in water-rich northern landscapes.
