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Redox Processes of Organic and Mineral Geochemical Phases at Aquatic Interfaces

  • In this thesis, rates and extend as well as the ecological implications of electron exchange reactions that involve redox-active moieties in organic matter (OM) were explored. The research builds on earlier findings that confirmed that OM may act as terminal electron acceptor (TEA) for electrons released in microbial respiration. This property was associated with quinone moieties that are ubiquitously found in OM from terrestrial and aquatic environments and that may undergo reversible reduction to the respective hydroquinone. Earlier methodological advances allowed for a rapid, direct and precise quantification of the electron accepting and donating properties of quinones in dissolved OM (DOM) by mediated electrochemical analysis. In this work, the previously established mediated electrochemical analysis was adapted and used in the characterization of redox properties of particulate natural samples that contain redox active iron and organic matter ("geochemical phases"). For the first time, direct measurements confirmed that microorganisms transferred electrons (e) from microbial respiration to the organic and inorganic electron acceptors in the particulate phase. Particulate OM in the sediments was found to provide a capacity to accept or donate e of 650 µmol e/gC. An incubation experiment resolved the spatiotemporal dynamics of organic and inorganic TEA species (i.e., nitrate, sulfate, Fe- and Mn oxyhydroxides) in sediments upon changes in oxygen availability and hence redox conditions. Oxygen is consumed when the reduced species are oxidized and, by this means, re-generate their electron-accepting capacity. The use of mediated electrochemical analysis allowed for the quantification of the redox state of the geochemical phases during their reduction and re-oxidation. The electron fluxes initiated by the oxic re generation of the TEAs nitrate, sulfate, Fe(III), Mn(IV) and quinoid moieties in OM were therefore directly monitored instead of modeled from the species’ distribution profiles in interstitial waters. The cyclic reduction and re-oxidation of redox species exposed to oxygen fluctuations was suspected to be a critical component of many aquatic ecosystems. In stratified lakes, extended sediment volumes are exposed to oxygen only upon lake overturn. Lake oxygen budgets are therefore influenced by benthic redox processes. The combined field and laboratory study showed that lake overturn seasonally introduces a finite amount of oxygen to the hypolimnion and that about 50% of the subsequent sediment oxygen consumption is exclusively associated with the re-generation of TEA species. These species previously formed in the sediment when organic matter was microbially decomposed during anaerobia. While lake overturn can completely mix epi- and hypolimnetic waters, small-scaled dynamics in temperature and oxygen availability may confine discrete parts of the water column with oscillations in physicochemical conditions. In the studied lake, a transient thermocline cyclically introduces oxygen to hypoxic hyplimnetic waters close to the pelagic redox interface. In the lake, organic TEAs may represent an important component of the total pelagic electron acceptor capacity. Due to the rapid and reversible redox reactions of DOM, reduced organic TEAs are re-generated upon dislocation to oxic parts of the water column. Results show that diurnal fluctuations of oxycline depth shape a micro-environment selecting for microbial species that are released from TEA limitations by OM in oxidized state. Pelagic microbial communities subjected to the same amount of OM in different oxidation states differed by more than 50% after one day. This work substantiates earlier findings that suggested that OM may be an important TEA species in many aquatic and terrestrial ecosystems. OM reduction in microbial respiration was shown to directly affect critical system parameters as bacterial activity, oxygen budgets and aquatic biodiversity. Both the microbial reduction and subsequent abiotic oxidation of OM are sufficiently fast for relevant interaction with oxycline fluctuation on different timescales. Given that organic TEAs are cyclically regenerated, a significant share of ecosystem respiration could be linked to OM reduction. This thesis demonstrated the new and important role electron exchange reactions in OM-rich environments play and explored the mechanism of this previously neglected part of lake functioning. As of today, linking the chemistry of aquatic turnover processes with the microbiological and physical conditions at redox interfaces remains challenging. In conclusions, by providing several cases from aquatic environments, this thesis contributes to the mechanistic understanding of OM reduction in microbial respiration. The results prompt for further research regarding the competitive inhibition of other respiration pathways, including the reductive production of the potent greenhouse gas methane.
  • In der respirativen Oxidation von organischem Material (OM) werden Elektronen frei, die viele aquatische Mikroorganismen auch auf andere Elektronenakzeptoren als Sauerstoff übertragen können. Sogar OM selbst beinhaltet chemische Funktionalitäten, Quinone, die in ihrem oxidierten Zustand als Elektronenakzeptor genutzt werden. In Süßwassersedimenten finden sich häufig große Mengen organischen Materials in partikulärer Phase. Um Zugang zu Zusammensetzung und Reaktivität dieser Phasen zu erhalten, wurde im Rahmen der vorliegenden Arbeit eine elektrochemische Messtechnik herangezogen, die deren Redoxzustand direkt und präzise quantifizieren kann. Es zeigte sich, dass partikuläres Material einen erheblichen Beitrag zur Elektronenbilanz in aquatischen und terrestrischen Ökosystemen beitragen kann. Anhand einiger Beispiele wurde gezeigt, dass die Berücksichtigung der Redoxprozesse von gelöstem und partikulärem OM für das Verständnis des Material- und Energieumsatzes in Süßwassersystemen entscheidend sein kann.

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Author: Maximilian Peter Lau
Title Additional (German):Organo-mineralische, "geochemische" Phasen und ihre Reaktivität in aquatischen Grenzzonen
Advisor:Dr. Michael Hupfer, Dr. Jörg Gelbrecht, Prof. Dr. Fritz Scholz
Document Type:Doctoral Thesis
Date of Publication (online):2016/12/14
Granting Institution:Ernst-Moritz-Arndt-Universität, Mathematisch-Naturwissenschaftliche Fakultät (bis 31.05.2018)
Date of final exam:2016/10/25
Release Date:2016/12/14
Tag:Redox Biogeochemie, aquatische Grenzzonen
Aquatic Interfaces, Organic Matter, Redox Biogeochemistry
GND Keyword:Biogeochemie, Limnologie, Organischer Stoff
Faculties:Mathematisch-Naturwissenschaftliche Fakultät / Institut für Chemie und Biochemie
DDC class:500 Naturwissenschaften und Mathematik / 540 Chemie