DFG Research Unit 580: Electron transfer processes in anoxic aquifers

1. DFG Research Unit 580: Electron transfer processes in anoxic aquifers SP KAP: Redox processes involving humicsubstances Prof. Dr. Andreas Kappler, Geomicrobiology, Center for Applied Geosciences, University of Tuebingen • Hypothesis: •  Electron transfer via humic substances in the metabolic network in anoxic aquifers is controlled by the (bio-)geochemical conditions that exist in anoxic aquifers: • Identity of iron(III) minerals • Concentrations of humic substances & presence of silica and other ions (e.g. phosphate), • Different ratios of iron minerals to microbial cells & spatial distance between cells and iron minerals • Presence of humics-reducing bacteria in aquifer sediment Goals for 2ndperiod Reduction of Fe(III) minerals and O2 by humics Non-reduced humic substances transfer electrons only to poorly crystalline or dissolved Fe(III) compounds while reduced humics reduce even crystalline Fe(III) minerals. O2 is reduced by reduced humics but to a lesser extent than expected thermodynamically due to the O2/H2O redox potential (reason: unfavorable 1st e-transfer). • Quantify the amount of electrons that can be transferred via electron shuttling from microorganisms to biogenic iron(III) (hydr)oxides (SP HAD, SP Z). • Identify the iron mineral phases produced during electron shuttling from microorganisms to different abiotically and biotically produced iron minerals (Mössbauer spectroscopy, SP HAD, SP Z). Requirements for humics electron shuttling Goals for 2ndperiod A minimum amount of 5-10 mg DOC/L is necessary for electron transfer stimulation by humic substance electron shuttling. Redox activity of humic substances strongly depends on ionic strength: Fe(II) formation from reduction of ferrihydrite by reduced humics increases with ionic strength. • Quantify electron shuttling via HS at different ratios of microbial cells to iron(III) minerals. • Determine over which distances electrons can be transferred (via electron hopping) between bacteria and Fe(III) minerals via humics e-shuttles (SP PLA, SP PEI). • Quantify rates and extent of iron mineral reduction by humic substances in the presence of different concentrations of ions (e.g. phosphate), silica and humic substances (SP HAD, SP Z). • Determine changes in 3D-structure and exposure of redox-active functional groups in humics as consequence of humics concentration, the presence/absence of structural iron and varying geochemical conditions (salt concentration) by scanning transmission X-ray microscopy (synchrotron). Electron shuttling in anoxic aquifers Goals for 2ndperiod MPN plate of AQDS-reducers Most-probable-number (MPN) studies revealed low but significant numbers of humics-reducing and Fe(III)-reducing microorganisms in the liquid phase of anoxic aquifers (102-103 cells/ml). • Quantify the significance and importance of electron transfer to iron minerals via electron shuttling in aquifer sediment material (batch experiments) (SP MEC).