Abstract
Geobacter spp. enrichment biofilms were cultivated in batch using one-chamber and two-chamber bioelectrochemical reactors. Time-resolved substrate quantification was performed to derive physiological parameters as well as incremental coulombic efficiency (i.e., coulombic efficiency during one batch cycle, here every 6h) during early stage biofilm development. The results of one-chamber reactors revealed an intermediate acetate increase putatively due to the presence of acetogens. Total coulombic efficiencies of two-chamber reactors were considerable lower (19.6±8.3% and 49.3±13.2% for 1st and 2nd batch cycle, respectively) compared to usually reported values of mature Geobacter spp. enrichment biofilms presumably reflecting energetic requirements for biomass production (i.e., cells and extracellular polymeric substances) during early stages of biofilm development. The incremental coulombic efficiency exhibits considerable changes during batch cycles indicating shifts between phases of maximizing metabolic rates and maximizing biomass yield. Analysis based on Michaelis-Menten kinetics yielded maximum substrate uptake rates (vmax,Ac, vmax,I) and half-saturation concentration coefficients (KM,Ac,KM,I) based on acetate uptake or current production, respectively. The latter is usually reported in literature but neglects energy demands for biofilm growth and maintenance as well as acetate and electron storage. From 1st to 2nd batch cycle, vmax,Ac and KM,Ac, decreased from 0.0042–0.0051 mmol Ac− h−1 cm−2 to 0.0031–0.0037 mmol Ac− h−1 cm−2 and 1.02–2.61 mM Ac− to 0.28–0.42 mM Ac−, respectively. Furthermore, differences between KM,Ac/KM,I and vmax,Ac/vmax,I were observed providing insights into the physiology of Geobacter spp. enrichment biofilms. Notably, KM,I considerably scattered while vmax,Ac/vmax,I and KM,Ac remained rather stable indicating that acetate transport within biofilm only marginally affects reaction rates. The observed data variation mandates the requirement of a more detailed analysis with an improved experimental system, e.g., using flow conditions and a comparison with Geobacter spp. pure cultures.
Highlights
Electroactive microorganisms (EAM) possess the metabolic trait of performing extracellular electron transfer (EET) [1]
The progress of coulombic efficiency (CEi) of early stage Geobacter spp. enrichment biofilms was reported for the first time
The results indicate that Geobacter spp. switch metabolism between optimizing metabolic rate and biomass yield depending on acetate concentration and the progress of biofilm formation
Summary
Electroactive microorganisms (EAM) possess the metabolic trait of performing extracellular electron transfer (EET) [1]. Several mechanisms of EET are known [2,3] that can be assigned to either (i) direct extracellular electron transfer (DEET) or to (ii) mediated extracellular electron transfer (MEET). DEET requires a direct physical contact between microorganisms and insoluble TEA. Electrons can be transferred between phylogenetically different microorganisms with one being the electron donor and the other the electron acceptor [6]. This is termed direct interspecies electron transfer (DIET) and enables the creation of metabolic webs. Another related type of electron conduction was discovered in Desulfobulbaceae. By forming multicellular filaments, electrons are transported across distances of few centimeters connecting, for instance, oxic and anoxic zones of marsh [7]
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