Abstract
We investigated transformation pathways and determined rate constants in a continuously operated glycerol-fed bioelectrochemical system under chemostatic conditions by applying concentration pulses of various intermediates. Our methodology does not require the interruption of the continuous operation and is thus in principle suitable for elucidating processes in continuously operated bioreactors in industry as well as in laboratory studies. Specifically for the example of glycerol electrooxidation, pulse responses of current density and effluent concentrations reveal that glycerol is first fermented to acetate, which is then oxidized electrochemically by the anode respiring bacteria. Microbial community analysis confirms this division of labour with a bioanode dominated by Geobacter species 92.8 %) and a much more diverse fermenting community in the planktonic phase, containing mainly Desulfovibrio sp. (45.2 %) and Spiroaetales (18.1 %). Desulfovibrio and Geobacter species are identified as promising candidates for tailored communities for glycerol electro-oxidation. From an acetate concentration pulse experiment, growth rates and half saturation rate constants for the biofilm of K_S = 1.4 mol m^-3 and d(q_{max,Ac} X_{bf})/dt = 933 mmol m^-2 d^-2 are obtained. Furthermore, 1,3-propanediol and glycerol concentration pulse experiments show that the reaction from glycerol to 1,3-propanediol is reversed at high 1,3-propanediol concentrations. The presented methodology allows one to study pathways and extract rate constants through simple experiments in a running system without irreversibly altering the microbial community or destroying the biofilm.
Highlights
Understanding reaction pathways and process interactions within bioelectrochemical systems (BES) running on complex substrates is an important step toward performance optimization
The fact that a steady state can be maintained with only minor fluctuations and the fact that the system returns to steady state after each concentration pulse are important conditions for evaluating the concentration pulse experiments which are discussed
We described the concentration transients after the pulse by a simplified model according to Equation (4), with the reactor volume V, the flow rate Fout, and the concentration ci of the substance that was injected for the respective pulse experiment
Summary
Understanding reaction pathways and process interactions within bioelectrochemical systems (BES) running on complex substrates is an important step toward performance optimization. Many reports on complex substrates which are degraded by a defined artificial community of two or more species were able to assign clear roles to single bacterial species Even though a wide range of techniques is available for analysis of biofilms in BES (Harnisch and Rabaey, 2012), often little is known about metabolic pathways for complex substrates. There is a knowledge gap concerning pathways and kinetics for degeneration of complex substrates in BES by mixed cultures.
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