Screening for new cathodic biocatalysts and microbial dicarbonic acids conversion activity
- Screening für neue kathodische Biokatalysatoren und mikrobielle Dikarbonsäure-Umwandlungsaktivität
de Campos Rodrigues, Tatiana; Agler-Rosenbaum, Miriam (Thesis advisor); Blank, Lars Mathias (Thesis advisor)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2016
Bioelectrochemical systems (BES) have become an important technology in the field of biological and environmental engineering research, being regarded as a promising future technology for sustainable production of electricity, fuels and chemicals. Microbial fuel cells are used to produce electric power are the most studied systems and, for this reason, the main focus of microbial electrocatalysis research for a long time was on anodic reactions. However, cathodic microbial electrocatalysis, where microorganisms consume electrons from a cathode to perform biochemical reductions, is currently of high interest. So far, only few electrosynthetic microorganisms have been identified and further characterized. One aim of this thesis was to identify possible electroactive candidates for a proposed novel microbial electroreduction process of platform chemicals to fuel precursors within the Excellence Cluster "Tailor-made Fuels from Biomass - (TMFB)". For an effective electrochemical screening and identification of such organisms, a time-saving, medium-throughput, six-well based BES system was developed in this thesis in order to evaluate the microbial ability to accept reducing equivalents from a cathode. Of nine tested physiologically and phylogenetically different strains, five promising cathode-active strains were identified: Acidithiobacillus ferrooxidans (jmax = -50 µA/ cm2), Desulfosporosinus orientis (jmax = -4 µA/ cm2), Thiobacillus denitrificans and Sulfurimonas denitrificans (jmax = -2 µA/ cm2 for both) and Desulfovibrio piger (jmax = -1.6 µA/ cm2). Based on the screening results and the interests of this study, the candidate selected for further characterization was the sulfate-reducer D. orientis. This strain confirmed its cathodic activity in H-type bioelectrochemical reactors reaching a maximum cathodic current density of -148 µA/ cm2. During this process, carbon dioxide was consumed (max: 3.2 x 10-4mols) with acetate production (0.65 mM of which 60% were coming from CO2) and sulfate reduction. Sulfate served as terminal electron acceptor and collected the highest amount of all charge available in the system with 75% electron recovery, followed by acetate (16.4% electron recovery) and planktonic biomass formation (0.1% electron recovery). With these findings, this thesis for the first time shows the electrosynthetic ability of D. orientis, which utilized electrons from an electrode poised at -0.5 V vs. Ag/AgClsat to reduce CO2 into the product acetate using its natural electron acceptor sulfate. However, the operation of the D. orientis BES reactors was very difficult and future work will be required to define more stable operational conditions for a deeper physiological characterization of this strain. Another aim for this thesis was to evaluate the ability of the tested strains to reduce the TMFB platform chemicals itaconic and levulinic acid for the targeted microbial electroreduction approach. The screenings prior to the BES experiments showed that Cupriavidus necator was the strain with highest decrease in concentration for both acids (39 and 25% for itaconic and levulinic acid respectively). D. orientis was the second best in itaconic acid conversion (12%) followed by Acidithiobacillus thiooxidans (11%). For levulinic acid, Cupriavidus metallidurans showed the second highest activity (24%). The attempt to test D. orientis activity towards itaconic acid directly in a BES was not successful due to the high sensitivity of this strain to any variation in operational conditions. However, this work suggests D. orientis as a promising new microorganism that could provide new insights for electron transfer at the cathode, and be employed for future MES applications.