Prospection of culture conditions that promote graphene hydrogel synthesis by electrogenic bacteria

FLORENCIA PRETZ; AURELIEN GASNIER; M. LAURA PEDANO; GUSTAVO CURUTCHET; M. BELÉN PRADOS

Marseille

Conferencia; Seventh International Symposium on Environmental Biotechnology and Engineering; 2023

Bioelectrochemical systems (BES) represent a technological platform capable of ex-ploiting microbial oxidation-reduction mechanisms for different purposes, mainly theconversion of wastewater into a renewable energy source. These systems are basedon the ability of electrogenic microorganisms to transfer electrons to a solid surface,such as an electrode in an electrochemical cell, while oxidizing organic compounds.In this project, we seek to contribute to BES applications through the production ofnovel electrodes. The method consists of the reduction of graphene oxide (GO) andself-assembly of an hydrogel (GH) catalyzed by the metabolic activity of electrogenicmicroorganisms. These hydrogels could be employed as process specific electrodes,depending on the bacterial strain that catalyzed its synthesis.In this work, we explored several combinations of culture media and bacterial strainsthat could promote GH synthesis. Culture media were: (1) a rich medium (triptone,peptone and yeast extract); (2) a minimal medium supplemented with minerals,vitamins, acetate-Na (electron and carbon source) and fumarate-Na 35 mM or (3) 3.5mM and (4) the same minimal media as in (2) with the addition of lactate-Na.Bacterial strains tested were Shewanella sp., Aeromonas sp. (both strains isolated byus from an electrogenic community selected from a polluted river sediment,Reconquista, Argentina) and M4 (electrogenic community selected from a pollutedwetland, Río Negro, Argentina). Sterile vials were filled with culture media (Vf=10mL), inoculated with 10% of a bacterial culture (OD 600≈0.3) in the presence orabscence of GO (0.14 mg/mL). Control vials in the absence of bacteria or in thepresence of a sterylized bacterial culture were prepared. Vials were flushed withultrapure N2:CO2 (80:20) to remove oxygen and cultured at 30ºC in an incubator. GOreduction was studied by visual inspection (GO changes from brown to black) toavoid hydrogel disruption. The hydrogels were characterized when the structure wasstable by scanning electron microscopy (SEM), Raman and Fourier-transformedinfrared spectroscopy (FTIR).At 24, 48 and 120 h GO reduction was observed in every culture condition withShewanella sp., M4 and Aeromonas sp., respectively. The formation of a hydrogelwas evidenced after 7 days, when Shewanella sp. and M4 were grown in thepresence of fumarate 35 mM and it was stable 7 days later (invertion test). Infumarate 3,5 mM, the formation of the hydrogel was evidenced after 15 days. In theabsence of fumarate and in the rich medium, a colloidal reduced GO was observedeven after 30 days of culture with Shewanella sp. and M4. This same result wasobserved with Aeromonas sp. in every tested condition. The Raman and FTIRspectra of the hydrogels were similar to that obtained with a chemically reducedgraphene oxide, thus confirming GO reduction by bacteria. A 3D-architecture withbacteria inside a macroporous structure was observed by SEM. We are currentlystudying the performance of these GH as electrodes in BES.