Extracellular electron transfer in phototrophic microbial communities

Abstract

Microbial solar cells (MSCs) are bioelectrochemical systems where photosynthetic microorganisms catalyse solar energy conversion to electricity through oxidation of water/organic carbon at solid-state electrodes, coupled to reductive reactions at a counter electrode, typically oxygen reduction to water. MSCs have potential for niche applications,particularly generation of off-grid electricity to power devices in remote areas, including biosensors. The key advantage over other solar technologies is that steady electrical output can be produced 24 hours a day. This thesis explores MSCs as a potential technology, by studying the underlying principles of phototrophic electroactivity and by engineering high-throughput electrodes. The usage of microbial consortia rather than pure cultures was chosen on the basis of higher electrical output and higher robustness to fluctuating process conditions. This thesis studies the process of extracellular electron transfer (EET) using three-electrode photoelectrochemical cells, inoculated with environmental cultures from fresh and saltwater sources, at poised working electrode potential of +0.6 V vs. SHE under day/night cycles. The following research objectives were sought: (i) to investigate whether EET is a common feature in naturally occurring phototrophs; (ii) to elucidate the link between photosynthetic oxygen and the observed phenomenon of reversible currents; (iii) to elucidate the metabolic reasons for the occurrence of EET in mixed phototrophic communities; (iv) to enhance performance towards creation of MSC devices. The natural occurrence of EET in environmental phototrophic microorganisms was investigated for several fresh and seawater cultures. No electron flux to electrodes was observed with any fresh water cultures, and added mobile redox mediator (riboflavin) was required to establish EET. Microbial communities in two identified enriched fresh water cultures showed a majority of microalgae. It appears instead that EET is ubiquitous in seawater environments, with cyanobacteria dominance in two tested biofilms. Within the electrochemically active seawater cultures a link was found between the existence of a dual microbial community (cyanobacteria and gamma-proteobacteria), and the generation of fully reversible current, anodic at night and cathodic during daytime. A consortium without gamma-proteobacteria exhibited negligible daytime output. The detrimental effect of photosynthetically evolved oxygen on EET was investigated in the presence of riboflavin upon illumination of a freshwater-mixed phototrophic culture. The working electrode compartment produced an electrical current in response to day/night cycles over 12 months of operation, generating a maximum current density of 17.5 mA m−2 during the night phase, and a much lower current of approximately 2 mA m−2 during illumination. The cause of lower current generation under light exposure was found to be the high rates of re-oxidation of reduced riboflavin by the oxygen produced during photosynthesis. A similar detrimental effect was observed in seawater cultures without mediator addition. The metabolic reason for EET in phototrophs was studied using previously enriched and electrochemically active marine microbial biofilms dominated by cyanobacteria. Inorganic carbon (Ci) was supplied by addition of NaHCO3 to the medium and/or by sparging CO2 gas. At high Ci conditions, anodic EET was observed only during the night phase, indicating the occurrence of a form of night-time respiration that can use insoluble electrodes as terminal electron acceptors. At low or no Ci conditions however, EET also occurred during illumination, suggesting that, in the absence of their natural electron acceptor, some cyanobacteria are able to utilise solid electrodes as an electron sink. This may be a natural survival mechanism for cyanobacteria to maintain redox balance in environments with limited CO2 and/or high light intensity. The performance enhancement was conducted by co-immobilisation of mat-building seawater photosynthetic consortia and polymeric redox mediators (polymeric osmium complexes and polymeric azine mediators, including polymethylene blue and green, polythionine) onto anodes. Importantly, by this co-immobilisation, the previously observed detrimental effect of oxygen to photo current generation disappeared – a higher anodic current was exhibited during the day than at night – with uninterrupted anodic current generation during the day/night cycles. The largest improvement of anodic current outputs was achieved with electrodes immobilised by polymeric osmium complex [Os(2,2’-bipyridine)2(polyvinyl-imidazole)10Cl]Cl, to 320 ± 28 mA m-2 (64 ± 6-fold) under illumination and 317 ± 29 mA m-2 (43 ± 8-fold) at dark, compared with the bare graphite bioelectrode. This thesis offers an analysis and insight on the biological complexity that drives the electron transfer processes at MSC anodes. It also describes a number of technological advances that enabled the enhancement of power outputs by one order of magnitude compared to previous work.