Microbial Electrochemical System: extracellular electron transfer from photosynthesis and respiration to electrode

The electrochemical communication between microorganisms and electrodes has substantial implications both for basic understanding of biological electron transfer as well as in diverse applications, such as, microbial electrochemical system (MES), microbial biosensors and in production of valuable chemicals. In these systems the extracellular electron transfer (EET) from microbial metabolism to electrodes is restricted due to the insulated cellular exterior made of lipid structures. To obtain the electrochemical wiring of biomaterials with electrodes, osmium redox polymers (ORPs) was used as an efficient electron transfer (ET) mediator.

In this thesis a systematic study on EET from a variety of biomaterials is demonstrated. The EET from the most metabolically versatile purple bacterium, i.e., Rhodobacter capsulatus, grown under both heterotrophically and photoheterotrophically conditions was studied. Shewanella oneidensis MR-1 is a metal ion reducing bacterium and was highly studied in microbial electrochemical systems (MES) due to its direct ET competence. We have shown that S. oneidensis MR-1 can be coupled with ORP modified graphite electrodes that results in an enhanced current density.

A secured supply of cost effective sustainable energy is on of the greatest challenges in the 21st century. Biological photovoltaics (BPVs) is emerging as a potential energy generating technology to convert solar energy into electrical energy. To harness solar energy we studied the photo-excited EET from water oxidation via thylakoid membranes, the site of photosynthesis in green plants and algae. In addition three prokaryotic cyanobacteria, two different Leptolyngbya sp., and Chroococcales sp. were photo-electrochemically wired with electrodes. The photo-electrochemical communication of a multicellular eukaryotic alga was assumed to be challenging, since here photosynthesis occurs in a specially designed subcellular organelle called chloroplast. We have shown the photoelectrochemical communication of the eukaryotic alga, Paulschulzia pseudovolvox immobilized on a graphite electrode wired with ORPs.

Although a great deal of research is focussed on MES, however, to seek any potential practical application their performance due the low power output and restricted stability need to be improved many folds. Our findings could have substantial implications in MES, such as microbial fuel cells (MFC), microbial biosensors, photosynthetic energy generation and in other light sensitive bioelectrochemical devices.