Narendran Sekar

Electrochemical impedance spectroscopy for microbial fuel cell characterization.

Electrochemical impedance spectroscopy is an efficient, non-intrusive and semi-quantitative technique to characterize the performance of bio-electrochemical systems such as microbial fuel cells and enzymatic fuel cells. Indeed, quantitative interpretation of the impedance data can be obtained with the help of mechanistic models using meaningful equivalent circuits. The production of maximum power using such systems has been limited by their higher internal resistance. The contribution of several different resistances to the overall internal resistance of the system can be ascertained through the measurement of impedance using EIS, which is greatly required for understanding and engineering of its principle components leading to better enhancement of its performance. EIS has been successfully employed in most of the MFC researches helping in advancement of the field through emergence of many novel MFC designs with greater power generating capacity. In a nutshell, impedance spectroscopy provides a valuable addition to the existing biochemical and spectroscopic techniques to better optimize the electrochemical behavior of the biological system.

Enhanced photo‐bioelectrochemical energy conversion by genetically engineered cyanobacteria

Photosynthetic energy conversion using natural systems is increasingly being investigated in the recent years. Photosynthetic microorganisms, such as cyanobacteria, exhibit light-dependent electrogenic characteristics in photo-bioelectrochemical cells (PBEC) that generate substantial photocurrents, yet the current densities are lower than their photovoltaic counterparts. Recently, we demonstrated that a cyanobacterium named Nostoc sp. employed in PBEC could generate up to 35 mW m(-2) even in a non-engineered PBEC. With the insights obtained from our previous research, a novel and successful attempt has been made in the current study to genetically engineer the cyanobacteria to further enhance its extracellular electron transfer. The cyanobacterium Synechococcus elongatus PCC 7942 was genetically engineered to express a non-native redox protein called outer membrane cytochrome S (OmcS). OmcS is predominantly responsible for metal reducing abilities of exoelectrogens such as Geobacter sp. The engineered S. elongatus exhibited higher extracellular electron transfer ability resulting in approximately ninefold higher photocurrent generation on the anode of a PBEC than the corresponding wild-type cyanobacterium. This work highlights the scope for enhancing photocurrent generation in cyanobacteria, thereby benefiting faster advancement of the photosynthetic microbial fuel cell technology.

Detection of methyl salicylate using bi-enzyme electrochemical sensor consisting salicylate hydroxylase and tyrosinase

Volatile organic compounds have been recognized as important marker chemicals to detect plant diseases caused by pathogens. Methyl salicylate has been identified as one of the most important volatile organic compounds released by plants during a biotic stress event such as fungal pathogen infection. Advanced detection of these marker chemicals could help in early identification of plant diseases and has huge significance for agricultural industry. This work describes the development of a novel bi-enzyme based electrochemical biosensor consisting of salicylate hydroxylase and tyrosinase enzymes immobilized on carbon nanotube modified electrodes. The amperometric detection using the bi-enzyme platform was realized through a series of cascade reactions that terminate in an electrochemical reduction reaction. Electrochemical measurements revealed that the sensitivity of the bi-enzyme sensor was 30.6±2.7µAcm(-2)µM(-1) and the limit of detection and limit of quantification were 13nM (1.80ppb) and 39nM (5.39ppb) respectively. Interference studies showed no significant interference from the other common plant volatile compounds. Synthetic analyte studies revealed that the bi-enzyme based biosensor can be used to reliably detect methyl salicylate released by unhealthy plants.

Electricity generation by Pyrococcus furiosus in microbial fuel cells operated at 90°C

Hyperthermophiles are microorganisms that thrive in extremely hot environments with temperatures near and even above 100°C. They are the most deeply rooted microorganisms on phylogenetic trees suggesting they may have evolved to survive in the early hostile earth. The simple respiratory systems of some of these hyperthermophiles make them potential candidates to develop microbial fuel cells (MFC) that can generate power at temperatures approaching the boiling point. We explored extracellular electron transfer in the hyperthermophilic archaeon Pyrococcus furiosus (Pf) by studying its ability to generate electricity in a two‐chamber MFC. Pf growing in defined medium functioned as an anolyte in a MFC operated at 90°C, generating a maximum current density of 2 A m−2 and a peak power density of 225 mW m−2 without the addition of any external redox mediator. Electron microscopy and electrochemical impedance spectroscopy of the anode with the attached Pf biofilm demonstrated bio‐electrochemical behavior that led to electricity generation in the MFC via direct electron transfer. This proof of concept study reveals for the first time that a hyperthermophile such as Pf can generate electricity in MFC at extreme temperatures. Biotechnol. Bioeng. 2017;114: 1419–1427.

Role of respiratory terminal oxidases in the extracellular electron transfer ability of cyanobacteria

Cyanobacteria are used as anode catalysts in photo-bioelectrochemical cells to generate electricity in a sustainable, economic, and environmental friendly manner using only water and sunlight. Though cyanobacteria (CB) possess unique advantage for solar energy conversion by virtue of its robust photosynthesis, they cannot efficiently perform extracellular electron transfer (EET). The reasons being, unlike dissimilatory metal reducing bacteria (that are usually exploited in microbial fuel cells to generate electricity), (1) CB do not possess any special features on their outer membrane to carry out EET and, (2) the electrons generated in photosynthetic electron transport chain are channeled into competing respiratory pathways rather than to the anode. CB, genetically engineered to express outer membrane cytochrome S (OmcS), was found to generate ∼nine-fold higher photocurrent compared to that of wild-type cyanobacterium in our previous work. In this study, each of the three respiratory terminal oxidases in Synechococcus elongatus PCC7942 namely bd-type quinol oxidase, aa3 -type cytochrome oxidase, and cbb3 -type cytochrome oxidase was knocked-out one at a time (cyd- , cox- , and cco- respectively) and its contribution for extracellular ferricyanide reduction and photocurrent generation was investigated. The knock-out mutant lacking functional bd-type quinol oxidase (cyd- ) exhibited greater EET by reducing more ferricyanide compared to other single knock-out mutants as well as the wild type. Further, cyd- omcs (the cyd- mutant expressing OmcS) was found to generate more photocurrent than the corresponding single knock out controls and the wild-type. This study clearly demonstrates that the bd-quinol oxidase diverted more electrons from the photosynthetic electron transport chain towards respiratory oxygen reduction and knocking it out had certainly enhanced the cyanobacterial EET.