Electrocatalytic Mechanism Involving Michaelis-Menten Kinetics at the Preparative Scale: Theory and Applicability to Photocurrents from a Photosynthetic Algae Suspension With Quinones

Investigation of quinone reduction by microalgae using fluorescence - do "lake" and "puddle" mechanisms matter?

Photosynthesis is a fundamental process used by Nature to convert solar energy into chemical energy. For the last twenty years, many solutions have been explored to provide electrical power from the photosynthetic chain. In this context, the coupling between microalgae and exogenous quinones is an encouraging strategy because of the capability of quinones to be reduced by the photosynthetic chain. The ability of a quinone to be a good or bad electron acceptor can be evaluated by fluorescence measurements. Fluorescence analyses are thus a convenient tool helping to define a diverting parameter for some quinones. However, this parameter is implicitly designed on the basis of a particular light capture mechanism by algae. In this paper, we propose to revisit previous fluorescence experimental data by considering the two possible mechanisms (lake vs. puddle) and discussing their implication on the conclusions of the analysis. In particular, we show that the maximum extraction efficiency depends on the mechanism (in the case of 2,6-dichlorobenzoquinone - 2,6-DCBQ, (0.45 ± 0.02) vs (0.61 ± 0.03) for lake and puddle mechanisms respectively) but that the trends for different quinones remain correlated to the redox potentials independently of the mechanism.

Decomposing biophotovoltaic current density profiles using the Hilbert-Huang transform reveals influences of circadian clock on cyanobacteria exoelectrogenesis

Electrons from cyanobacteria photosynthetic and respiratory systems are implicated in current generated in biophotovoltaic (BPV) devices. However, the pathway that electrons follow to electrodes remains largely unknown, limiting progress of applied research. Here we use Hilbert–Huang Transforms to decompose Synechococcus elongatus sp. PCC7942 BPV current density profiles into physically meaningful oscillatory components, and compute their instantaneous frequencies. We develop hypotheses for the genesis of the oscillations via repeat experiments with iron-depleted and 20% CO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${_2}$$\end{document}2 enriched biofilms. The oscillations exhibit rhythms that are consistent with the state of the art cyanobacteria circadian model, and putative exoelectrogenic pathways. In particular, we observe oscillations consistent with: rhythmic D1:1 (photosystem II core) expression; circadian-controlled glycogen accumulation; circadian phase shifts under modified intracellular %ATP; and circadian period shortening in the absence of the iron-sulphur protein LdpA. We suggest that the extracted oscillations may be used to reverse-identify proteins and/or metabolites responsible for cyanobacteria exoelectrogenesis.

Mediator-Microorganism Interaction in Microbial Solar Cell: a Fluo-Electrochemical Insight

Microbial solar cells that mainly rely on the use of photosynthesic organisms are a promising alternative to photovoltaics for solar electricity production. In that way, we propose a new approach involving electrochemistry and fluorescence techniques. The coupled setup Electro-Pulse-Amplitude-Modulation ("e-PAM") enables the simultaneous recording of the produced photocurrent and fluorescence signals from the photosynthetic chain. This methodology was validated with a suspension of green alga Chlamydomonas reinhardtii in interaction with an exogenous redox mediator (2,6-dichlorobenzoquinone; DCBQ). The balance between photosynthetic chain events (PSII photochemical yield, quenching) and the extracted electricity can be monitored overtime. More particularly, the nonphotochemical quenching induced by DCBQ mirrors the photocurrent. This setup thus helps to distinguish the electron harvesting from some side effects due to quinones in real time. It therefore paves the way for future analyses devoted to the choice of the experimental conditions (redox mediator, photosynthetic organisms, and so on) to find the best electron extraction.

A kinetic model for redox-active film based biophotoelectrodes

Redox-active films are advantageous matrices for the immobilization of photosynthetic proteins, due to their ability to mediate electron transfer as well as to achieve high catalyst loading on an electrode for efficient generation of electricity or solar fuels.

Redox-active films are advantageous matrices for the immobilization of photosynthetic proteins, due to their ability to mediate electron transfer as well as to achieve high catalyst loading on an electrode for efficient generation of electricity or solar fuels. A general challenge arises from various charge recombination pathways along the light-induced electron transfer chain from the electrode to the charge carriers for electricity production or to the final electron acceptors for solar fuel formation. Experimental methods based on current measurement or product quantification are often unable to discern between the contributions from the photocatalytic process and the detrimental effect of the short-circuiting reactions. Here we report on a general electrochemical model of the reaction–diffusion processes to identify and quantify the “bottlenecks” present in the fuel or current generation. The model is able to predict photocurrent–time curves including deconvolution of the recombination contributions, and to visualize the corresponding time dependent concentration profiles of the product. Dimensionless groups are developed for straightforward identification of the limiting processes. The importance of the model for quantitative understanding of biophotoelectrochemical processes is highlighted with an example of simulation results predicting the effect of the diffusion coefficient of the charge carrier on photocurrent generation for different charge recombination kinetics.

Investigation of photocurrents resulting from a living unicellular algae suspension with quinones over time

Plants, algae, and some bacteria convert solar energy into chemical energy by using photosynthesis. In light of the current energy environment, many research strategies try to benefit from photosynthesis in order to generate usable photobioelectricity. Among all the strategies developed for transferring electrons from the photosynthetic chain to an outer collecting electrode, we recently implemented a method on a preparative scale (high surface electrode) based on a Chlamydomonas reinhardtii green algae suspension in the presence of exogenous quinones as redox mediators. While giving rise to an interesting performance (10-60 μA cm-2) in the course of one hour, this device appears to cause a slow decrease of the recorded photocurrent. In this paper, we wish to analyze and understand this gradual fall in performance in order to limit this issue in future applications. We thus first show that this kind of degradation could be related to over-irradiation conditions or side-effects of quinones depending on experimental conditions. We therefore built an empirical model involving a kinetic quenching induced by incubation with quinones, which is globally consistent with the experimental data provided by fluorescence measurements achieved after dark incubation of algae in the presence of quinones.