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

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Investigation of photocurrents resulting froma living unicellular algae suspension with quinones over time, Chem. Sci.2018

 

Sunlight is the most abundant and sustainable source of energy available on Earth.That is why strategies have been developed for many years in order to take advantage of solar energy. In this respect, photosynthesis, the biological process evolved by nature to feed the biosphere with reduced carbon, is nowadays a source of inspiration for implementing new and promising conversion strategies. However, despite the very high efficiency of the photochemical converters Photosystems I and II, only a few percent of the total energy available from sunlight is converted into chemical energy. This apparently disappointing performance leads to two opposite considerations.On the one hand, it could suggest that directly exploiting a very complex system like photosynthesis is not the easiest way to harness solar energy. As a consequence, chemical and electrochemical tools can be used to perform “artificial photosynthesis”, i.e. to build photoelectrochemical systems mimicking the basic principles of photosynthesis. On the other hand, this lack of efficiency could also suggest that photosynthesis is an unexploited fuel-producing factory. From this point of view, electrochemical tools can be used to harvest electrons from “real” photosynthetic systems (particularly integral and whole systems and under high-light conditions), i.e. to benefit from “natural photosynthesis”.

 

 

In conclusion, we demonstrated through fluorescence and electrochemical measurements that during photosynthetic electron harvesting from a unicellular algae suspension with quinones, a decrease in terms of performance took place because of three different phenomena. The first one is related to the light conditions, i.e. to possible photoinactivation. The other two are side-effects due to the added quinones as redox mediators. On the one hand, a kinetic quenching is observed for all the quinones, especially for PPBQ. On the other hand, an alteration of the ratio of open centers is also observed for chloroquinones that may lead to the decrease of the photochemical rate and a corresponding decrease of recorded photocurrents. Controlling the experimental conditions (nature of the quinone, incubation time, and excitation light) discriminate the different kinds of degradation (from quinones or from light). Therefore, a compromise needs to be found at long timescales between the ability of the quinone to harvest electrons and its poisoning effect. As an example, poor PSII acceptor quinones like DMBQ will not induce any alteration but will require appropriate mutations of the photosynthetic organism to increase its ability to extract electrons from the exogenous quinone acceptor. 

To the best of our knowledge, investigations of quinone effects on these biophotoelectrochemical cells are rather scarce. While the different mechanisms of action still remain to be investigated in detail, the results reported here pave the way for future optimizations of quinone use for harvesting bioelectricity from photosynthetic organisms.

 

Pour plus d'informations, consulter le communiqué de presse associé à cet article  : Étudier la photoélectricité produite par des algues !

 

Résumé: 

Chem. Sci.2018

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.

 

 

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

 

Guillaume Longatte,  Adnan Sayegh,  Jérôme Delacotte, Fabrice Rappaport, Francis-André Wollman,  Manon Guille-Collignon and Frédéric Lemaître

 

Chem. Sci.2018

 

DOI: 10.1039/c8sc03058h