Abstract
Biophotovoltaic (BPV) devices employ the photosynthetic activity of microalgae or cyanobacteria to harvest light energy and generate electrical current directly as a result of the release of electrons from the algal cells. NADPH oxidases (NOX) are plasma-membrane enzymes that transport electrons from the cytosol to generate extracellular superoxide anions, and have been implicated in BPV output. In this study, we investigated NOX activity in the diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana in an attempt to understand and enhance NOX and BPV function. We found that NOX activity was linked to defined growth regimes and growth phases, and was light dependent. Crucially, current output in a BPV device correlated with NOX activity, and levels of up to 14μA per 106 cells (approximately 500mA·m−2) were obtained. Expression of two putative P. tricornutum NOX genes (PtNOX1 and PtNOX2) was found to correspond with the observed growth patterns of superoxide anion production and power output, suggesting that these are responsible for the observed patterns of NOX activity. Crucially, we demonstrate that NOX activity levels could be enhanced via semi-continuous culturing, pointing to the possibility of maintaining long-term power output in BPV devices.
Highlights
To cope with an increasing world population and energy demand there has been extensive research into new ways of exploiting renewable biological sources of energy [1]
We investigated if superoxide anion production would correlate with current generation, and power output, by measuring the steady-state currents generated by the diatoms in a BPV device [3]
We have shown that two marine diatoms, P. tricornutum and T. pseudonana, generate extracellular superoxide anion, and that this correlates with their ability to generate current in a BPV device
Summary
To cope with an increasing world population and energy demand there has been extensive research into new ways of exploiting renewable biological sources of energy [1] One such technology is the microbial fuel cell, which makes use of electron-producing catalytic processes in heterotrophic microbes to generate electricity [2]. An alternative is to operate such devices with photosynthetic microorganisms such as microalgae and cyanobacteria, which require only sunlight as energy and water as a source of electrons, converting these into electricity and creating, in essence, a biological solar panel [2] In such devices (termed biological photovoltaic (BPV) devices [3]), electrons are extracted from water by the photosynthetic machinery and are transported to and across the outer cell membranes [4]. The plasma membrane is a significant barrier for electron extrusion, so studying mechanisms for electron transport across this membrane will be key for understanding how algal cells operate in BPV devices
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