Photoelectrochemistry of Photosystem II in Vitro vs in Vivo

Abstract

Factors governing the photoelectrochemical output of photosynthetic microorganisms are poorly understood, and energy loss may occur due to inefficient electron transfer (ET) processes. Here, we systematically compare the photoelectrochemistry of photosystem II (PSII) protein-films to cyanobacteria biofilms to derive: (i) the losses in light-to-charge conversion efficiencies, (ii) gains in photocatalytic longevity, and (iii) insights into the ET mechanism at the biofilm interface. This study was enabled by the use of hierarchically structured electrodes, which could be tailored for high/stable loadings of PSII core complexes and Synechocystis sp. PCC 6803 cells. The mediated photocurrent densities generated by the biofilm were 2 orders of magnitude lower than those of the protein-film. This was partly attributed to a lower photocatalyst loading as the rate of mediated electron extraction from PSII in vitro is only double that of PSII in vivo. On the other hand, the biofilm exhibited much greater longevity (>5 days) than the protein-film (<6 h), with turnover numbers surpassing those of the protein-film after 2 days. The mechanism of biofilm electrogenesis is suggested to involve an intracellular redox mediator, which is released during light irradiation.

The Development of Biophotovoltaic Systems for Power Generation and Biological Analysis

“…Furthermore, this electrode architecture enabled stable and enhanced integration of a range of redox active guests, including various oxidoreductases and co‐immobilisation with redox polymers . This hierarchical structure was then adopted for intact Synechocystis cells, with 10 μm macropores and 3 μm interconnecting channels, at a thickness of 40 μm . This gave rise to stable non‐mediated photocurrents (0.3 μA cm −2 , 680 nm at 1 mW cm −2 ) that grew over 5 days.…”

“…Good electrical wiring between the biological material and the anode is also important, or fast kinetic extracellular electron transfer may be missed. Good wiring may be easier to achieve with third‐generation electrodes, revealing complex shapes in the photocurrent profile that may otherwise be difficult to capture . For all studies, controlled environmental parameters and cell growth conditions are important, and experimental design should be matched with the properties of the system under study.…”

“…They also suggested a role for reactive oxygen species. Zhang et al inferred through CV the secretion of a redox species with a midpoint potential of 0.34 V versus SHE, and suggested it may correspond to benzoquinone or flavin derivatives. Saper et al .…”

“…It therefore seems likely that at least some cyanobacteria may use a molecule such as a quinone or a flavin for endogenous IEET. It has also been suggested based on chronoamperometry profiles that transporters or gated ion channels are involved in exporting an endogenous mediator that would otherwise be unable to cross the cytoplasmic membrane . Identification of any endogenous mediators for IEET is clearly a high priority for the field.…”

Underpotential Photoelectrooxidation of Water by SnS2−Laccase Co‐catalysts on Nanostructured Electrodes with Only Visible‐Light Irradiation

“…In fact, the only biological route to synthesize O 2 from H 2 O in nature is through photosynthesis, driven by photosystem II, an enzymatic complex that comprises around 20 subunits, where O 2 is a by‐product in the photocatalytic reduction of NADP + to NADPH. Therefore, most research efforts within photobiocatalysis have been focused on achieving photoelectrochemical optimization of PSII using transparent, nanostructured electrodes ,. Laccases are enzymes that reduce O 2 to H 2 O, but their natural biocatalytic activity is irreversible and requires special conditions and a high external energy input to perform the reverse reaction .…”