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.

Figure 1

(A) Photograph of an IO-ITO electrode hosting Synechocystis. (B) Scheme of a cyanobacterium on the electrode. (C) Scanning electron microscopy (SEM) image of the IO-ITO electrode. (D) Proposed ET pathways at the biofilm–electrode interface.^, Pathway 1: direct ET from a membrane-bound redox protein. Pathway 2: indirect ET due to the efflux of intracellular redox species into the extracellular matrix. Pathway 3: indirect ET via pili. X: unknown redox mediator.

Figure 2

Stepped chronoamperometry scans of (A) IO-ITO|PSII and (B) IO-ITO|biofilm in the absence of exogenous mediators (DET) and under chopped light irradiation. MES electrolyte solution (pH 6.5) and BG11 medium (pH 8.0) were used in experiments involving IO-ITO|PSII and IO-ITO|biofilm, respectively. All experiments were carried out under Ar at 25 °C, using a red light source (λ_685 nm: 1 mW cm^–2).

Figure 3

TON of IO-ITO|PSII (MET) and IO-ITO|biofilm (MET and DET) over 5 days of 12 h light–dark cycling. Inset: corresponding plot of overall charge passed. Biofilm experiments were carried out under air, and protein-film experiments were carried out under Ar, at 25 °C, using a red light source (λ_685 nm: 1 mW cm^–2).

Figure 4

(A) Representative photocurrent profile of IO-ITO|biofilm after 12 h light irradiation (E_app: 0.3 V vs SHE). (B) Difference between CV scans (ν = 10 mV s^–1) of IO-ITO|biofilm under light irradiation and under dark-adapted anaerobic conditions. The CV scans obtained in the dark were subtracted from those obtained under light (Figure S12). CV scans were carried out under Ar purged conditions at 25 °C, using a red light source (λ_685 nm: 1 mW cm^–2).