A system-oriented strategy to enhance electron production of Synechocystis sp. PCC6803 in bio-photovoltaic devices: experimental and modeling insights

Bio-photovoltaic devices (BPVs) harness photosynthetic organisms to produce bioelectricity in an eco-friendly way. However, their low energy efficiency is still a challenge. A comprehension of metabolic constraints can result in finding strategies for efficiency enhancement. This study presents a systemic approach based on metabolic modeling to design a regulatory defined medium, reducing the intracellular constraints in bioelectricity generation of Synechocystis sp. PCC6803 through the cellular metabolism alteration. The approach identified key reactions that played a critical role in improving electricity generation in Synechocystis sp. PCC6803 by comparing multiple optimal solutions of minimal and maximal NADH generation using two criteria. Regulatory compounds, which controlled the enzyme activity of the key reactions, were obtained from the BRENDA database. The selected compounds were subsequently added to the culture media, and their effect on bioelectricity generation was experimentally assessed. The power density curves for different culture media showed the BPV fed by Synechocystis sp. PCC6803 suspension in BG-11 supplemented with NH4Cl achieved the maximum power density of 148.27 mW m−2. This produced power density was more than 40.5-fold of what was obtained for the BPV fed with cyanobacterial suspension in BG-11. The effect of the activators on BPV performance was also evaluated by comparing their overpotential, maximum produced power density, and biofilm morphology under different conditions. These findings demonstrated the crucial role of cellular metabolism in improving bioelectricity generation in BPVs.


Material and methods
Metabolic model and in silico simulation. A genome-scale metabolic network of Synechocystis, named iJN678 16 , was used to investigate the metabolic pathways. The COBRA Toolbox and the GLPK package were applied to MATLAB 2017b to run the metabolic modeling 17 . Optimizing the intracellular electrons generation rates was done by defining the NADH production rates as the objective function. The lower bounds of intracellular reversible and irreversible reactions were applied to -1000 and 0 mmol/gDCW/h, respectively. Moreover, the upper bounds of all intracellular reactions were set to 1000 mmol/gDCW/h. Photoautotrophic conditions were simulated by constraining the photon uptake rate to 52 mmol/gDCW/h, and bicarbonate uptake rate was set to 3.7 mmol/gDCW/h as the only carbon source. The lower and upper bounds of exchange reactions including Co 2+ , Fe 2+ , Fe 3+ , H + , Na + , Ni 2+ , Cu 2+ , Zn 2+ , Ca 2+ , Mg 2+ , Mn 2+ , O 2 , H 2 O, Molybdate, Potassium, Nitrate, Phosphate, and Sulfate remained unconstrained by -1000 and 1000 mmol/gDCW/h, respectively. Hence the uptake and secretion of the mentioned metabolites were allowed 16 . Identifying effective reactions and designing the regulatory defined medium. As mentioned earlier 14 , LAMOS 18 was employed to calculate multiple optimal flux distributions and determine effective reactions for NADH overproduction. Besides, it was previously reported that the biomass concentration of Synechocystis in the electrochemical cell should not reduce significantly because it would lead to a low power generation 19 ; therefore, reactions that their regulation causes a high reduction of the growth rate are not appropriate candidates for manipulation. According to this point, the optimal biomass formation rate was calculated using FBA 20 (0.0884 h −1 ), and the growth rate was bound to 90% of the optimal growth rate in the metabolic model to ensure finding bottleneck genes in which manipulating their enzymes for overproduction of NADH does not result in a significant decrease in cell density. Moreover, a threshold of 10 −8 was considered to determine active reactions. Thus, 10,000 optimal solutions were found for two conditions, including the maximum and the minimum rates of NADH production (i.e., NADH → NAD + + H + + 2e − ). Then, for each mentioned condition, the flux variability for each reaction was determined (Fig. 1), and two criteria were applied to identify the candidate reactions for up or down regulations.
As demonstrated in Fig. 1, a reaction is suitable to be up-regulated if its minimum flux in the maximum case (i.e., maximum NADH production rate) is higher than the maximum flux in the minimum case (i.e., minimum NADH production rate). In contrast, a reaction is proper to be down-regulated if its minimum flux in the minimum case is higher than the maximum flux in the maximum one. www.nature.com/scientificreports/ The activity difference for each reaction was regarded as the second criterion and was defined as a fraction of entire optimal solutions wherein the reaction was indicated to be active. Therefore, those reactions were taken into consideration that possessed the absolute value of activity difference equal to 1. In this regard, the activity difference of 1 for a reaction indicated that the reaction was entirely active in every 10,000 optimal solutions of maximum NADH production while it was completely inactive in all optimal solutions of minimum NADH production. On the other hand, the reaction, which activated and inactivated entirely in every 10,000 optimal solutions of minimum and maximum NADH production, respectively, possessed the activity difference of -1.
The effect of up (down)-regulation of the predicted reactions was experimentally investigated by adding regulators to the culture medium. These regulatory compounds found in the BRENDA database affected the target enzyme activity controlling the target reaction, which is predicted to result in the overproduction of NADH and, consequently, electricity enhancement. Therefore, the investigation of the regulators' impact on electrons overproduction by Synechocystis is another goal of the study.
Bio-photovoltaic cell (BPV) assembly. The air-cathode single chamber BPV with a working volume of 8.8 cm 3 , height of 2.2 cm, and electrodes surface of 4 cm 2 was fabricated. The anodic and cathodic chambers were fabricated from polymethyl methacrylate (PMMA). The graphite-coated stainless steel mesh (mesh size 400) was used as the anode and cathode electrodes 21 . The cathode was treated by the procedures described in 22 to attain 0.5 mg cm −2 Pt loading. A schematic of the BPV is shown in Fig. 2. Microorganism and culture media. Synechocystis was obtained from Ariyan Gostar Research Corporation. All cultivation experiments conducted in the BG-11 medium described in the work of Bombelli et al. 7 . In this regard, liquid seed culture was obtained periodically, which was then used for the inoculation of fresh liquid culture. Subsequently, 2 ml of seed culture was inoculated into 100 ml flasks containing 18 ml of fresh BG-11. All incubations were carried out at 150 rpm and 29 °C, which were continuously kept under the illumination of a red light bulb (630 nm) with 6000 lux lighting. It was demonstrated that red light wavelengths have a significant impact on maintaining photosynthetic reactions 23 . Inoculation and BPV operation. In all experiments, after five days of cultivation, cyanobacterial cells were harvested during exponential growth ( OD 750 = 3.5 ± 0.1 , Fig. S1) and centrifuged for 15 min at 3000 g. Then, regarding the experiment to be studied, Synechocystis cells resuspended in fresh BG-11 or regulatory BG-11. With regard to the regulatory BG-11 medium, the selected regulatory compounds were supplemented to fresh BG-11 according to the concentrations obtained from Brenda to evaluate their effect on electricity genera-

Results
Identifying the effective reactions in NADH production. The appropriate reactions contributing to an increase in the intracellular NADH generation of Synechocystis were predicted and shown in Fig. 3A. The reactions depicted on the top possessed the activity difference of 1. Furthermore, their minimum flux under maximum NADH production rate is more than their maximum flux under the minimum NADH production rate (Fig. 1A), and thus there were proposed for up-regulation. In contrast, down-regulation should be considered for those reactions revealed on the bottom with the activity difference of -1 in which their minimum flux under minimum NADH production rate is more than the maximum flux under maximum NADH production rate (Fig. 1B). The maximum and minimum NADH production rates were calculated to be 2.1 and 0 mmol/ gDCW/h, respectively. The predicted reactions were prepared in more detail in the Supplementary file (Tables S1  and S2). Moreover, a complete name of all reactions and metabolites were presented in the Supplementary file (Tables S3 and S4), respectively. The predicted effective metabolic pathways for enhancing NADH are schematically represented in Fig. 3B. Herein, two crucial metabolic pathways for converting acetate (ac) to acetyl-CoA (accoa) that possessed a significant activity difference were selected. As shown in Fig. 3C, acetate could be consumed via the acetyl-CoA synthetase (ACS) by converting ATP to AMP. Besides, this metabolite can also be used through a series of reactions, including acetate kinase (ACKr_f) and phosphotransacetylase (PTAr_b), by conversion of ATP to ADP (Fig. 3C). Thus, acetate kinase and phosphotransacetylase were recommended to produce acetyl-CoA, which then enters the TCA cycle and causes NADH to form via malate dehydrogenase (MDH_f).
The systemic approach also recommended that glycerol 3-phosphate (glyc3p) played a vital role in NADH generation. As demonstrated in Fig. 3B, a red dashed line that starts with glyoxalate carboligase pathway (GLXCL) and ends with glycerol kinase (GLYK), consumed three moles NADH along with one mole of ATP to produce glycerol 3-phosphate. These pathways occur as a series of reactions that each reaction possesses the same flux through the metabolic network leading to 0 mmol/gDCW/h NADH generation (Table 1). Therefore, they are regarded as proper candidates to be down-regulated. Moreover, glycerol 3-phosphate could also be produced by glycerol-3-phosphate dehydrogenase (G3PD2_b), which consumes only one mole of NADH without ATP consumption. Green arrows show this pathway in Fig. 3B. Glycerol-3-phosphate dehydrogenase possesses the same reaction rates as the mentioned series reactions but leads to 2.1 mmol/gDCW/h NADH generation (Table 1). Thus, its up-regulation should be taken to prevent NADH and ATP loss via other metabolic pathways.
A further investigation into metabolic pathways depicted in (Fig. 3B), showed that the generated glycerol 3-phosphate produces acyl carrier protein (acp), which is then used to form CoA, which can be utilized by (PTAr_b). Furthermore, the hypothetical glycerol-3-phosphate acyltransferase reactions (G3PAT Rxns) comprise nine reactions generating acyl carrier protein, in which their details were mentioned in the Supplementary file (Table S5).
Moreover, the mechanism of extracellular electron transfer (EET) in cyanobacteria has been a subject of study in recent years, leading to conflicting reports 24 .
Gorby et al. 25 reported the presence of conductive nanowire appendages for Synechocystis, which was formed under low CO 2 conditions. The filaments thickness was reported about 100-150 nm, which was not consistent with individual type IV pili. The type IV pili of Synechocystis reported a thickness of 6-8 nm thick 26 . Later on, Thirumurthy et al. 27 reported that type IV pili were not required for photocurrent generation. McCormick et al. 5 www.nature.com/scientificreports/ revealed direct electron transfer of Synechocystis by developing a mediatorless BPV system. The direct electron  (Tables S3 and S4). www.nature.com/scientificreports/ transfer of Synechocystis to the anode was also reported by Cereda et al. 9 . However, it was not yet cited which cellular components responsible for transferring electrons to the electrode. Mao et al. 11 used computational metabolic techniques to study the maximum current outputs of Synechocystis in different electron transfer cases. The highest current output production was reported for plastoquinol and c-type cytochrome. Saper et al. 28 reported that Synechocystis could secrete some undefined small molecules to shuttle electrons to the anode. Similarly, Shlosberg et al. 29 showed that Synechocystis employed endogenous NADPH as an electron mediator to generate bioelectricity in bio-photoelectrochemical cells.
According to the mentioned points, the main extracellular electron transfer mechanism of Synechocystis has not been recognized yet. To this end, future studies should be carried out to investigate which mechanism mostly governs the extracellular electron transport of Synechocystis.
Microbial enrichment and open circuit potential (OCP) monitoring. With regard to the introduced reactions by metabolic modeling in Fig. 3A, phosphotransacetylase and glutamate dehydrogenase were taken into consideration. As reported in Brenda and previously published studies, 40 mM NH 4 Cl resulted in threefold stimulation of phosphotransacetylase 30 , and 50 mM KCl led to an increase in 170% activity of glutamate dehydrogenase 31 . Hence, three suspensions for microbial enrichment were used including Synechocystis + BG-11, Synechocystis + BG-11 + 50 mM KCl, and Synechocystis + BG-11 + 40 mM NH 4 Cl. Having concluded an effective biofilm formation during open circuit conditions 32 , which established a uniform biofilm facilitating substrate diffusion and electron transfer, the microbial enrichment of the BPV has been done by monitoring the open circuit potential (OCP). Thus, the OCP evolution of the three culture media was shown in Fig. 4.
Considering the OCP of the sole culture medium of BG-11, the addition of activators (i.e., KCl or NH 4 Cl) bring about a significant increase in the OCP of the BPV. Since Nernst's equation depicting the effective parameters on the electrochemical OCP, the higher cell potentials strongly depend on the activation of redox species implying the critical role of KCl and NH 4 Cl in this increment.
By observing a noticeable decrease in OCP, the fresh medium was injected into the BPV, as shown by arrows in Fig. 4. The replacement of the fresh culture medium with the old one causes an increase in the OCP trend. Since the BPV operated in the batch mode with high cell density ( OD 750 = 3.5 ± 0.1 ), the potential reduction can be attributed to the depletion of nutrients; thus, fresh medium injection compensates for the potential drop. This phenomenon was also shown previously in the work of Madiraju et al. 19 , in which replenishing minerals led to an increase in power density. Moreover, for the culture media with activator, this abrupt growth in the OCP was higher than the sole culture medium of BG-11. A comparison between Fig. 4A, B reveal that the culture medium of Synechocystis + BG-11 + 40 mM NH 4 Cl obtained the higher OCP and more prolonged stationary phase. This issue puts the emphasis on the more impact of this culture medium on the cell potential compared to the BG-11 + 50 mM KCl.
Polarization and power density curves. The effect of external resistance on the BPV performance fed with various culture media types was investigated by monitoring current evolution. Figure 5A depicted the current evolution of Synechocystis for BG-11, BG-11 + KCl, and BG-11 + NH 4 Cl culture media at 500, 500, and 750 kΩ external resistances, respectively. The presence of KCl increased the maximum produced current of the BPV by more than 8.8% compared with its value when the initial culture medium of BG-11 was fed. The addition of NH 4 Cl to BG-11 had a remarkable influence on the current production and increased the maximum current density of the cell more than twofold compared with the BPV fed by BG-11. This incremental phenomenon was obtained at a higher external resistance of 750 kΩ, which was comparable to the other media implying more increase have to be obtained in the same external resistance (i.e., 500 kΩ). www.nature.com/scientificreports/ Additionally, the stationary phase in the current evolution (shown by two dashed horizontal arrows in Fig. 5A) of the BG-11 + NH 4 Cl fed BPV was longer compared to the BPV cultured BG-11 and BG-11 + KCl, indicating a stable current density by extension feed replacement.
The BPV overpotentials (including activation, ohmic, and concentration) can be investigated by applying different external resistances to polarize the cell. The polarization curves of four types of BPV feed was illustrated in Fig. 5B. The abrupt reductions of cell potentials in the polarization curves of BG-11 + Synechocystis and BG-11 + KCl + Synechocystis indicate the high activation overpotential to extract the electron from the cellular metabolism. This type of overpotential was remarkably higher than two other overpotentials (i.e., ohmic and concentration overpotentials) characterized at the middle and end of polarization curves. Activation overpotential represents the energy that microbes required to transfer electrons from their surface to the electrode 33 . These high drops corroborate the hypothesis presented in previous researches 5,9 , indicating the incapability of Synechocystis to transfer electrons from its membrane due to the competition of numerous metabolic pathways for energy www.nature.com/scientificreports/ resources. Moreover, the absence and presence of cyanobacteria in the BG-11 culture medium demonstrated the incapability of Synechocystis in generating electrons. Although the addition of KCl increased the current density, the initial sharp reduction could not be compensated. However, for the BPV fed BG-11 + NH 4 Cl + Synechocystis, the cell potential reduction was exceedingly made up, revealing the impressive role of NH 4 Cl to compensate for the activation loss of metabolic reactions and extract electrons from light and water. Furthermore, the higher current densities obtained at lower external resistances show the augmented electron production and accentuate the stimulation of cell electrogenesis metabolic pathways. Low activation losses owned to the fact that phosphotransacetylase had a crucial role in converting acetate to acetyl-CoA, assisting the cell to maintain a balance between biosynthesis and energy production 34 . Therefore, the overexpression of this enzyme by NH 4 Cl could facilitate electrons transfer.
Moreover, the occurrence of the overshoot phenomenon resulting from sudden electron depletion at low external resistances that cannot be compensated by microorganisms 35 was observed in the BPV fed with all four types of culture media (Fig. 5B). The overshoot in the BPV fed with BG-11 + KCl + Synechocystis led to a more than 31% decrease in the current density (from 0.1913 to 0.1306 mA m −2 ). The culture medium used NH 4 Cl as an activator showed a better performance in compensating electron depletions and only brought about a 6.6% decrease in the produced current. This is another evidence emphasizing the crucial role of NH 4 Cl, which improves Synechocystis electrogenesis metabolic pathways.
The power density curves for three culture media were shown in Fig. 5C-E. The BPV fed by BG-11 + NH 4 Cl + Synechocystis achieved the maximum power density of 148.27 mW m −2 , which is more than 40.5-fold of what was obtained for the BPV fed with BG-11 + Synechocystis (Fig. 5C).
The maximum produced power density for BG-11 + KCl + Synechocystis was 10.97 mW m −2 (Fig. 5D). Despite the higher salt concentration, this power density was 13.5 times lower than the BPV fed by BG-11 + NH 4 Cl + Synechocystis. Achieving a higher power density by NH 4 Cl compared to KCl can be related to the fact that KCl had an adverse effect on some critical metabolic pathways, which is discussed in the Supplementary file (Table S6). Therefore, with regard to the role of KCl in the stimulation of glutamate dehydrogenase, it cannot be helpful in electrogenesis metabolism as much as NH 4 Cl.
Furthermore, to scrutinize the effect of NH 4 Cl on diminishing intracellular constraints, Synechocystis was removed from the BPV. In this case, the maximum produced power density for BG-11 + NH 4 Cl was 13.72 mW m −2 (Fig. 5E). This power density was more than tenfold lower than the maximum power density of the BPV fed with BG-11 + NH 4 Cl + Synechocystis. The abiotically produced power density is mainly attributed to the medium salinity 33,36 . Therefore, the presence of NH 4 Cl results in the metabolic stimulation of Synechocystis rather than the anolyte conductivity improvement. Thus, the addition of NH 4 Cl can play a critical role in shifting the Synechocystis metabolism to produce more electricity.
Finally, the average operating time of Synechocystis for each external resistance was about 70 h for BG-11 + NH 4 Cl, 52 h for BG-11 + KCl, and 40 h for BG-11. The polarization curves of Synechocystis for BG-11 + NH 4 Cl was obtained over 36 days compared to approximately 20 days for BG-11 + KCl and over 16 days for BG-11. Thus, the overall period of BPV operation under different culture media was also increased by the addition of regulators.
Biofilm morphology. FESEM micrographs of Synechocystis in BG-11 and BG-11 + NH 4 Cl were illustrated in Fig. 6. In both cases, a small portion of cells attached to the anode's surface. This low biofilm formation www.nature.com/scientificreports/ reported here is similar to those investigated by 5 in which 8.5% of Synechocystis attached to the anode's surface made of indium tin oxide-coated polyethylene terephthalate (ITO-PET). Consequently, the addition of NH 4 Cl to BG-11 had a small impact on the biofilm morphology, and the observed power density enhancement is mainly attributed to the improvement of electrogenesis metabolic pathways.

Discussion
The proposed systemic approach based on metabolic modeling identified bottleneck genes in metabolism for enhancing bioelectricity generation. To this end, LAMOS and BRENDA were applied for the initial screening of existing compounds for switching metabolism to improve NADH production, and therefore increase power density generation (Fig. 7). It is important to note that proposed system-oriented strategy is a screening that seeks to make suggestions from a large number of available components which the effect of their addition to the medium can be evaluated. This strategy suggested the components affect the enzyme of key bottleneck genes, and thus, increasing the chance of improving bioelectricity production. In this regard, LAMOS identified the reactions that had the highest potential in improving bioelectricity production, listed in Tables S1 and S2. Then, for each reaction, the potential regulators in the BRENDA database were searched. Besides, in addition to the experimentally reported effect of the regulator on the bottleneck genes, the regulator may be effective on some other enzymes; therefore, assessing the effect on the final product, which is the production of electrons, maybe the best solution. This method has also been implemented successfully in previous studies 13,37 leading to improved production. After an in-depth screening of all components in the BRENDA database, phosphotransacetylase, and glutamate dehydrogenase were selected to be experimentally investigated due to the following reasons. Firstly, since it is undeniable that regulators can affect other enzymes' activity as well 13 ; thus, the adverse effect of regulators on other metabolic pathways in improving power generation was investigated. In other words, as it was shown in Table S6, ammonium chloride, which is an activator of phosphotransacetylase, did not have a substantial adverse effect on other critical metabolic pathways. However, potassium chloride, an activator of glutamate dehydrogenase, possessed an adverse effect on some critical metabolic pathways, including the inhibitory effect on oxidative phosphorylation pathways (Table S6). It was concluded that using the regulators that had an adverse effect on crucial pathways could decrease the bioelectricity production, albeit its positive effect on the targeted reaction. It is worth mentioning that the detailed effects of additives on cellular metabolism cannot be accurately evaluated, but with the aid of BRENDA, an attempt has been made to provide an estimate of the regulator's effect on cellular metabolism, and components screening has been carried out based on this. Furthermore, for some enzymes, such as glycerol-3-phosphatedehydrogenase, no regulators were specified in the BRENDA database. Most recently, Lai et al. 24 also developed a recipe for BG11 (nBG11), compatible with the BPV operation. The nBG11 medium was modified by removing organic pH buffers (HEPES/TES), optimizing MnCl 2 concentration, and chelating trace metal mix solution (TMM) with EDTA. Optimizing culture medium using 1 mM ferricyanide as the mediator led to the long-term photo-current generation of about 20 µA from Synechocystis during repeated light-dark cycles for over 12 days. Similarly, in our study, redesigning the culture medium, particularly the supplementation of NH 4 Cl, generated a longer stationary phase (Fig. 4A), which shows a more stable current density. Moreover, Bombelli et al. 38 were also supplemented BG-11 with NaCl to study bioelectricity generation of Synechocystis; therefore, the closest minerals to NaCl were obtained and studied. www.nature.com/scientificreports/ Our results demonstrate that adding activators could provoke the Synechocystis as a strain engaged in noticeable intrinsic metabolic losses to reduce its activation overpotential significantly and accelerate bioelectricity generation. The overexpression of the phosphotransacetylase by NH 4 Cl exceedingly facilitates electrons transfer and stimulates cell electrogenesis metabolic pathways, which brings about lower activation losses. Although KCl led to an increase in glutamate dehydrogenase activity, the experimental results showed that it could not be helpful in electrogenesis metabolism as much as NH 4 Cl. The investigation of overshoot occurrence in the BPV also demonstrated the culture medium used NH 4 Cl as an activator produced a better performance in compensating electron depletions. Furthermore, the addition of NH 4 Cl had a small effect on the biofilm morphology.
Interestingly, our experimental studies showed that the addition of 40 mM NH 4 Cl resulted in a 49% decrease in optical density (OD750) in the strain culture, which is consistent with the previous study, representing a photodamage 39 . On the other hand, it was reported that ammonium chloride saves up to 30% of the reducing equivalents produced by the photosynthetic light reactions in cyanobacteria 40 . More specifically, nitrate needs to be reduced to ammonium via two sequential reactions catalyzed by nitrate reductase and nitrite reductase to be assimilated by cyanobacteria. The reductions of nitrate to nitrite to ammonium are Fd-dependent, consuming two and eight electrons (Table S7), respectively 41 . Thus, although ammonium chloride was reported to cause photodamage and reducing cell density, its role in reducing intracellular constraints, saving reducing equivalents, and facilitating electrons transfer is undeniable.
Additionally, this study focused on reducing intracellular constraints to enhance bioelectricity generation. However, some researches were conducted to minimize extracellular constraints to improve the BPV performance. Wenzel et al. 42 enhanced current generation from photosynthetic biofilms of Nostoc punctiforme and Synechocystis by using porous translucent electrodes in which a twofold increase in current generation was observed. Bombelli et al. 38 harnessed a high power density of 105 mW m −2 for Synechocystis by miniaturizing geometries using microfluidic BPV. In microfluidic devices, due to the reduction of the distance between the electrodes, ohmic losses are reduced 33 , and achieving the maximum power density is more feasible. Thus, diminishing intracellular constraints by the method presented in this study together with optimizing the BPV to reduce extracellular constraints, would be a big step towards the industrialization of this technology.
Future research should also aim to improve the electrogenesis metabolic pathways by implementing systems metabolic engineering approach 43 . Zhu et al. 44 developed a BPV with a constrained electron flow based on a d-lactate mediated consortium consisting of engineered Synechococcus elongatus UTEX 2973 and Shewanella oneidensis MR-1. The cyanobacteria harvested solar energy to produce d-lactate, which is subsequently used by the exoelectrogenic microorganism, releases electrons from it, and transfers them to the anode for bioelectricity generation. A power density of 150 mW·m −2 in a temporal separation setup was produced by implementing this method. Thus, unlike the addition of mediators such as potassium ferricyanide, which is unsuitable for large-scale 7 , it is more interesting to implement the systemic approach to introduce novel microbial interactions capable of sustainable bioelectricity production.
Besides, instead of supplementing the classic inhibitors such as DCMU and DBMIB, which were used to investigate the source of electrons in the photosynthetic organism, the addition of regulators identified by the systemic approach would bring about more benefits by improving electrical power generation. Finally, it is worth to mention that the produced power density of Synechocystis is still relatively lower than exoelectrogenic bacteria such as Shewanella or Geobacter. However, it would be interesting to implement this method to enhancing exoelectrogenic bacteria bioelectricity generation.