Abstract:

The development of renewable energy technologies has become a crucial strategic direction in the face of the dual challenges of climate change and energy crises. Bio-photovoltaic (BPV) systems are an emerging solar technology that harnesses microorganisms to convert light energy into electrical energy, demonstrating significant potential in terms of cost-effectiveness and environmental friendliness. Traditional BPV systems predominantly utilize a single algal species (either cyanobacteria or eukaryotic algae) as photosynthetic microorganisms. These systems rely on intracellular electron transport chains (ETC) and extracellular electron transfer (EET) mechanisms to transfer high-energy electrons to the anode, thereby generating an electric current. However, the relatively weak EET capacity of algae has limited further performance improvements, prompting researchers to explore various optimization strategies. In recent years, researchers have achieved a notable enhancement in BPV system performance by constructing a photosynthetic-electrogenic microbial combination system composed of cyanobacteria and Shewanella, realizing the spatiotemporal decoupling of photosynthetic charging and biological discharge. Moreover, as an energy technology closely related to biological processes, BPV systems are significant not only in the energy field but also in relation to the carbon cycle mechanisms of ecosystems. Therefore, in-depth research on the regulatory mechanisms of light-driven processes in carbon cycling and energy conversion is of significant ecological importance. In lake ecosystems, the carbon cycle is a key process for maintaining ecological balance, with the interaction between phytoplankton (represented by algae) and heterotrophic microorganisms being the core factor that drives this process. The carbon balance between photosynthesis and respiratory decomposition mediated by these interactions governs carbon cycling in lakes and determines the accumulation and release of carbon in lake sediments. The relationship between algae and heterotrophic microorganisms is of paramount ecological significance in aquatic ecosystems. It exerts a profound influence on the cycling of carbon and nutrients and affects the exchange of climate-related chemicals between ecosystems and the atmosphere. Thus, this study speculates that in BPV systems, the combination of heterotrophic bacteria and algae may present a different balance between power generation performance and CO₂ fixation ability compared to traditional BPV systems. However, the mechanisms and system efficiencies of these combinations remain unknown. Bacillus subtilis has garnered significant attention because of its powerful ability to decompose organic substrates, quorum-sensing regulatory mechanisms, and biofilm formation characteristics. These traits may enable it to influence carbon flow distribution within the system by reshaping algal-bacterial interaction networks. Based on this, the present study selected B. subtilis as the representative of heterotrophic microorganisms and Chlamydomonas reinhardtii and Microcystis aeruginosa as representatives of algae to construct five groups of BPV systems: M. aeruginosa+B. subtilis (M+BS), C. reinhardtii+B. subtilis (C+BS), M. aeruginosa+blank (M+BL), C. reinhardtii+blank (C+BL), and blank+B. subtilis (BL+BS). This study aimed to explore the differentiated utilization ability of B. subtilis for algal metabolites and whether this ability could enhance the CO₂ fixation efficiency of the system by optimizing the carbon metabolic network. The findings revealed that the two groups without B. subtilis (M+BL and C+BL) exhibited weak power generation capabilities and low current densities in the three-chamber BPV system. In contrast, the three groups with B. subtilis (M+BS, C+BS, and BL+BS) demonstrated significantly enhanced power generation capabilities and more stable current densities throughout the experimental period than the other groups did. Moreover, the total coulombic volumes in the M+BS and C+BS groups increased by 7.9 and 6.5 times, respectively, compared with those in the M+BL and C+BL groups. The cyclic voltammetry (CV) curves show distinct oxidation peaks near 0.9 V (vs. Ag/AgCl), indicating the oxidation of extracellular polymers and aromatic electron shuttles. The C+BS system exhibited the highest response current, suggesting that its electroactive bacteria possessed superior extracellular electron transfer (EET) capability. In algae-bacteria interactive BPV systems, energy conversion is achieved through the synergistic metabolism of both organisms. The algal biomass in the control groups without bacterial inoculation accumulated slowly, whereas it decreased in the algae-bacteria coupling systems, particularly in M. aeruginosa. The metabolic activities of B. subtilis intensified the nutrient-limiting pressures within the system, resulting in a decline in algal biomass. In the bacterium-containing systems, the OD₆₀₀ values of B. subtilis decreased, with a relatively slower decline observed in the algae-bacteria combination systems. This could be attributed to the limitations of horizontal mass transfer on the cross-chamber transfer efficiency of algal organic matter and the uncertain suitability of algal metabolic products for the growth of B. subtilis. Additionally, M. aeruginosa may produce toxic secondary metabolites, microcystin toxins, which migrate to the anode chamber and inhibit the growth of B. subtilis. Regarding CO₂ fixation, the net CO₂ concentration in the C+BS group, which combined C. reinhardtii and B. subtilis, remained consistently negative and significantly lower than that of the single-algal systems, indicating that the presence of B. subtilis markedly enhanced the CO₂ fixation ability of C. reinhardtii. In terms of TOC concentration changes, the TOC in the algal chambers of the systems without B. subtilis exhibited dynamic changes corresponding to the light/dark cycle. After the introduction of the dual-species algae-bacteria systems, the variation range of TOC concentration in each algal and anode chamber increased significantly, showing large fluctuations, which reflected the synergistic effects between algae and bacteria. Carbon balance analysis indicated that the calculable carbon content in the systems containing B. subtilis showed a marked downward trend, suggesting that B. subtilis plays a significant role in carbon transformation in algal BPV systems. Observations of microbial adhesion on the anode surface revealed that a certain degree of anode biofilm attachment was observed in the systems inoculated with B. subtilis, with the C+BS system exhibiting a more pronounced anode biofilm than the C+S system. Metabolite analysis showed that the composition of metabolites in the algal chambers was primarily dominated by B. subtilis, and its metabolic activities affected the growth and metabolism of algae, thereby influencing the carbon cycle of the system. Overall, algae and bacteria interact through the secretion of specific metabolites that affect their growth and, consequently, the carbon flow distribution in the entire system. These findings provide a theoretical basis for the functional design and optimization of microbial communities in complex BPV systems and offer insights into improving power generation efficiency and enhancing CO₂ fixation capacity through multispecies collaboration.

Key words: Chlamydomonas reinhardtii, Microcystis aeruginosa, Bacillus subtilis, bio-photovoltaic (BPV), CO₂ fixation, electrogenesis

摘要：

生物光伏（BPV）是利用微生物将光能转化为电能的生物电化学系统，具有明显的成本效益和环境友好特性。在BPV系统中，藻类和异养菌的组合可能在产电性能与CO₂固定能力间呈现新的平衡关系，但其作用机制和系统效能仍需进一步研究。以铜绿微囊藻（Microcystis aeruginosa）和莱茵衣藻（Chlamydomonas reinhardtii）为藻类代表，以枯草芽孢杆菌（Bacillus subtilis）为异养细菌代表，构建不同组合的藻菌互作型BPV系统。通过监测体系中的产电性能、CO₂含量、溶解性有机碳等的动态变化，评估两种藻分别耦合B. subtilis的产电能力与CO₂固定效应。 结果表明，M. aeruginosa和C. reinhardtii与B. subtilis的组合均表现出一定程度的产电能力，其峰值电流密度为36.7 mA·m⁻²。藻菌组合体系中的库伦总量较单一藻体系提高了6－7倍。在CO₂固定方面，C. reinhardtii+B. subtilis组的CO₂固定能力相较于单一C. reinhardtii组提高了3.32倍。这表明藻菌耦合系统不仅表现出较好的产电性能，而且还具备更强的CO₂固定能力。这也为在复杂系统中强化不同菌群职能，利用多物种协同提高产电效率与强化CO₂固定能力提供了思路。

关键词: 莱茵衣藻, 铜绿微囊藻, 枯草芽孢杆菌, 生物光伏（BPV）, CO₂固定, 产电