Overexpressing Ferredoxins in Chlamydomonas reinhardtii Increase Starch and Oil Yields and Enhance Electric Power Production in a Photo Microbial Fuel Cell

Whole-cell biophotovoltaic systems for renewable energy generation: A systematic analysis of existing knowledge

The development of carbon-neutral fuel sources is an essential step in addressing the global fossil energy crisis. Whole-cell biophotovoltaic systems (BPVs) are a renewable, non-polluting energy-generating device that utilizes oxygenic photosynthetic microbes (OPMs) to split water molecules and generate bioelectricity under the driving of light energy. Since 2006, BPVs have been widely studied, with the order magnitudes of power density increasing from 10-4 mW/m2 to 103 mW/m2. This review examines the extracellular electron transfer (EET) mechanisms and regulation techniques of BPVs from biofilm to external environment. It is found that the EET of OPMs is mainly mediated by membrane proteins, with terminal oxidase limiting the power output. Synechocystis sp. PCC6803 and Chlorella vulgaris are two species that produce high power density in BPVs. The use of metal nanoparticles mixing, 3D pillar array electrodes, microfluidic technology, and transient-state operation models can significantly enhance power density. Challenges and potential research directions are discussed, including a deeper analysis of EET mechanisms and dynamics, the development of modular devices, integration of multiple regulatory components, and the exploration of novel BPV technologies.

The Microalgae Chlamydomonas for Bioremediation and Bioproduct Production

The extensive metabolic diversity of microalgae, coupled with their rapid growth rates and cost-effective production, position these organisms as highly promising resources for a wide range of biotechnological applications. These characteristics allow microalgae to address crucial needs in the agricultural, medical, and industrial sectors. Microalgae are proving to be valuable in various fields, including the remediation of diverse wastewater types, the production of biofuels and biofertilizers, and the extraction of various products from their biomass. For decades, the microalga Chlamydomonas has been widely used as a fundamental research model organism in various areas such as photosynthesis, respiration, sulfur and phosphorus metabolism, nitrogen metabolism, and flagella synthesis, among others. However, in recent years, the potential of Chlamydomonas as a biotechnological tool for bioremediation, biofertilization, biomass, and bioproducts production has been increasingly recognized. Bioremediation of wastewater using Chlamydomonas presents significant potential for sustainable reduction in contaminants and facilitates resource recovery and valorization of microalgal biomass, offering important economic benefits. Chlamydomonas has also established itself as a platform for the production of a wide variety of biotechnologically interesting products, such as different types of biofuels, and high-value-added products. The aim of this review is to achieve a comprehensive understanding of the potential of Chlamydomonas in these aspects, and to explore their interrelationship, which would offer significant environmental and biotechnological advantages.

Enhancing biophotovoltaic efficiency: Study on a highly productive green algal strain Parachlorella kessleri MACC-38

Biophotovoltaic (BPV) devices are a potential decentralized and environmentally friendly energy source that harness solar energy through photosynthesis. BPV devices are self-regenerating, promising long-term usability. A practical strategy for enhancing BPV performance is to systematically screen for highly exoelectrogenic algal strains capable of generating large electric current density. In this study, a previously uncharacterized green algal strain - Parachlorella kessleri MACC-38 was found to generate over 340 µA mg-1 Chl cm-2. This output is approximately ten-fold higher than those of Chlamydomonas reinhardtii and Chlorella species. The current production of MACC-38 primarily originates from photosynthesis, and the strain maintains its physiological integrity throughout the process. MACC-38 exhibits unique traits such as low extracellular O2 and Fe(III) reduction, substantial copper (II) reduction, and significant extracellular acidification during current generation, contributing to its high productivity. The exoelectrogenic and growth characteristics of MACC-38 suggest that it could markedly boost BPV efficiency.

Roots Fuel Cell Produces and Stores Clean Energy

In recent years, extensive scientific efforts have been conducted to develop clean bioenergy technologies. A promising approach that has been under development for more than a hundred years is the microbial fuel cell (MFC) which utilizes exoelectrogenic bacteria as an electron source in a bioelectrochemical cell. The viability of bacteria in soil MFCs can be maintained by integrating plant roots, which release organic materials that feed the bacteria. In this work, we show that rather than organic compounds, roots also release redox species that can produce electricity in a biofuel cell. We first studied the reduction of the electron acceptor Cytochrome C by green onion roots. We integrate green onion roots into a biofuel cell to produce a continuous bias-free electric current for more than 24 h in the dark. This current is enhanced upon irradiation of the onion's leaves with light. We apply cyclic voltammetry and 2D-fluorescence measurements to show that NADH and NADPH act as major electron mediators between the roots and the anode, while their concentrations in the external root matrix are increased upon irradiation of the leaves. Finally, we show that roots can contribute to energy storage by charging a supercapacitor.

Genetic engineering of microalgae for enhanced lipid production

Microalgae have the potential to become microbial cell factories for lipid production. Their ability to convert sunlight and CO₂ into valuable lipid compounds has attracted interest from cosmetic, biofuel, food and feed industries. In order to make microalgae-derived products cost-effective and commercially competitive, enhanced growth rates and lipid productivities are needed, which require optimization of cultivation systems and strain improvement. Advances in genetic tool development and omics technologies have increased our understanding of lipid metabolism, which has opened up possibilities for targeted metabolic engineering. In this review we provide a comprehensive overview on the developments made to genetically engineer microalgal strains over the last 30 years. We focus on the strategies that lead to an increased lipid content and altered fatty acid profile. These include the genetic engineering of the fatty acid synthesis pathway, Kennedy pathway, polyunsaturated fatty acid and triacylglycerol metabolisms and fatty acid catabolism. Moreover, genetic engineering of specific transcription factors, NADPH generation and central carbon metabolism, which lead to increase of lipid accumulation are also reviewed.

Performance evaluation of a photosynthetic microbial fuel cell (PMFC) using Chlamydomonas reinhardtii at cathode

This study reports the use of Chlamydomonas reinhardtiiat the cathode in a photosynthetic microbial fuel cell (PMFC). The PMFC produced power and current density of 15.21 W m^-3 and 39 A m^-3, respectively. The specific growth rate of algae culture at the cathode was 0.284 day^-1. The system achieved COD removal at 73.30% with a Coulombic efficiency of 9.068%. The usability of algae biomass was assessed in terms of its total phenol content (157.69 mg GAE/g algae DW), antioxidant activity (IC₅₀ = 67.07 µg/ml), total Chlorophyll (18.95 mg/g), total Carotenoids (2.40 mg/g), and antibacterial properties against known pathogens. Overall, the study's findings suggested thatC. reinhardtiisupports high power output from a PMFC and is highly resourceful in terms of value-added products.

Enhancement of exopolysaccharides production and reactive oxygen species level of Nostoc flagelliforme in response to dehydration

Nostoc flagelliforme is a remarkable drought-resistant terrestrial cyanobacterium whose exopolysaccharides (EPS) have been found to exert important physiological and ecological functions, and the EPS are known to improve soil physicochemical properties. In this study, we used physiological and molecular methods to investigate the influences of three moisture loss levels on EPS production and the antioxidant system in N. flagelliforme. The aim was to reveal the EPS production mechanism involved in the gene differential expression and antioxidant system of N. flagelliforme in response to drought. Our results showed that EPS contents increased by 13% and 22% after 6-h and 48-h dehydration (6HAD and 48HAD) compared with 4-h rehydration (4HAR), respectively. The same trends were also detected for most EPS synthesis genes, especially glycosyltransferases. Furthermore, the intracellular reactive oxygen species (ROS) levels in N. flagelliforme were generally higher at 6HAD and 48HAD than at 4HAR. Superoxide dismutase (SOD) and peroxidase (POD) activities were restricted in N. flagelliforme under 6HAD and 48HAD compared with 4HAR, but the opposite result was found in catalase (CAT) activity. These results provide a new foundation for understanding the mechanism of EPS accumulation in N. flagelliforme in response to drought.

Comparative secretome analysis between salinity-tolerant and control Chlamydomonas reinhardtii strains

Secretome analysis of a salt-tolerant and control Chlamydomonas reinhardtii revealed 514 differentially expressed proteins. Membrane transport and trafficking, signal transduction and channel proteins were up-regulated in the ST secretome. Salinity is a major abiotic stress that limits crop production worldwide. Multiple adverse effects have been reported in many living organisms exposed to high-saline concentrations. Chlamydomonas reinhardtii is known for secreting proteins in response to many environmental stresses. A salinity-tolerant (ST) strain of Chlamydomonas has been developed, whose cells were able to grow at 300 mM NaCl. The current study analyzed the secretomes of ST grown in TAP medium supplemented with 300 mM NaCl and the laboratory strain CC-503 grown in TAP medium without NaCl supplement. In total, 514 secreted proteins were identified of which 203 were up-regulated and 110 were down-regulated. Bioinformatic analysis predicted 168 proteins to be secreted or in the conventional secretory pathway. Out of these, 70 were up-regulated, while 51 proteins were down-regulated. Proteins involved in membrane transport and trafficking, signal transduction and channel proteins were altered in their expression in the ST secretome, suggesting the response of saline stress acts toward not only the intracellular pool of proteins but also the extracellular proteins. This also suggested that the secreted proteins might have roles in the extracellular space. Signal peptide (SP) prediction revealed that almost 40% of the predicted secreted proteins contained a signal peptide; however, a high proportion of proteins lacked an SP, suggesting that these proteins might be secreted through an unconventional protein secretion pathway.

NADPH performs mediated electron transfer in cyanobacterial-driven bio-photoelectrochemical cells

Previous studies have shown that live cyanobacteria can produce photocurrent in bio-photoelectrochemical cells (BPECs) that can be exploited for clean renewable energy production. Electron transfer from cyanobacteria to the electrochemical cell was proposed to be facilitated by small molecule(s) mediator(s) whose identity (or identities) remain unknown. Here, we elucidate the mechanism of electron transfer in the BPEC by identifying the major electron mediator as NADPH in three cyanobacterial species. We show that an increase in the concentration of NADPH secreted into the external cell medium (ECM) is obtained by both illumination and activation of the BPEC. Elimination of NADPH in the ECM abrogates the photocurrent while addition of exogenous NADP⁺ significantly increases and prolongs the photocurrent production. NADP⁺ is thus the first non-toxic, water soluble electron mediator that can functionally link photosynthetic cells to an energy conversion system and may serve to improve the performance of future BPECs.

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NADPH is the electron mediator in cyanobacterial bio-photoelectrochemical cells

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Operation of the electrochemical cell induces NADPH release from cyanobacteria

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Addition of exogenous NADP⁺ to cyanobacteria enhances photocurrent production

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NADPH is released by different fresh or sea water cyanobacterial species

Salinity Stress Responses and Adaptation Mechanisms in Eukaryotic Green Microalgae

High salinity is a challenging environmental stress for organisms to overcome. Unicellular photosynthetic microalgae are especially vulnerable as they have to grapple not only with ionic imbalance and osmotic stress but also with the generated reactive oxygen species (ROS) interfering with photosynthesis. This review attempts to compare and contrast mechanisms that algae, particularly the eukaryotic Chlamydomonas microalgae, exhibit in order to immediately respond to harsh conditions caused by high salinity. The review also collates adaptation mechanisms of freshwater algae strains under persistent high salt conditions. Understanding both short-term and long-term algal responses to high salinity is integral to further fundamental research in algal biology and biotechnology.

The Development of Biophotovoltaic Systems for Power Generation and Biological Analysis

Biophotovoltaic systems (BPVs) resemble microbial fuel cells, but utilise oxygenic photosynthetic microorganisms associated with an anode to generate an extracellular electrical current, which is stimulated by illumination. Study and exploitation of BPVs have come a long way over the last few decades, having benefited from several generations of electrode development and improvements in wiring schemes. Power densities of up to 0.5 W m-2 and the powering of small electrical devices such as a digital clock have been reported. Improvements in standardisation have meant that this biophotoelectrochemical phenomenon can be further exploited to address biological questions relating to the organisms. Here, we aim to provide both biologists and electrochemists with a review of the progress of BPV development with a focus on biological materials, electrode design and interfacial wiring considerations, and propose steps for driving the field forward.

Ferredoxin5 Deletion Affects Metabolism of Algae during the Different Phases of Sulfur Deprivation

Ferredoxin5 (FDX5), a minor ferredoxin protein in the alga Chlamydomonas (Chlamydomonas reinhardtii), helps maintain thylakoid membrane integrity in the dark. Sulfur (S) deprivation has been used to achieve prolonged hydrogen production in green algae. Here, we propose that FDX5 is involved in algal responses to S-deprivation as well as to the dark. Specifically, we tested the role of FDX5 in both the initial aerobic and subsequent anaerobic phases of S-deprivation. Under S-deprived conditions, absence of FDX5 causes a distinct delay in achieving anoxia by affecting photosynthetic O2 evolution, accompanied by reduced acetate uptake, lower starch accumulation, and delayed/lower fermentative metabolite production, including photohydrogen. We attribute these differences to transcriptional and/or posttranslational regulation of acetyl-CoA synthetase and ADP-Glc pyrophosphorylase, and increased stability of the PSII D1 protein. Interestingly, increased levels of FDX2 and FDX1 were observed in the mutant under oxic, S-replete conditions, strengthening our previously proposed hypothesis that other ferredoxins compensate in response to a lack of FDX5. Taken together, the results of our omics and pull-down experiments confirmed biochemical and physiological results, suggesting that FDX5 may have other effects on Chlamydomonas metabolism through its interaction with multiple redox partners.

Antimicrobial properties of acrylic resins doped with Undaria pinnatifida exposed to light-emitting diode: In silico and in vitro assessments on multispecies biofilm-producing microbiota

BACKGROUND

This study sought to evaluates the efficiency of anti-microbial activity of acrylic resins doped with different concentrations of Undaria pinnatifida after activation with light-emitting diode (LED) at producing photodynamic damage to multispecies biofilm-producing microbiome.

MATERIAL AND METHODS

In this study, bioinformatics tools and computer simulation molecular modeling were used to evaluate the capacity of ferredoxin (FDX), an electron acceptor in metabolic pathways of U. pinnatifida, which can discharge electrons produced from photo-excited chlorophyll-a (Chl-a) by LED irradiation. Acrylic resin discs containing different concentration of U. pinnatifida (0, 0.5, 1, and 2%) were fabricated and were subjected to LED irradiation immediately before each experiment. After continuously rinsed (up to 30 days), the antimicrobial activity of acrylic resins doped with U. pinnatifida following photo-activation was determined by disc agar diffusion, biofilm formation inhibition, and eluted component assays versus bacterial species linked to caries that constitute a mixed biofilm including Streptococcus mutans, S. sanguinis, and Lactobacillus acidophilus, as well as Candida albicans as main etiology of candidal stomatitis.

RESULTS

Modeling and a virtual screening analysis of FDX indicated that it is a stable protein with an iron-sulfur center that can discharge electrons produced from photo-excited Chl-a and transfers them to FDX-NADP+ reductase for NADP+ reduction in photosystem I, which is essential in the Calvin cycle for carbon assimilation. FDX acts as an electron transfer agent in the redox reactions. The results showed that growth inhibition zones were not seen around acrylic resin discs in any group. In biofilm test, the colony counts of all test microorganisms significantly decreased (36%-87%) by an increase in the percentage of U. pinnatifida in acrylic resins after photo-activation (P < 0.05). Acrylic resins doped with 2% wt. U. pinnatifida following photo-activation using LED was inhibited biofilm formation by the test microorganisms, up to 30 days of rinsing.

CONCLUSION

Based on the results presented here, an acrylic resin containing U. pinnatifida, even at the lowest concentration, following photo-activation using LED have antimicrobial properties against planktonic and biofilm forms of the cariogenic microorganisms as well as C. albicans.

Enhancing Stability of Microalgae Biocathode by a Partially Submerged Carbon Cloth Electrode for Bioenergy Production from Wastewater

The electricity output from microbial fuel cell (MFC) with a microalgae assisted cathode is usually higher than that with an air cathode. The output of electricity from a photosynthetic microalgae MFC was positively correlated with the dissolved oxygen (DO) level in the microalgae assisted biocathode. However, DO is highly affected by the photosynthesis of microalgae, leading to the low stability in the electricity output that easily varies with the change in microalgae growth. In this study, to improve the electricity output stability of the MFC, a partially submerged carbon cloth cathode electrode was first investigated to use oxygen from both microalgae and air, with synthetic piggery wastewater used as the anolyte and anaerobically digested swine wastewater as the catholyte. When the DO levels dropped from 13.6–14.8 to 1.0–1.6 mg/L, the working voltages in the MFCs with partially submerged electrodes remained high (256–239 mV), whereas that for the conventional completely submerged electrodes dropped from 259 to 102 mV. The working voltages (average, 297 ± 26 mV) of the MFCs with the 50% submerged electrodes were significantly (p < 0.05) higher than with other partially or completely submerged electrodes. The associated maximum lipid production from wastewater was 250 ± 42 mg/L with lipid content of 41 ± 6% dry biomass. Although the partially submerged electrode had no significant effects on lipid production or nitrogen removal in wastewater, there was significant improvement in the stability of the electricity generated under variable conditions.

Investigation of photocurrents resulting from a living unicellular algae suspension with quinones over time

Plants, algae, and some bacteria convert solar energy into chemical energy by using photosynthesis. In light of the current energy environment, many research strategies try to benefit from photosynthesis in order to generate usable photobioelectricity. Among all the strategies developed for transferring electrons from the photosynthetic chain to an outer collecting electrode, we recently implemented a method on a preparative scale (high surface electrode) based on a Chlamydomonas reinhardtii green algae suspension in the presence of exogenous quinones as redox mediators. While giving rise to an interesting performance (10-60 μA cm-2) in the course of one hour, this device appears to cause a slow decrease of the recorded photocurrent. In this paper, we wish to analyze and understand this gradual fall in performance in order to limit this issue in future applications. We thus first show that this kind of degradation could be related to over-irradiation conditions or side-effects of quinones depending on experimental conditions. We therefore built an empirical model involving a kinetic quenching induced by incubation with quinones, which is globally consistent with the experimental data provided by fluorescence measurements achieved after dark incubation of algae in the presence of quinones.

Photosynthetic Microbial Fuel Cells

This chapter presents the current state of research on bioelectrochemical systems that include phototrophic organisms. First, we describe what is known of how phototrophs transfer electrons from internal metabolism to external substrates. This includes efforts to understand both the source of electrons and transfer pathways within cells. Second, we consider technological progress toward producing bio-photovoltaic devices with phototrophs. Efforts to improve these devices by changing the species included, the electrode surfaces, and chemical mediators are described. Finally, we consider future directions for this research field.