Graphitic carbon nitride (g-C3N4) has emerged as a promising metal-free semiconductor for solar-driven environmental applications. However, its performance in photoelectrochemical systems is often limited by rapid charge recombination and poorly understood interfacial charge transport processes. In particular, the occurrence of both anodic and cathodic photocurrents in g-C3N4-based photoelectrodes under different applied biases has been widely reported, revealing a dual photoresponse behavior that remains mechanistically unexplained. In this work, g-C3N4 thin films were fabricated on fluorine-doped tin oxide substrates by electrophoretic deposition using guanidinium chloride and urea as nitrogen-rich precursors. Structural and surface characterization confirmed the formation of a porous, defect-rich carbon nitride framework with compact in-plane packing and mesoporous features. Optical analysis revealed an average band gap of 2.82 eV with limited visible-light absorption. The photoelectrochemical performance of the films was systematically evaluated as a function of film thickness and illumination direction, either through the electrode–electrolyte or substrate–electrode interface. Open-circuit potential measurements and transient photocurrent analyses demonstrated a bias-dependent switching between anodic and cathodic photocurrent responses at approximately 0.18 V versus saturated calomel electrode. Back illumination consistently produced higher photocurrent densities, suggesting that charge transport limitations within the film significantly influence the dominant photoresponse mechanism. Kinetic analysis of photocurrent transients indicated that recombination processes, rather than carrier generation, govern the observed dual behavior. These findings provide new insight into the interfacial charge transport mechanisms in carbon nitride thin films and establish synthesis–performance relationships relevant for the design of systems for solar-driven water treatment and environmental remediation.

Purple phototrophic bacteria (PPB) exhibit diverse metabolic strategies to cope with excess carbon and metabolic constraints. Here, we report a previously unrecognized overflow mechanism in PPB: the transient extracellular release of crotonate as a metabolic escape valve. In batch photoheterotrophic cultures of enriched PPB, crotonate accumulated under carbon-excess conditions when conventional metabolic sinks were constrained. Crotonate excretion coincided with depletion of polyhydroxybutyrate and H₂ production and was reversible upon addition of an alternative electron acceptor (DMSO), indicating a regulated overflow rather than irreversible fermentation. Metaproteomic analysis showed that dominant Rhodopseudomonas species redirect acetyl-CoA metabolism toward crotonyl-CoA formation when glyoxylate shunt activity is suppressed. A CoA-transferase was identified as a candidate enzyme enabling conversion of crotonyl-CoA to free crotonate for excretion. These findings reveal crotonate excretion as a naturally emerging overflow phenotype in wild-type PPB under defined metabolic constraints.

Refinery oily sludge is typically managed via the API-DAF-biological treatment train, generating residual sludges rich in hydrocarbons and nutrients that are often disposed of with limited valorization. In this work, a photobiorefinery concept coupling wet air oxidation (WAO) with purple phototrophic bacteria (PPB) is evaluated using a realistic mixture of API, DAF and biological sludges at their production ratio. WAO effluents obtained at different severities were screened for biodegradability and further investigated under selected conditions. Effluent generated at 200 degrees C supported robust PPB growth, with WAO liquors at 33% (v/v) providing the best compromise between substrate availability and inhibition. In kinetic batch experiments, PPB exhibited similar maximum growth rates when treating WAO effluents obtained at 200 degrees C and 1-3 h, with soluble COD removals of 1453-1545 mg/L (45-48%) together with 26-37% of NH4+, but achieving higher biomass yields and faster carboxylic acid removal at 1 h. In contrast, ammonium and soluble COD reached partial plateaus, consistent with a persistent soluble fraction enriched in oxidized aromatic and N-heterocyclic compounds. Amplicon sequencing revealed a stable PPB core (Cereibacter, Rhodopseudomonas and Rhodocista), complemented by heterotrophic taxa associated with degradation of complex organics. Overall, these results delineate a WAO operating window that maximizes PPB-treatable carbon and support integrating WAO and PPB for valorization of real refinery sludge mixtures.

Electrochemical biosensors based on purple phototrophic bacteria (PPB) offer a low-cost, real-time alternative to conventional toxicant detection methods. This study explored a single-chamber PPB-based biosensor for early-range detection of glyphosate in aquatic environments (1–30 mg/L). The results show that the pure culture Rhodobacter capsulates (R. capsulatus) is highly sensitive to low-to-medium-high glyphosate concentrations, whereas the mixed PPB-enriched consortium system exhibits stronger responses at low concentrations and maintains stable signals at higher concentrations, displaying characteristic cross-over behavior that reflects functional differentiation within the microbial systems. Mechanistic analysis indicates that glyphosate suppresses substrate metabolism and photosynthetic pigment synthesis, thereby reducing electron donor generation while increasing resistance to extracellular electron transfer, ultimately leading to decreased electron flux. Principal component analysis further confirms that the current signal arises from the coupling of metabolic activity and electrochemical processes rather than a single concentration-dependent response. The system requires no electrode modification, achieves a detection limit of 0.121 mg/L, and exhibits good stability and adaptability in complex aquatic environments. This study provides a new bioelectrochemical strategy for rapid screening and tiered monitoring of high-load pollutants.

The management of pig slurry is associated with environmental concerns due to its high nitrogen content and greenhouse gas emissions. This work proposes a photobiorefinery concept integrating steam explosion pretreatment, ammonia recovery via gas-permeable membranes, anaerobic digestion, and microbial protein production using purple phototrophic bacteria (PPB). Pretreatment at 145 °C for 30 min led to 57% solubilization of organic matter and 29% reduction of total solids. More than 99% of ammoniacal nitrogen was recovered from the hydrolysate in less than 3 h. Methane production increased by up to 350% after pretreatment. The recovered nitrogen was used as a nutrient for PPB growth, enabling CO2 assimilation from biogas under photoautotrophic conditions and microbial protein production. The results demonstrate the feasibility of coupling thermal, biological, and phototrophic processes for the integrated valorization of pig slurry, with simultaneous recovery of energy and nutrients.

Purple phototrophic bacteria (PPB) are renowned for versatile metabolic strategies to maintain redox balance, including CO₂ fixation, H₂ production, and polymer storage. Here we report a previously unknow redox-balancing mechanism in PPB: the extracellular release of crotonate as a metabolic overflow. In batch photoheterotrophic cultures of enriched PPB, crotonate accumulated transiently under conditions of carbon excess. Crotonate excretion coincided with the depletion of other electron sinks (polyhydroxybutyrate and H₂) and was reversed when an alternate electron acceptor (DMSO) was provided, indicating a regulated overflow rather than irreversible fermentation. Using metaproteomics, we found that dominant Rhodopseudomonas species redirect metabolism toward crotonyl-CoA production when conventional acetyl-CoA assimilation routes are limited. Key enzymes of the glyoxylate cycle were suppressed, while enzymes converting acetyl-CoA to crotonyl-CoA were up-regulated, leading to crotonyl-CoA accumulation. Notably, a CoA-transferase enzyme was identified as a candidate responsible for converting crotonyl-CoA to free crotonate, enabling excretion. These results reveal crotonate as an “escape valve” metabolite that PPB naturally deploy to dissipate excess reductant. The discovery of crotonate overflow expands our understanding of microbial redox homeostasis and highlights a novel facet of PPB metabolism with potential implications for optimizing biotechnological processes such as bioplastic production

Purple Phototrophic Bacteria (PPB), owing to their unique metabolism and electron transfer capabilities, hold great promise for application in microbial electrochemical systems (MES). This study proposes a new strategy by incorporating solid waste-derived materials (HySludge, HyGreen, and HyOrange) produced by HydroThermal Carbonization (HTC), as functional electroactive carbonaceous materials in PPB-based MES. The study focuses on evaluating the impact of these materials on PPB growth, electrochemical reaction, and microbial community composition under both non-polarized and polarized conditions, with Graphite serving as a control. This study addresses 3 core issues: a) the potential of hydrochars to serve as a stable platform for attachment and electron exchange between PPB and electroactive bacteria (EAB); b) the feasibility of achieving effective extracellular electron transfer (EET) through surface functional groups, despite low electrical conductivity of materials; and c) the capacity of hydrochars to generate electron output under light-driven conditions. The results indicate that HySludge (sludge-derived hydrochar) supported efficient PPB growth and nutrient uptake under non-polarized conditions, achieving removal efficiencies of 95.2 % for acetate and 91.9 % for NH4+. Polarization further enhanced the synergistic coexistence of photoelectroactive and EAB communities such as Rhodopseudomonas, Cereibacter, and Pseudomonas in HySludge systems. It achieved complete removal of acetate and NH4+, generated current density of 1.6 A/m3 with a coulombic efficiency of 1.1 %. Although its conductivity is inferior to that of Graphite, HySludge still demonstrated electrochemical functionality and biological compatibility, indicating its potential as a viable alternative to conventional electrode material.

In this study, a conceptual bioelectrochemical cell coupled with electroactive constructed wetland (ECW) and purple phototrophic bacteria (PPB) (termed as PPB-ECW) was constructed to systematically investigate its mechanisms and operational characteristics in wastewater treatment and electricity generation. A porous pottery pot was employed to replace the conventional proton exchange membrane, forming an open and electroactive environment. In the anode region, typical electroactive bacteria (EAB), represented by Hydrogenophaga and Ignavibacterium, were enriched and primarily facilitated efficient pollutant removal through the oxidation of organic substrates and electron release. In contrast, the cathode was dominated by Rhodopseudomonas palustris, a phototrophic and electroactive bacterium, which established an electrosyntrophic metabolic network characterized by electron transfer and light-driven energy utilization. The direct electron transfer (DET) of cathode PPB mainly depends on the outer membrane c-type cytochrome combined with light-driven cyclic electron flow. This coupling mechanism significantly enhanced pollutant degradation efficiency, with removal efficiencies of COD, TN, NH4+-N, and NO3--N reaching 98 %, 87 %, 88 %, and 100 %, respectively, among which NO3--N exhibited the most pronounced removal. The system achieved a maximum voltage output of 246 mV and a peak power density of 31 mW/m3. Compared with existing bioelectrochemical systems studies based on phototrophic microorganisms, this study systematically explained the synergistic mechanisms between anodic and cathodic microbial communities, and revealed the advantages of PPB in electron uptake and metabolism at the cathode. These findings provide both conceptual support and potential practical implications to further expand the scope of bioelectrochemical-based CW for wastewater treatment and energy recovery.

Reducing greenhouse gas emissions is critical for humanity nowadays, but it can be beneficial by developing engineered systems that valorize CO2 into commodities, thus mimicking nature's wisdom. Purple phototrophic bacteria (PPB) naturally accept CO2 into their metabolism as a primary redox sink system in photo-heterotrophy. Dedicated use of this feature for developing sustainable processes (e.g., through negative-emissions photo-bioelectrosynthesis) requires a deep knowledge of the inherent metabolic mechanisms. This work provides evidence of tuning the PPB metabolic mechanisms upon redox stressing through negative polarization (-0.4 and -0.8 V vs. Ag/AgCl) in photo-bioelectrochemical devices. A mixed PPB-culture upregulates its ability to capture CO2 from organics oxidation through the Calvin-Besson-Bassam cycle and anaplerotic pathways, and the redox imbalance is promoted to polyhydroxyalkanoates production. The ecological relationship of PPB with mutualist bacteria stabilizes the system and opens the door for future development of photo-bioelectrochemical devices focused on CO up-cycling.

BACKGROUNDPoly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) is the most promising biopolymer for replacing petroleum-derived plastics from polyhydroxyalkanoates (PHA) copolymers. The mechanical properties of PHBV improve as the content of the 3-hydroxyvalerate monomer (3HV) increases. However, there is no information about the effect of light/dark cycles on PHA composition produced by phototrophic purple bacteria. This work aimed to improve the molar fraction of 3HV in the PHBV copolymer formed by a mixed phototrophic culture enriched in Rhodopseudomonas palustris, combining the effect of the illumination regime and using odd-chain fatty acids and cheese whey as carbon sources.RESULTSUnder continuous lighting, the PHBV concentration (mg L-1) and 3HV molar fraction (%) were 508 +/- 25 mg L-1 (1%), 193 +/- 10 mg L-1 (22%), 252 +/- 10 mg L-1 (1%), and 286 +/- 10 mg L-1 (3%) for valerate, propionate, lactate, and fermented cheese whey, respectively. Interestingly, under light/dark cycles, the 3HV molar fraction increased 23-fold when using valerate (light/dark cycle of 30 min), 12-fold with lactate, and 1.5-fold with fermented cheese whey. The PHBV content (% w/w) reached was 20-25%, 14-16%, 12-15%, and 8-12% for valerate, propionate, lactate, and fermented cheese whey, respectively.CONCLUSIONCombining odd-chain fatty acids and light-dark cycles was an excellent strategy to improve the 3HV molar fraction in PHBV. This study shows for the first time that the 3HV molar fraction production is positively affected by light/dark cycles using odd-chain fatty acids. Using light/dark cycles brings a double benefit: improving the molar fraction of 3HV in the polymer and decreasing the illumination-associated cost. (c) 2024 Society of Chemical Industry (SCI).

Hydrothermal carbonization (HTC) is a highly efficient and valuable technology for treating wet solid wastes and producing solid carbon-based materials named hydrochar. In this work, a hydrochar coming from the HTC of an anaerobic digestion sludge of wastewater treatment plant was used to assess the influence of several activation agents, a base (KOH) and different chloride salts (FeCl3, ZnCl2, and CuCl2) with the exact molar quantities, to develop materials with enhanced surface area and potential inclusion of metal active species for application in wet air oxidation processes. The KOH as an activating agent increased the surface area of hydrochar up to ca. 1000 m2/g of BET surface area. The employment of CuCl2 and FeCl3 as activating agents allows Cu- and Fe-rich doped materials of remarkable surface areas with 49.1 and 42.5 wt.% of each metal, respectively. Likewise, the catalytic behavior of the different synthesized carbon-based materials as metal-free and metal-doped catalysts was evaluated for the Catalytic Wet Air Oxidation (CWAO) of a HTC aqueous liquor from a HTC process of animal manure to produce a valuable stream of higher biochemical methane potential in anaerobic digestion. CWAO effluents increased the proportion of carboxylic acids as final by-products due to the oxidation of more complex organic compounds of the initial effluent (ketones, phenols, aromatics and olefins). The CWAO treatments improve the anaerobic digestion rate in biochemical methane potential tests, although the methane production was limited by the lower TOC concentration of the treated streams after CWAO. This research contributes to developing sustainable and efficient strategies for the HTC-liquor treatment, using its solid hydrochar as catalysts, closing the loop of a Circular Economy.

Purple phototrophic bacteria (PPB) show an underexplored potential for resource recovery from wastewater. Raceway reactors offer a more affordable full-scale solution on wastewater and enable useful additional aerobic processes. Current mathematical models of PPB systems provide useful mechanistic insights, but do not represent the full metabolic versatility of PPB and thus require further advancement to simulate the process for technology development and control. In this study, a new modelling approach for PPB that integrates the photoheterotrophic, and both anaerobic and aerobic chemoheterotrophic metabolic pathways through an empirical parallel metabolic growth constant was proposed. It aimed the modelling of microbial selection dynamics in competition with aerobic and anaerobic microbial community under different operational scenarios. A sensitivity analysis was carried out to identify the most influential parameters within the model and calibrate them based on experimental data. Process perturbation scenarios were simulated, which showed a good performance of the model.

In this study, a photoanaerobic PPB-based biological system for the treatment of refinery wastewater and oily sludge pretreated by thermal hydrolysis has been studied within the framework of sustainability.

This work shows the potential of a new way of co-treatment of domestic wastewater (DWW) and a liquid stream coming from the thermal hydrolysis of the organic fraction of municipal solid waste (OFMSW) mediated by a mixed culture of purple phototrophic bacteria (PPB) capable of assimilating carbon and nutrients from the medium. The biological system is an open single-step process operated under microaerophilic conditions at an oxidative reduction potential (ORP) < 0 mV with a photoperiod of 12/24 h and fed during the light stage only so the results can be extrapolated to outdoor open pond operations by monitoring the ORP. The effluent mostly complies with the discharge values of the Spanish legislation in COD and p-values (<125 mg/L; <2 mg/L), respectively, and punctually on values in N (<15 mg/L). Applying an HRT of 3 d and a ratio of 100:7 (COD:N), the presence of PPB in the mixed culture surpassed 50% of 16S rRNA gene copies, removing 78% of COD, 53% of N, and 66% of P. Furthermore, by increasing the HRT to 5 d, removal efficiencies of 83% of COD, 65% of N, and 91% of P were achieved. In addition, the reactors were further operated in a membrane bioreactor, thus separating the HRT from the SRT to increase the specific loading rate. Very satisfactory removal efficiencies were achieved by applying an HRT and SRT of 2.3 and 3 d, respectively: 84% of COD, 49% of N, and 93% of P despite the low presence of PPB due to more oxidative conditions, which step-by-step re-colonized the mixed culture until reaching >20% of 16S rRNA gene copies after 49 d of operation. These results open the door to scaling up the process in open photobioreactors capable of treating urban wastewater and municipal solid waste in a single stage and under microaerophilic conditions by controlling the ORP of the system.

Bioplastics offer a promising sustainable alternative to petroleum-based plastics due to their biodegradability as well as favourable thermal and mechanical properties. Among different types of biobased polymers, the production of polyhydroxyalkanoates (PHA) using purple phototrophic bacteria (PPB) and low-value substrates has gained increasing interest. Despite the momentum, challenges regarding the scalability and environmental feasibility of this biopolymer production pathway remain. In response, this study employs an exploratory LCA approach to quantitatively assesses the potential environmental implications of PHA production in powder form and the joint management of the organic fraction of municipal solid waste (OFMSW) through a novel photobiorefinery system that uses PPB mixed cultures. Environmental impacts were tested under multiple improvement scenarios and benchmarked against the production of conventional fossil-based granulate or unprocessed plastics, including low density polyethylene (LDPE), polyethylene terephthalate (PET) and polyurethane (PU). The photobiorefinery stage was found to have the greatest contribution to the impact categories, particularly due to direct emissions, consumption of electricity and production of extractive chemical agents used. These factors accounted for over 70% of the photobiorefinery impact in all cases. Avoided impacts provided net favourable outcomes in terms of carbon footprint and fossil resources when comparing PHA production to conventional plastics, especially PET and PU, with impact reductions ranging from 30% to 60%, respectively. However, when considering other impact categories like eutrophication, this situation was less favourable. The exploration of alternative scenarios offered significant impact reductions, especially when renewable electricity or an environmentally friendly extraction agent is used. Moreover, minimizing methane losses or co-producing hydrogen in the photobiorefinery had a notably positive effect on the carbon footprint, reducing the impact by more than 2 t of CO2 eq per t of PHA powder compared to the base case. Therefore, the implementation of feasible improvement measures in the short term can position PHA produced by mixed cultures as a sustainable alternative to petroleum-based plastics.

Polyhydroxyalkanoate (PHA) production at a large scale by purple phototrophic bacteria (PPB) is hindered due to high production costs and limited recovery due to its consumption during starvation periods. The present study identified cost-effective inorganic acids as inactivation methods for PPB to obtain higher PHA recovery. The study was performed on reactors of different scales (10 L and 0.5 L) to grow PPB and recover PHA subsequently. The permanent feast strategy was adopted to obtain higher PHA in an anaerobic environment. As a result, the study achieved 33% (dry weight) PHA recovery using inorganic acid inactivation, while formaldehyde inactivation (traditional method) achieved significantly lower PHA recovery (20% only). The results from inorganic acid inactivation were further examined for their stability. The samples were stable even after day 14, and the PHA recovery was the same as on day 0. This pioneering study shows that inorganic acids can be used to inactivate the PPB metabolism to obtain higher PHA recovery; inorganic acid inactivation could be economical for large-scale PHA production. Graphical Abstract

Refinery wastewater (RWW) treatment is outdated since new wastewater management and reuse challenges require more environmental-friendly and cheap alternatives. Conventional biological treatments focused on activated sludge are highly energy-intensive and resource-dissipating processes. However, anaerobic treatments are an excellent alternative to reduce costs derived from aeration and carbon footprint. This work proposes a novel strategy for the treatment of RWW involving a photoanaerobic membrane bioreactor (PAnMBR) with a mixed culture of purple phototrophic bacteria (PPB). PPB upcycles the organic matter, nitrogen, and phosphorus in an assimilative way, leading to a much higher biomass yield and nutrient removal than aerobic cultures. The enriched PPB culture was generated from the RWW as the sole substrate without specific PPB inoculation. The RWW (exempted from sufficient nutrients) was successfully treated with additional ammonium and phosphates provided by domestic wastewater (DWW). Preliminary batch tests determined the best DWW/RWW volumetric mixing ratio at 25:75. The PAnMBR was operated for 144 days under different specific loading rates (SLR) by modifying hydraulic and solid retention times. The maximum specific loading rate (SLR) for the efficient RWW/DWW mix treatment was 0.3 mgCOD(inlet)/mgCOD(biomass).d. The COD consumption was mainly mediated by Rhodopseudomonas sp. and Rhodobacter sp. PPB genera. The PPB-based photo-anaerobic membrane reactor was able to comply with regulated parameters for wastewater discharge for the more restrictive use of reclaimed water according to the European legislation in force.

Purple phototrophic bacteria (PPB) are a novel driver to recover organics and nutrients from wastewater by assimilative growth. Depending on the source, assimilated resources from the PPB biomass can still be recovered after a releasing step. Anaerobic digestion (AD) releases carbonand nutrients, but the release is incomplete. Thermal hydrolysis (TH) as a pretreatment before AD improves the digestibility, release, and subsequent recovery potentials. This work determines the effects of TH in batch and continuous modes regarding methane potential, nutrients' release efficiencies, volatile solids destruction, degradability, and hydrolysis rates. Continuous runs over 165 days (d) confirmed enhanced recovery potentials, achieving up to 380 LCH4/kgVS (83 % solids destruction) and 73 % N release, respectively. The TH pretreatment is energy-intensive, but with appropriate heat recovery and increased methane production in the AD of the pretreated biomass, a combined configuration is energy positive.