Microalgal capture of carbon dioxide: A carbon sink or source?

Fluorescent protein-based anaerobic reporter for construction of promoter libraries in Clostridium autoethanogenum

Clostridium autoethanogenum, a key organism for syngas fermentation, has great industrial potential as an anaerobic microbe. However, tools for monitoring and characterizing gene expression, such as fluorescent protein-based anaerobic reporters (FPARs), and promoter libraries for regulating expression intensity, are lacking. In this study, we developed a fluorescent protein-based anaerobic reporter (FPAR) tailored for C. autoethanogenum. The FPAR enabled intuitive and precise assessment of promoter activity, facilitating the creation of libraries of constitutive promoters with varying expression strengths, as well as lactose-inducible promoter libraries. The strongest constitutive promoter exhibited approximately 7.5-fold greater activity than the weakest, while the strongest inducible promoter demonstrated a 10-fold increase compared to the weakest. This work not only establishes an efficient FPAR system for C. autoethanogenum, but also provides key genetic elements for advancing metabolic engineering and optimizing industrial processes involving this microbe.

Less toxic combined microplastics exposure towards attached Chlorella sorokiniana in the presence of sulfamethoxazole while massive microalgal nitrous oxide emission under multiple stresses

Microalgae-based wastewater treatment could realize simultaneous nutrients recovery and CO2 sequestration. However, impacts of environmental microplastics (MPs) and antibiotic co-exposure on microalgal growth, nutrients removal, intracellular nitric oxide (NO) accumulation and subsequent nitrous oxide (N2O) emission are unclarified, which could greatly offset the CO2 sequestration benefit. To reveal the potential impacts of environmental concentrations of MPs and antibiotic co-exposure on microalgal greenhouse gas mitigation, this study investigated the effects of representative MPs (PE, PVC, PA), antibiotic sulfamethoxazole (SMX), and nitrite (NO2--N) in various combinations on attached Chlorella sorokiniana growth, nutrients removal, anti-oxidative responses, and N2O emission originated from intracellular NO build-up. Microalgal biofilm growth was more inhibited under 10 μg/L MPs than 100 μg/L SMX, and MPs+SMX co-exposure displayed toxicity antagonism while MPs+MPs co-exposure caused toxicity synergism (up to 66 % growth inhibition). Extracellular polysaccharides content correlated well with microalgal biofilm density under various stresses, while SMX involved stresses displayed chlorophyll a content reduction. Microalgal assimilation and MPs adsorption contributed to nutrients removal, and phosphorus removal displayed less variance among different stresses (residual phosphorus <0.5 mg/L) than nitrogen. Intracellular NO conversion to N2O almost doubled during the co-exposure processes, and N2O emission under NO2--N + PE+PVC co-exposure could offset the contribution of microalgal CO2 sequestration by as high as 176.2 %. Results of this study appealed for urgent concern regarding environmental MPs and antibiotic co-exposure on primary producers' growth characteristics and their greenhouse gas mitigation properties.

Wastewater as a resource for carbon capture: A comprehensive overview and perspective

As two important but energy-intense processes, carbon capture and wastewater treatment always attract wide research interests to improve their operational efficiency and technological feasibility. Consequently, utilizing wastewater for carbon capture or integrating carbon capture plants into wastewater treatment facilities has become a promising concept drawing great attention to investigate and demonstrate its feasibility and efficiency. In this study, recent research progress and concept validation studies of utilizing wastewater for carbon capture were briefly reviewed and summarized with the status and main challenges of this concept provided accordingly. Three integration strategies for combining carbon capture with wastewater treatment-utilization of wastewater as the absorbent to capture CO2, biological pathway for simultaneous carbon capture and wastewater treatment, and electrochemical approach to integrate wastewater purification with carbon capture-were primarily reviewed and discussed in this study. Meanwhile, the perspectives of these integrated technology strategies were also discussed providing guidance for future investigations and development of carbon capture with wastewater treatment. Based on our study, the integrated wastewater treatment and carbon capture shows promising prospects in terms of reducing energy consumption and cost of carbon capture and wastewater treatment. However, more relevant studies and demonstrations are still necessary to improve efficiency and reduce possible carbon emissions. As a promising technology contributing to achieving net-zero emission and mitigating global warming, the integration of wastewater treatment and carbon capture will attract more attention in the future.

Unveiling underlying factors for optimizing light spectrum to enhance microalgae growth

Emerging research highlights the potential of specific light spectral regions to significantly enhance microalgae biomass production compared to conventional white light illumination. However, conflicting results of existing studies on the most optimal wavelengths reveal a knowledge gap regarding the underlying factors for optimal spectrum. The present paper aims to address this gap by critically analyzing existing studies on light spectral quality and its impact on microalgae growth. The analysis focuses on identifying the key factors determining an optimal light spectrum for microalgae cultivation. The study critically evaluates the effects of narrow wavelengths, assessing whether monochromatic light may be effective in maximizing biomass yield. While wavelength manipulation has a high potential, a deeper investigation into combining narrow wavelengths at varying ratios to determine the most effective spectral composition for maximizing growth is required. The study aims to provide insights into designing an optimal light spectrum for sustainable and efficient microalgae cultivation.

Evaluating the performance and stability of microalgal-bacterial granular sludge in municipal wastewater treatment plants

The microalgal-bacterial granular sludge (MBGS) process shows potential for carbon-neutral wastewater treatment, yet its application in wastewater treatment plants remains underexplored. This study attempted to use a continuous-flow raceway reactor to treat real municipal wastewater using the MBGS process. The results showed that the removal efficiencies of organics peaked on the fifth day, while declining trends were observed for nitrogen and phosphorus removal. Microbial community and functional gene analyses indicated that the removal of organics, nitrogen, and phosphorus might be heavily influenced by Proteobacteria, suggesting that fluctuations in their abundance significantly impacted the performance of MBGS. Bacteroidota and Actinobacteria played a vital role in cellulose decomposition via the cbhA gene. Moreover, energy shortages caused by light attenuation due to wastewater turbidity and environmental fluctuations disrupted the microbial balance, shifting metabolic activity towards carbon pathways. Key challenges for the broader application of the MBGS process include managing wastewater turbidity and ensuring process stability. These findings highlight the need for pretreatment measures and robust operational strategies to mitigate environmental fluctuations and maintain system performance.

Simultaneous CO2 and biogas slurry treatment using a newly isolated microalga with high CO2 tolerance

The fixation of carbon dioxide (CO2) using microalgae is a promising CO2 capture and utilization technology. Microalgae have also been suggested as a potential treatment for biogas slurry (BS). This study screened microalgae capable of tolerating both high CO2 concentrations and BS, assessed their CO2 fixation and pollutant removal capabilities, and evaluated the potential use of the resulting algal biomass. Chlamydopodium sp. HS01, which showed the highest tolerance to 15% CO2 and BS, was selected due to its strong growth, CO2 fixation, and ammonia nitrogen removal abilities. The generated biomass also demonstrated significant potential for bioenergy production. Metabolomics analysis revealed that the lipid composition of HS01 underwent substantial changes under 15% CO2 alone and in combination with BS, likely as a stress adaptation strategy. Overall, HS01 presents high potential for resource utilization of CO2 coupled with actual BS.

Enhancing the performance of microalgal-bacterial systems with sodium bicarbonate: A step forward to carbon neutrality of municipal wastewater treatment

The microalgal-bacterial granular sludge (MBGS) process, enhanced with sodium bicarbonate (NaHCO3), offers a sustainable alternative for wastewater treatment aiming for carbon neutrality. This study demonstrates that NaHCO3, which can be derived from the flue gases and alkaline textile wastewater, significantly enhances pollutant removal and biomass production. Optimal addition of NaHCO3 was found to achieve an inorganic-to-organic carbon ratio of 1.0 and a total carbon-to-nitrogen ratio of 5.0. Metagenomic analysis and structural equation modeling showed that NaHCO3 addition increased dissolved oxygen concentrations and pH levels, creating a more favorable environment for key microbial communities, including Proteobacteria, Chloroflexi, and Cyanobacteria. Confocal laser scanning microscopy further confirmed enhanced interactions between Cyanobacteria and Proteobacteria/Chloroflexi, facilitating the MBGS process. These microbes harbored functional genes (gap2, GLU, and ppk) critical for removing organics, nitrogen, and phosphorus. Carbon footprint analysis revealed significant reductions in CO2 emissions by the NaHCO3-added MBGS process in representative countries (China, Australia, Canada, Germany, and Morocco), compared to the conventional activated sludge process. These findings highlight the effectiveness of NaHCO3 in optimizing MBGS process, establishing it as a key strategy in achieving carbon-neutral wastewater treatment globally.

Scale-up of microalgal systems for decarbonization and bioproducts: Challenges and opportunities

The threat of global climate change presents a significant challenge for humanity. Microalgae-based carbon capture and utilization (CCU) technology has emerged as a promising solution to this global issue. This review aims to comprehensively evaluate the current advancements in scale-up of microalgae cultivation and its applications, specifically focusing on decarbonization from flue gases, organic wastewater remediation, and biogas upgrading. The study identifies critical challenges that need to be addressed during the scale-up process and evaluates the economic viability of microalgal CCU within the carbon market. Additionally, it analyzes the commercial status of microalgae-derived products and highlights those with high market demand. This review serves as a crucial resource for researchers, industry professionals, and policymakers to develop and implement innovative approaches to enhance the efficiency of microalgae-based CO2 utilization while addressing the challenges associated with the scale-up of microalgae technologies.