Improvement of Photoautotrophic Algal Biomass Production after Interrupted CO2 Supply by Urea and KH2PO4 Injection

Principles, challenges, and optimization of indigenous microalgae-bacteria consortium for sustainable swine wastewater treatment

Indigenous microalgae-bacteria consortium (IMBC) offers significant advantages for swine wastewater (SW) treatment including enhanced adaptability and resource recovery. In this review, the approaches for enriching IMBC both in situ and ex situ were comprehensively described, followed by symbiotic mechanisms for IMBC which involve metabolic cross-feeding and signal transmission. Strategies for enhancing treatment efficiencies of SW-originated IMBC were then introduced, including improving SW quality, optimizing system operating conditions, and adjusting microbial activities. Recommendations for maximizing treatment efficiencies were particularly proposed using a decision tree approach. Moreover, removal/recovery mechanisms for typical pollutants in SW using IMBC were critically discussed. Ultimately, a technical route termed SW-IMBC-Crop-Pig was proposed, to achieve a closed-loop economy for pig farms by integrating SW treatment with crop cultivation. This review provides a deeper understanding of the mechanism and strategies for IMBC's resource recovery from SW.

Microalgae are not an umbrella solution for power industry waste abatement but could play a role in their valorization

Microalgae have long been regarded as a promising solution for biological carbon abatement from the power industry, offering renewable biomass without competing for land or water resources used for food crops. In this study, we extensively examined the application of photosynthetic microorganisms for closing carbon, nitrogen, and micronutrient loops in the power industry. Subsequently, we explored the bottom-up integration of algal biorefineries into power industry waste streams for increased economic benefits and reduced environmental impacts. Analysis of the available data indicated that microalgae integration with the power industry is primarily performed using flue-gas-assisted cultivation. This approach allows for carbon sequestration typically below one gram per liter per day, too low to significantly impact carbon abatement at achievable scales of microalgae cultivation. Alternative approaches are also being explored. For example, soluble bicarbonate platforms allow for higher biomass productivity and temporary carbon storage. Meanwhile, the use of ashes and waste heat and thermophilic strains can result in lower cultivation costs and better control of cultivation conditions. These approaches offer further incremental improvement to microalgae-based carbon abatement systems in the power industry but are unlikely to be an umbrella solution for carbon reduction. Consequently, in the near term, microalgae-based carbon valorization systems are likely to be limited to niche applications involving the synthesis of high-value products. For microalgae to truly transform carbon abatement processes radical improvements in both biology and engineering approaches are urgently needed.

Enhancement for the synthesis of bio-energy molecules (carbohydrates and lipids) in Desmodesmus subspicatus: experiments and optimization techniques

Microalgae are regarded as renewable resources of energy, foods and high-valued compounds using a biorefinery approach. In the present study, we explored isolated microalgae (Desmodesmus subspicatus) for the production of bio-energy molecules (carbohydrate and lipid). Optimizations of media (BG-11) components have been made using the Taguchi orthogonal array (TOA) technique to maximize biomass, carbohydrate and lipid production. Optimized results showed that biomass, carbohydrates and lipid productivity increased by 1.3 times at optimal combinations of media components than standard BG-11 media. Further, the influence of various carbon and nitrogen sources as nutritional supplement with optimum media composition under different light intensities was investigated for productivity of carbohydrate and lipid. Results demonstrated that 1.5 times higher productivity of carbohydrate and lipids were achieved in the presence optimum BG-11 under a broad range of light intensities (84-504 µmol m-2 s-1). Among different nitrogen sources, glycine was found to give higher productivity (1.5 times) followed by urea. Use of the cellulose as a carbon source in the media significantly increases biomass (2.4 times), carbohydrates (2.3 times) and lipids (2.3 times) productivity. Investigations revealed that cultivating Desmodesmus subspicatus under optimum culture conditions has the potential for large-scale bio-ethanol and bio-diesel production.

A Low-Cost Fertilizer Medium Supplemented with Urea for the Lutein Production of Chlorella sp. and the Ability of the Lutein to Protect Cells against Blue Light Irradiation

This study aimed to investigate the use of organic fertilizers instead of modified f/2 medium for Chlorella sp. cultivation, and the extracted lutein of the microalga to protect mammal cells against blue-light irradiation. The biomass productivity and lutein content of Chlorella sp. cultured in 20 g/L fertilizer medium for 6 days were 1.04 g/L/d and 4.41 mg/g, respectively. These values are approximately 1.3- and 1.4-fold higher than those achieved with the modified f/2 medium, respectively. The cost of medium per gram of microalgal biomass reduced by about 97%. The microalgal lutein content was further increased to 6.03 mg/g in 20 g/L fertilizer medium when supplemented with 20 mM urea, and the cost of medium per gram lutein reduced by about 96%. When doses of ≥1 μM microalgal lutein were used to protect mammal NIH/3T3 cells, there was a significant reduction in the levels of reactive oxygen species (ROS) produced by the cells in the following blue-light irradiation treatments. The results show that microalgal lutein produced by fertilizers with urea supplements has the potential to develop anti-blue-light oxidation products and reduce the economic challenges of microalgal biomass applied to carbon biofixation and biofuel production.

Enhancement of microalgal biomass productivity through mixotrophic culture process utilizing waste soy sauce and industrial flue gas

Wastewater treatment plants are indispensable facilities, which emit a massive amount of greenhouse gases. To boost CO₂ mitigation and wastewater treatment performance, mixotrophic microalgae cultivation using wastewater has recently been proposed. In this study, food industry wastewater (waste soy sauce) was applied to Chlorella sorokiniana UTEX 2714 cultivation. By using a medium with 20% (v/v) of 10-fold diluted soy sauce, the biomass and fatty acid methyl ester (FAME) productivity enhanced by 1.93 and 1.76 times, respectively. Biomass productivity increased up to 5.2 times when using medium with high soy sauce content under high-intensity light that inhibits cell growth in photoautotrophic environments. Furthermore, industrial flue gas treatment with wastewater was demonstrated by outdoor semi-continuous cultivation with 42% improved biomass production. Consequently, these results suggest that mixotrophic microalgal cultivation has great potential to address both climate change and water pollution while producing valuable products and can contribute to building a sustainable society.

Novel effective bioprocess for optimal CO2 fixation via microalgae-based biomineralization under semi-continuous culture

In this study, the effects of microalgae-based biomineralization in a semi-continuous process (M-BSP) on biomass productivity and CO2 fixation rate were investigated. M-BSP significantly improved biomass production and CO2 fixation rate at the second stage of induction by sustaining relatively high photosynthetic rate without exposure to toxic substances (e.g., chlorellin) from aging cells using the microalgae Chlorella HS2. In conventional systems, cells do not receive irradiated light evenly, and many cells age and burst because of the long culture period. In contrast, in the M-BSP, the photosynthesis efficiency increases and biomass production is not inhibited because most of the cells can be harvested during shorter culture period. The accumulated biomass production and CO2 fixation rate of the HS2 cells cultured under M-BSP increased by 4.67- (25 ± 1.09 g/L) and 10.9-fold (30.29 ± 1.79 g/L day-1), respectively, compared to those cultured without the CaCl2 treatment.

A newly isolated microalga Chlamydomonas sp. YC to efficiently remove ammonium nitrogen of rare earth elements wastewater

The aim of this study was to establish a practical approach to remove ammonium nitrogen of rare earth elements (REEs) wastewater by an indigenous photoautotrophic microalga. Firstly, a new microalgal strain was successfully isolated from REEs wastewater and identified as Chlamydomonas sp. (named Chlamydomonas sp. YC). The obtained results showed that microalga could completely remove the NH₄⁺-N of 10% REEs wastewater after 10 days of cultivation; however, the highest NH₄⁺-N removal rate was attained by microalga to treat undiluted REEs wastewater. Then, three cultivation modes including batch, semi-continuous and continuous cultivation methods were developed to evaluate the ability of NH₄⁺-N removal rate by this microalga to treat diluted (10%) and undiluted REEs wastewater. It was found that, Chlamydomonas sp. YC exhibited superior performance towards NH₄⁺-N removal rates (32.75-61.05 mg/(L·d)) by semi-continuous and continuous processes for the treatments of 10% and undiluted REEs wastewater in comparison to the results (19.50-30.38 mg/(L·d) by batch process. Interestingly, under the same treatment conditions, among the three cultivation modes, microalga exhibited the highest removal rates of NH₄⁺-N in undiluted REEs wastewater by semi-continuous (61.05 mg/(L·d)) and continuous (57.10 mg/(L·d) processes. In term of the biochemical analysis, microalgal biomass obtained from the wastewater treatment had 35.40-44.40% carbohydrate and 4.97-6.03% lipid, which could be potential ingredients for sustainable biofuels production. And the highest carbohydrate and lipid productivities attained by Chlamydomonas sp. YC in the continuous mode were 226.36 mg/(L·d) and 32.98 mg/(L·d), respectively. Taken together, the established processes mediated with Chlamydomonas sp. YC via continuous cultivation was the great promising approaches to efficiently remove NH₄⁺-N of REEs wastewater and produce valuable biomass for sustainable and renewable biofuels in a simultaneous manner.

Evaluation of Microalgal Bacterial Dynamics in Pig-Farming Biogas Digestate under Impacts of Light Intensity and Nutrient Using Physicochemical Parameters

Determination of the dynamics between microalgae and bacteria in pig farming biogas digestate is vital for a consistent and reliable application towards sustainable wastewater treatment and biofuel production. This study assesses the reliability of using physicochemical parameters as indicators for the rapid evaluation of microalgal bacterial dynamics in real digestate under impacts of light, nutrient loads, and N:P ratios. The relationship between variation profiles of nutrients, biomass and physicochemical properties in each experiment was analyzed. High light and high nutrient load enhanced biomass growth and nutrient removal rate. Ammonium addition (high N:P ratio) elevated NH3 level which inhibited the growth of microalgae, subsequently reducing the biomass growth and nutrient removal. Low N:P ratio triggered the accumulation of phosphorus and the growth of chlorophyll-a but exerted little influence on treatment. Variation profiles of dissolved oxygen, nutrient and biomass were highly consistent in every experiment allowing us to identify the shift from microalgal to bacterial predomination under unfavorable conditions including low light intensity and high N:P ratio. Strong linear correlation was also found between total nitrogen removal and electrical conductivity (R2 = 0.9754). The results show the great potential of rapid evaluation of microalgal bacterial dynamics for large scale system optimization and modelling.

An acceleration of carotenoid production and growth of Haematococcus lacustris induced by host-microbiota network interaction

Haematococcus lacustris is a chlamydomonadalean with high biotechnological interest owing to its capacity to produce astaxanthin, a valuable secondary carotenoid with extraordinary antioxidation properties. However, its prolonged growth has limited its utility commercially. Thus, rapid growth to attain high densities of H. lacustris cells optimally producing astaxanthin is an essential biotechnological target to facilitate profitable commercialisation. Our study focused on characterising the bacterial communities associated with the alga's phycosphere by metagenomics. Subsequently, we altered the bacterial consortia in combined co-culture with key beneficial bacteria to optimise the growth of H. lacustris. The algal biomass increased by up to 2.1-fold in co-cultures, leading to a 1.6-fold increase in the astaxanthin yield. This study attempted to significantly improve the H. lacustris growth rate and biomass yield via Next-Generation Sequencing analysis and phycosphere bacterial augmentation, highlighting the possibility to overcome the hurdles associated with astaxanthin production by H. lacustris at a commercial scale.

Sustainable production of polyhydroxybutyrate from autotrophs using CO2 as feedstock: Challenges and opportunities

Due to industrialization and rapid increase in world population, the global energy consumption has increased dramatically. As a consequence, there is increased consumption of fossil fuels, leading to a rapid increase in CO₂ concentration in the atmosphere. This accumulated CO₂ can be efficiently used by autotrophs as a carbon source to produce chemicals and biopolymers. There has been increasing attention on the production of polyhydroxybutyrate (PHB), a biopolymer, with focus on reducing the production cost. For this, cheaper renewable feedstocks, molecular tools, including metabolic and genetic engineering have been explored to improve microbial strains along with process engineering aspects for scale-up of PHB production. This review discusses the recent advents on the utilization of CO₂ as feedstock especially by engineered autotrophs, for sustainable production of PHB. The review also discusses the innovations in cultivation technology and process monitoring while understanding the underlying mechanisms for CO₂ to biopolymer conversion.

Concurrent enhancement of CO2 fixation and productivities of omega-3 fatty acids and astaxanthin in Haematococcus pluvialis culture via calcium-mediated homeoviscous adaptation and biomineralization

Haematococcus pluvialis has attracted interest as a bio-platform for producing omega-3 fatty acids (ω-3 FA) and astaxanthin that have a great potential as anti-inflammatory drugs. This study aimed to concurrently enhance the CO₂ fixation and the productivities of ω-3 FA and astaxanthin, which have been difficult to achieve because of the dissimilar culture methods for each goal, via calcium-mediated homeoviscous adaptation and biomineralization. As a result of 3 mM of Ca²⁺ addition, ω-3 FA content was improved by 31% due to Ca²⁺-induced homeoviscous adaptation. Biomineralization was promoted by the extracellular carbonic anhydrase, which resulted in 46.3% improvement in CO₂ fixation. CaCO₃ from the biomineralization was beneficially re-used in the H. pluvialis culture and triggered 178- and 522-fold increased biomass productivity and astaxanthin content, respectively, thanks to its anisotropic nature. The Ca²⁺-based productivity enhancement strategy was applied to large-scale culture which resulted improvement in overall bioprocess performance.