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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 CO 2 concentration in the atmosphere. This accumulated CO 2 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 CO 2 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 CO 2 to biopolymer conversion., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

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 CO 2 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 Ca 2+ addition, ω-3 FA content was improved by 31% due to Ca 2+ -induced homeoviscous adaptation. Biomineralization was promoted by the extracellular carbonic anhydrase, which resulted in 46.3% improvement in CO 2 fixation. CaCO 3 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 Ca 2+ -based productivity enhancement strategy was applied to large-scale culture which resulted improvement in overall bioprocess performance., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Recognized as the third-generation biomass of the future, microalgae are increasingly viewed as a promising solution for the sustainable production of biofuels, often referred to as "green gold." Extensive research is being conducted across the upstream, midstream, and downstream sectors to develop fundamental technologies that enable efficient and economical large-scale microalgae cultivation. Recent studies suggest that microalgae-based biofuels have the potential to meet global energy demands. However, challenges such as spatial constraints in site selection and the high cost of transporting CO2—an essential component for pH regulation and photosynthesis—pose obstacles. Here, this study demonstrates that by supplementing air-only medium with CaCO3, Chlorella sorokiniana can effectively utilize airborne CO2 to produce biomass. In laboratory-scale culture conditions supplied only with air, adding 5 mM CaCO3 (pH 7.8) could maintain the pH stably compared to the untreated conditions (pH 9.5) and improved the biomass concentration and lipid content by 17.68-fold and 9.58-fold, respectively. In bench-scale conditions, cultures supplemented with 5 mM CaCO3 exhibited a 9-fold increase in the biomass and a 7.15-fold increase in the lipid concentrations compared to those cultivated with air alone. With microalgae emerging as an essential resource for future generations, cultivation technology utilizing CaCO3 will be a critical technology that enables commercial-scale microalgae cultivation using only air, without artificial CO2 supply facilities. [ABSTRACT FROM AUTHOR]

[Display omitted] • Calcite crystals were generated on the cell surface of Chlorella sp. HS2. • Biomineralization minimized culture period by increasing photosynthetic efficiency. • Reaction was successful under mildly alkaline pH and mild CaCl 2 concentration. • Biomass and lipid production were successfully maintained for cultivation period. • CO 2 removal rate increased considerably, and required area for mitigation decreased. In this study, the effects of microalgae-based biomineralization in a semi-continuous process (M-BSP) on biomass productivity and CO 2 fixation rate were investigated. M-BSP significantly improved biomass production and CO 2 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 CO 2 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 CaCl 2 treatment. [ABSTRACT FROM AUTHOR]

[Display omitted] • Commercial use of H. pluvialis to produce astaxanthin and ω-3 FA has been reviewed. • Possible contaminations in different cultivation systems are discussed. • Biological pollutant species and their contamination modes are presented. • Decontamination technologies according to biocontaminants are introduced. • Further cost-effective strategies on controlling the contaminants are necessary. With industrialization, anthropogenic mishandlings have resulted in the discharge of abundant amount of CO 2 into the atmosphere. This has triggered an unnatural warming that has dramatically increased the Earth's temperature in a short duration. This problem can be addressed by the biological conversion of CO 2 ; several studies have been conducted using H. pluvialis culture that produces high value-added materials, such as astaxanthin and omega-3 fatty acids. However, although H. pluvialis has a high market value, the market size is quite small. Because H. pluvialis cells are susceptible to contamination due to its slow growth rate, hence large-scale culture of H. pluvialis without reliable contamination control strategies poses significant risks. This review comprehensively discusses the contamination that occurs during the culturing of H. pluvialis in various culture systems under different culture conditions. The review also discusses the strategies in controlling the biotic contaminants, such as bacteria and fungi. [ABSTRACT FROM AUTHOR]

Microalgae-derived biomass is currently considered a sustainable feedstock for making biofuels, including biodiesel and direct combustion fuel. The photoautotrophic cultivation of microalgae using flue gas from power plants has been continuously investigated to improve the economic feasibility of microalgae processes. The utilization of waste CO2 from power plants is advantageous in reducing carbon footprints and the cost of carbon sources. Nonetheless, the sudden interruption of CO2 supply during microalgal cultivation leads to a severe reduction in biomass productivity. Herein, chemical fertilizers including urea and KH2PO4 were added to the culture medium when CO2 supply was halted. Urea (5 mM) and KH2PO4 (5 mM) were present in the culture medium in the form of CO2/NH4+ and K+/H2PO4−, respectively, preventing cell growth inhibition. The culture with urea and KH2PO4 supplementation exhibited 10.02-fold higher and 7.28-fold higher biomass and lipid productivity, respectively, compared to the culture with ambient CO2 supply due to the maintenance of a stable pH and dissolved inorganic carbon in the medium. In the mass cultivation of microalgae using flue gas from coal-fired power plants, urea and KH2PO4 were supplied while the flue gas supply was shut off. Consequently, the microalgae were grown successfully without cell death. [ABSTRACT FROM AUTHOR]

Novel 3D-printed buoyant structures can be applied in various environmental processes because of their considerable advantages. Microalgae cultivation in photobioreactors, directly supplemented by industrial CO 2 , enables environmental pollution mitigation/cleanup and sustainable energy production. However, in photobioreactor systems, biofilm formation due to gas bubbling decreases microalgal productivity. Therefore, in this study, we aimed to develop a novel 3D-printed buoyant structure to suppress biofilm formation. The 10 mm-sized spherical buoyant structure reduced the height and area of the biofilm by 58.3% and 82.5%, respectively. The structure decreased space where bubble burst occurred and controlled the bubble size, reducing the overall biomass loss by 58.7%. It did not reduce photobioreactor performance noticeably during semi-continuous cultivation, indicating the possibility of long-term applicability. In large-scale outdoor microalgae cultivation using flue gas CO 2 , the buoyant structure improved the cell density and biodiesel production potential without contamination. This study provides a promising strategy to contribute to biological CO 2 mitigation through the utilization of flue gas CO 2 for enhanced microalgal production, paving the way for energy and environmental sustainability. [Display omitted] • 3D-printed buoyant structure prevented biofouling in a polymeric photobioreactor. • The 10 mm-sized spherical structure decreased the height and area of the biofilm. • Buoyant structure prevented bursting of bubbles and generation of large bubbles. • Biofouling was controlled in semi-continuous culture without structure replacement. • Biofouling suppression contributed to microalgal valorization of flue gas CO 2. [ABSTRACT FROM AUTHOR]