Historical perspective on microalgal and cyanobacterial acclimation to low- and extremely high-CO(2) conditions

The Freshwater Cyanobacterium Synechococcus elongatus PCC 7942 Does Not Require an Active External Carbonic Anhydrase

Under standard laboratory conditions, Synechococcus elongatus PCC 7942 lacks EcaASyn, a periplasmic carbonic anhydrase (CA). In this study, a S. elongatus transformant was created that expressed the homologous EcaACya from Cyanothece sp. ATCC 51142. This additional external CA had no discernible effect on the adaptive responses and physiology of cells exposed to changes similar to those found in S. elongatus natural habitats, such as fluctuating CO2 and HCO3- concentrations and ratios, oxidative or light stress, and high CO2. The transformant had a disadvantage over wild-type cells under certain conditions (Na+ depletion, a reduction in CO2). S. elongatus cells lacked their own EcaASyn in all experimental conditions. The results suggest the presence in S. elongatus of mechanisms that limit the appearance of EcaASyn in the periplasm. For the first time, we offer data on the expression pattern of CCM-associated genes during S. elongatus adaptation to CO2 replacement with HCO3-, as well as cell transfer to high CO2 levels (up to 100%). An increase in CO2 concentration coincides with the suppression of the NDH-14 system, which was previously thought to function constitutively.

De novo transcriptome and lipidome analysis of Desmodesmus abundans under model flue gas reveals adaptive changes after ten years of acclimation to high CO2

Microalgae's ability to mitigate flue gas is an attractive technology that can valorize gas components through biomass conversion. However, tolerance and growth must be ideal; therefore, acclimation strategies are suggested. Here, we compared the transcriptome and lipidome of Desmodesmus abundans strains acclimated to high CO2 (HCA) and low CO2 (LCA) under continuous supply of model flue gas (MFG) and incomplete culture medium (BG11-N-S). Initial growth and nitrogen consumption from MFG were superior in strain HCA, reaching maximum productivity a day before strain LCA. However, similar productivities were attained at the end of the run, probably because maximum photobioreactor capacity was reached. RNA-seq analysis during exponential growth resulted in 16,435 up-regulated and 4,219 down-regulated contigs in strain HCA compared to LCA. Most differentially expressed genes (DEGs) were related to nucleotides, amino acids, C fixation, central carbon metabolism, and proton pumps. In all pathways, a higher number of up-regulated contigs with a greater magnitude of change were observed in strain HCA. Also, cellular component GO terms of chloroplast and photosystems, N transporters, and secondary metabolic pathways of interest, such as starch and triacylglycerols (TG), exhibited this pattern. RT-qPCR confirmed N transporters expression. Lipidome analysis showed increased glycerophospholipids in strain HCA, while LCA exhibited glycerolipids. Cell structure and biomass composition also revealed strains differences. HCA possessed a thicker cell wall and presented a higher content of pigments, while LCA accumulated starch and lipids, validating transcriptome and lipidome data. Overall, results showed significant differences between strains, where characteristic features of adaptation and tolerance to high CO2 might be related to the capacity to maintain a higher flux of internal C, regulate intracellular acidification, active N transporters, and synthesis of essential macromolecules for photosynthetic growth.

Nuclear mutagenesis and adaptive evolution improved photoautotrophic growth of Euglena gracilis with flue-gas CO2 fixation

To effectively improve biomass growth and flue-gas CO2 fixation of microalgae, acid-tolerant Euglena gracilis was modified with cobalt-60 γ-ray irradiation and polyethylene glycol (PEG) adaptive screening to obtain the mutant strain M800. The biomass dry weight and maximum CO2 fixation rate of M800 were both 1.47 times higher than that of wild strain, which was attributed to a substantial increase in key carbon fixation enzyme RuBisCO activity and photosynthetic pigment content. The high charge separation quantum efficiency in PSII reaction center, efficient light utilization and energy regulation that favors light conversion, were the underlying drivers of efficient photosynthetic carbon fixation in M800. M800 had stronger antioxidant capacity in sufficient high-carbon environment, alleviating lipid peroxidation damage. After adding 1 mM PEG, biomass dry weight of M800 reached 2.31 g/L, which was 79.1 % higher than that of wild strain. Cell proliferation of M800 was promoted, the apoptosis and necrosis rates decreased.

Testing for terrestrial and freshwater microalgae productivity under elevated CO2 conditions and nutrient limitation

BACKGROUND

Microalgae CO₂ fixation results in the production of biomass rich in high-valuable products, such as fatty acids and carotenoids. Enhanced productivity of valuable compounds can be achieved through the microalgae's ability to capture CO₂ efficiently from sources of high CO₂ contents, but it depends on the species. Culture collections of microalgae offer a wide variety of defined strains. However, an inadequate understanding of which groups of microalgae and from which habitats they originate offer high productivity under increased CO₂ concentrations hampers exploiting microalgae as a sustainable source in the bioeconomy.

RESULTS

A large variety of 81 defined algal strains, including new green algal isolates from various terrestrial environments, were studied for their growth under atmospheres with CO₂ levels of 5-25% in air. They were from a pool of 200 strains that had been pre-selected for phylogenetic diversity and high productivity under ambient CO₂. Green algae from terrestrial environments exhibited enhanced growth up to 25% CO₂. In contrast, in unicellular red algae and stramenopile algae, which originated through the endosymbiotic uptake of a red algal cell, growth at CO₂ concentrations above 5% was suppressed. While terrestrial stramenopile algae generally tolerated such CO₂ concentrations, their counterparts from marine phytoplankton did not. The tests of four new strains in liquid culture revealed enhanced biomass and chlorophyll production under elevated CO₂ levels. The 15% CO₂ aeration increased their total carotenoid and fatty acid contents, which were further stimulated when combined with the starvation of macronutrients, i.e., less with phosphate and more with nitrogen-depleted culture media.

CONCLUSION

Green algae originating from terrestrial environments, Chlorophyceae and Trebouxiophyceae, exhibit enhanced productivity of carotenoids and fatty acids under elevated CO₂ concentrations. This ability supports the economic and sustainable production of valuable compounds from these microalgae using inexpensive sources of high CO₂ concentrations, such as industrial exhaust fumes.

Role of cyclic electron transport mutations pgrl1 and pgr5 in acclimation process to high light in Chlamydomonas reinhardtii

Light is crucial for photosynthesis, but the amount of light that exceeds an organism's assimilation efficacy can lead to photo-oxidative damage and even cell death. In Chlamydomonas (C). reinhardtii cyclic electron flow (CEF) is very important for the elicitation of non-photochemical quenching (NPQ) by controlling the acidification of thylakoid lumen. This process requires the cooperation of proton gradient regulation (PGR) proteins, PGRL1 and PGR5. Here, we compared the growth pattern and photosynthetic activity between wild type (137c, t222+) and mutants impaired in CEF (pgrl1 and pgr5) under photoautotrophic and photoheterotrophic conditions. We have observed the discriminative expression of NPQ in the mutants impaired in CEF of pgrl1 and pgr5. The results obtained from the mutants showed reduced cell growth and density, Chl a/b ratio, fluorescence, electron transport rate, and yield of photosystem (PS)II. These mutants have reduced capability to develop a strong NPQ indicating that the role of CEF is very crucial for photoprotection. Moreover, the CEF mutant exhibits increased photosensitivity compared with the wild type. Therefore, we suggest that besides NPQ, the fraction of non-regulated non-photochemical energy loss (NO) also plays a crucial role during high light acclimation despite a low growth rate. This low NPQ rate may be due to less influx of protons coming from the CEF in cases of pgrl1 and pgr5 mutants. These results are discussed in terms of the relative photoprotective benefit, related to the thermal dissipation of excess light in photoautotrophic and photoheterotrophic conditions.

The effects of CO2-induced acidification on Tetraselmis biomass production, photophysiology and antioxidant activity: A comparison using batch and continuous culture

A Tetraselmis sp. was selected for its antioxidant activity owing to its high lipid peroxidation inhibition capacity. With the aim to monitor culture conditions to improve antioxidant activity, effects of CO₂-induced acidification on Tetraselmis growth, elemental composition, photosynthetic parameters and antioxidant activity were determined. Two pH values were tested (6.5 and 8.5) in batch and continuous cultures in photobioreactors. Acidification enhanced cell growth under both culture methods. However, the microalgae physiological state was healthier at pH 8.5 than at pH 6.5. Indeed, photosynthetic parameters measured with pulse amplitude modulated (PAM) fluorometry showed a decrease in the photosystem II (PSII) efficiency at pH 6.5 in batch culture. Yet, with the exception of the PSII recovering capacity, photosynthetic parameters were similar in continuous culture at both pH. These results suggest that lowering pH through CO₂-induced acidification may induce a lower conversion of light to chemical energy especially when coupled with N-limitation and/or under un-balanced culture conditions. The highest antioxidant activity was measured in continuous culture at pH 6.5 with an IC₅₀ of 3.44 ± 0.6 μg mL^-1, which is close to the IC₅₀ of reference compounds (trolox and α-tocopherol). In addition, the principal component analysis revealed a strong link between the antioxidant activity and the culture method, the photophysiological state and the nitrogen cell quota and C:N ratio of Tetraselmis sp.. These results highlight Tetraselmis sp. as a species of interest for natural antioxidant production and the potential of PAM fluorometry to monitor culture for production of biomass with a high antioxidant activity.

Using polyethylene glycol to promote Nannochloropsis oceanica growth with 15 vol% CO2

CO₂ capture with microalgae has been put forward in response to global concern on greenhouse gas emission. However, the short residence time and slow diffusion of CO₂ in water limits the growth of microalgae. In order to improve CO₂ transfer from gas phase to liquid phase and utilization by algal cells, polyethylene glycol 200 (PEG 200) was used as CO₂ absorbent to promote growth of Nannochloropsis oceanica with the bubbling of 15 vol% CO₂. Total inorganic carbon (TIC) absorbed in culture medium remained constant at 5.6 mM when 15 vol% CO₂ was bubbled continuously. PEG 200 in the medium provided additional CO₂ absorption from 0.6 to 4.8 mM when PEG 200 concentration increased from 0.5 to 4 mM. The specific growth rate of N. oceanica reached the maximum (1.41 d^-1) with 1 mM PEG 200 in culture medium, which was 21.5% higher than the specific growth rate without PEG 200. About 79% of the increase in biomass was attributed to the increased TIC with more CO₂ dissolution in culture medium because of PEG 200, and about 21% was attributed to PEG 200 itself utilized as an organic carbon source. In conclusion, PEG 200 as a CO₂ absorbent can effectively capture flue-gas CO₂ for algal growth.

Screening High CO2-Tolerant Oleaginous Microalgae from Genera Desmodesmus and Scenedesmus

Microalgae from genus Scenedesmus sensu lato (including Desmodesmus and Scenedesmus) were reported to be particularly suitable candidates for CO₂ biomitigation. In this study, 16 strains from Scenedesmus sensu lato were obtained from different climate zones of China and their phylogenetic positions were determined. Seven strains out of the 16 showed high CO₂ tolerance and grew much faster under 20% CO₂ than air condition. Two representatives from genera Desmodesmus (NMD46) and Scenedesmus (HBX310) respectively were selected due to their higher lipid productivity, and the maximum value of 146 mg L^-1 day^-1 was achieved in NMD46. Triacylglycerols increased with the rising of CO₂ levels from 0.04 to 15% in NMD46, while they changed little in HBX310. High CO₂ level decreased the polyunsaturated fatty acid content in NMD46 but increased it in HBX310. NMD46 is more suitable for standardized biodiesel production in view of its lipid and fatty acid composition responses to high CO₂.

CO2 biofixation and fatty acid composition of two indigenous Dunaliella sp. isolates (ABRIINW-CH2 and ABRIINW-SH33) in response to extremely high CO2 levels

Global warming, as a result of atmospheric CO₂ increase, is regarded as an important universal concern. Microalgae are considered as appropriate microorganisms for CO₂ assimilation. Here we aimed to investigate carbon biofixation ability of two indigenous isolates of Dunaliella spp. (ABRIINW-CH2 and ABRIINW-SH33) under elevated CO₂ concentrations of 10, 20, and 30% (v/v) as well as their lipid content, productivity, and fatty acid profile under adjusted pH conditions. The maximum biomass production and CO₂ biofixation rates were obtained under 10% CO₂. High CO₂ concentrations were favorable for the accumulation of lipids, lipid productivity, and polyunsaturated fatty acids formation. The highest lipid content and lipid productivity was obtained at 10% CO₂. The highest fraction of the fatty acids (FA) profile was allocated to omega-3 FAs at 20% CO₂. Accordingly, these isolates were able to tolerate extremely high CO₂ concentrations and present even enhanced growth as well as formation of valuable products.

Calcinated MIL-100(Fe) as a CO2 adsorbent to promote biomass productivity of Arthrospira platensis cells

In order to solve the problems of short residence time and low diffusion of CO₂ gas in microalgal solution, calcinated metal-organic framework MIL-100(Fe) were first used as CO₂ adsorbents to promote the growth of Arthrospira platensis cells by increasing carbon fixation. The adsorbent (MIL-100(Fe)-4 h) containing unsaturated metal sites, improved the conversion of CO₂ to dissolved inorganic carbon by 52.3% and concentration of HCO₃^- by 20.0% in culture medium, as compared to the medium without CO₂ adsorbent added. The increased HCO₃^- concentration facilitated carboxysome accumulation (increased to 21.7 times) to activate the photosynthetic Calvin cycle in Arthrospira cells. The increased cell growth rate promoted cell volume by 132% and knot length by 102%, while the fractal dimension of the cell surface decreased by 13.5%. The biomass productivity of Arthrospira cells cultivated with the CO₂ adsorbent MIL-100(Fe)-4 h remarkably increased by 81.9%.

Green alga cultivation with nanofibers as physical adsorbents of carbon dioxide: Evaluation of gas biofixation and macromolecule production

The objective of this study was to evaluate the biofixation and production of biocompounds by Chlorella fusca LEB 111 cultivated with different concentrations of carbon dioxide (CO₂) adsorbent nanofibers in their free form or retained. Cultures were grown in 15% (v v^-1) CO₂ with 0.1, 0.3 and 0.5 g L^-1 nanofibers developed with 10% (w v^-1) polyacrylonitrile (PAN)/dimethylformamide (DMF), with or without nanoparticles; retained or not. The addition of 0.1 g L^-1 nanofibers with nanoparticles in their free form to the cultures promoted the accumulation of approximately 3 times more carbon in the medium (46.6 mg L^-1), a 45% higher biofixation rate (89.2 mg L^-1 d^-1) and increased carbohydrate production by approximately 2.3% (w w^-1) of that observed in cultures grown without nanofibers. Therefore, nanofibers showed promising potential as physical adsorbents of CO₂ in the cultivation to increase gas fixation and promote the synthesis of macromolecules.

Low pH rather than high CO2 concentration itself inhibits growth of Arthrospira

Microalga is a promising candidate for bio-mitigation of CO₂. It has been longtime recognized that high CO₂ concentration would impose stresses on microalga to suppress the growth. However, this concept was challenged in this research by investigating the growth, photosynthesis and anti-oxidant characteristics of Arthrospira platensis under independent effects of CO₂ concentrations and pH. Results showed the growth of A. platensis was only inhibited when broth was in acidic pH. Microalgal cells could deal with high CO₂ concentration readily if medium pH was maintained in favorite level. Photosynthesis was inhibited swiftly and significantly under acidified condition. The singlet oxygen was produced in low level for alkalic pH treatment, however it burst quickly after low pH stress was imposed. Accordingly, it was proposed that the phenomena of high CO₂ intolerance was caused by CO₂ induced pH decline rather than high CO₂ concentration itself. This finding has significance on large scale application of microalga based CO₂ mitigation and flue gas treatment since it proved concentrated CO₂ could be directly assimilated without dilution.

Elevated carbon dioxide levels lead to proteome-wide alterations for optimal growth of a fast-growing cyanobacterium, Synechococcus elongatus PCC 11801

The environmental considerations attributing to the escalation of carbon dioxide emissions have raised alarmingly. Consequently, the concept of sequestration and biological conversion of CO₂ by photosynthetic microorganisms is gaining enormous recognition. In this study, in an attempt to discern the synergistic CO₂ tolerance mechanisms, metabolic responses to increasing CO₂ concentrations were determined for Synechococcus elongatus PCC 11801, a fast-growing, novel freshwater strain, using quantitative proteomics. The protein expression data revealed that the organism responded to elevated CO₂ by not only regulating the cellular transporters involved in carbon-nitrogen uptake and assimilation but also by inducing photosynthesis, carbon fixation and glycolysis. Several components of photosynthetic machinery like photosystem reaction centers, phycobilisomes, cytochromes, etc. showed a marked up-regulation with a concomitant downshift in proteins involved in photoprotection and redox maintenance. Additionally, enzymes belonging to the TCA cycle and oxidative pentose phosphate pathway exhibited a decline in their expression, further highlighting that the demand for reduced cofactors was fulfilled primarily through photosynthesis. The present study brings the first-ever comprehensive assessment of intricate molecular changes in this novel strain while shifting from carbon-limited to carbon-sufficient conditions and may pave the path for future host and pathway engineering for production of sustainable fuels through efficient CO₂ capture.

Spermidine enhanced resistance of Chlorella to high levels of CO2 and light intensity for improving photosynthetic growth rate

In order to promote the photosynthetic growth rate of Chlorella in the presence of flue gas CO₂ from coal-fired power plants, spermidine was first used to enhance cellular resistance to a high CO₂ concentration (15%) and high light intensity (30 000 lux). It was found that low concentrations (100–300 μM) of spermidine significantly enhanced the photosynthetic growth rate of Chlorella. The accelerated cell division decreased the cell diameter from 3.64 μm to 2.71 μm and the fractal dimension from 1.60 to 1.49, and the activity of total superoxide dismutase (T-SOD) increased from 0.48 U mL⁻¹ to 5.33 U mL⁻¹. Expression levels of key enzymes of photosystems I and II, ATP synthase and transportase markedly increased, thereby enhancing the electron transport and energy supply that reduced oxidative damage. Finally, an enhanced cellular resistance to the high CO₂ concentration and high light intensity increased the biomass yield from 0.11 g L⁻¹ to 1.71 g L⁻¹ (300 μM).

Spermidine enhanced resistance of Chlorella to high levels of CO₂ and light intensity.

Bicarbonate supplementation enhances growth and biochemical composition of Dunaliella salina V-101 by reducing oxidative stress induced during macronutrient deficit conditions

The unicellular marine alga Dunaliella salina is a most interesting green cell factory for the production of carotenes and lipids under extreme environment conditions. However, the culture conditions and their productivity are the major challenges faced by researchers which still need to be addressed. In this study, we investigated the effect of bicarbonate amendment on biomass, photosynthetic activity, biochemical constituents, nutrient uptake and antioxidant response of D. salina during macronutrient deficit conditions (N⁻, P⁻ and S⁻). Under nutrient deficit conditions, addition of sodium bicarbonate (100 mM) significantly increased the biomass, carotenoids including β-carotene and lutein, lipid, and fatty acid content with concurrent enhancement of the activities of nutrient assimilatory and carbonic anhydrase enzymes. Maximum accumulation of carotenoid especially β-carotene (192.8 ± 2.11 µg/100 mg) and lipids (53.9%) was observed on addition of bicarbonate during nitrate deficiency compared to phosphate and sulphate deficiency. Supplementation of bicarbonate reduced the oxidative stress caused by ROS, lowered lipid peroxidation damage and improved the activities of antioxidant enzymes (SOD, CAT and APX) in D. salina cultures under nutrient stress.

Impact of Flue Gas Compounds on Microalgae and Mechanisms for Carbon Assimilation and Utilization

To shift the world to a more sustainable future, it is necessary to phase out the use of fossil fuels and focus on the development of low-carbon alternatives. However, this transition has been slow, so there is still a large dependence on fossil-derived power, and therefore, carbon dioxide is released continuously. Owing to the potential for assimilating and utilizing carbon dioxide to generate carbon-neutral products, such as biodiesel, the application of microalgae technology to capture CO₂ from flue gases has gained significant attention over the past decade. Microalgae offer a more sustainable source of biomass, which can be converted into energy, over conventional fuel crops because they grow more quickly and do not adversely affect the food supply. This review focuses on the technical feasibility of combined carbon fixation and microalgae cultivation for carbon reuse. A range of different carbon metabolisms and the impact of flue gas compounds on microalgae are appraised. Fixation of flue gas carbon dioxide is dependent on the selected microalgae strain and on flue gas compounds/concentrations. Additionally, current pilot-scale demonstrations of microalgae technology for carbon dioxide capture are assessed and its future prospects are discussed. Practical implementation of this technology at an industrial scale still requires significant research, which necessitates multidisciplinary research and development to demonstrate its viability for carbon dioxide capture from flue gases at the commercial level.

Enhancing cell growth and lutein productivity of Desmodesmus sp. F51 by optimal utilization of inorganic carbon sources and ammonium salt

The type and concentration of inorganic carbon and nitrogen sources were manipulated to improve cell growth and lutein productivity of Desmodesmus sp. F51. Using nitrate as nitrogen source, the better cell growth and lutein accumulation were obtained under 2.5% CO₂ supply when compared to the addition of NaHCO₃ or Na₂CO₃. To solve the pH variation problem of ammonium consumption, the strategy of using dual carbon sources (NaHCO₃ and CO₂) was explored. A lower bicarbonate-C: ammonium-N ratio led to a lower culture pH as well as lower lutein productivity, but significantly enhanced the auto-flocculation efficiency of the microalgal cells. The highest biomass productivity (939mg/L/d) and lutein productivity (5.22mg/L/d) were obtained when the bicarbonate-C/ammonium-N ratio and ammonium-N concentration were 1:1 and 150mg/L, respectively. The lutein productivity of 5.22mg/L/d is the highest value ever reported in the literature using batch phototrophic cultivation.

Transcriptome-based analysis on carbon metabolism of Haematococcus pluvialis mutant under 15% CO2

To elucidate the mechanism underlying the enhanced growth rate in the Haematococcus pluvialis mutated with ⁶⁰Co-γ rays and domesticated with 15% CO₂, transcriptome sequencing was conducted to clarify the carbon metabolic pathways of mutant cells. The CO₂ fixation rate of mutant cells increased to 2.57gL^-1d^-1 under 15% CO₂ due to the enhanced photosynthesis, carbon fixation, glycolysis pathways. The upregulation of PetH, ATPF0A and PetJ related to photosynthetic electron transport, ATP synthase and NADPH generation promoted the photosynthesis. The upregulation of genes related to Calvin cycle and ppdK promoted carbon fixation in both C3 and C4 photosynthetic pathways. The reallocation of carbon was also enhanced under 15% CO₂. The 19-, 14- and 3.5-fold upregulation of FBA, TPI and PK genes, respectively, remarkably promoted the glycolysis pathways. This accelerated the conversion of photosynthetic carbon to pyruvate, which was an essential precursor for astaxanthin and lipids biosynthesis.

Carbon dioxide (CO2) biofixation by microalgae and its potential for biorefinery and biofuel production

Carbon dioxide (CO₂) using biological process is one of the promising approaches for CO₂ capture and storage. Recently, biological sequestration using microalgae has gained many interest due to its capability to utilize CO₂ as carbon source and biomass produced can be used as a feedstock for other value added product for instance biofuel and chemicals. In this study, the CO₂ biofixation by two microalgae species, Chlorella sp. and Tetraselmis suecica was investigated using different elevated CO₂ concentration. The effect of CO₂ concentration on microalgae growth kinetic, biofixation and its chemical composition were determined using 0.04, 5, 15 and 30% CO₂. The variation of initial pH value and its relationship on CO₂ concentration toward cultivation medium was also investigated. The present study indicated that both microalgae displayed different tolerance toward CO₂ concentration. The maximum biomass production and biofixation for Chlorella sp. of 0.64gL^-1 and 96.89mgL^-1d^-1 was obtained when the cultivation was carried out using 5 and 15% CO_2, respectively. In contrast, the maximum biomass production and CO₂ biofixation for T. suecica of 0.72gL^-1 and 111.26mgL^-1d^-1 were obtained from cultivation using 15 and 5% CO₂. The pH value for the cultivation medium using CO₂ was between 7.5 and 9, which is favorable for microalgal growth. The potential of biomass obtained from the cultivation as a biorefinery feedstock was also evaluated. An anaerobic fermentation of the microalgae biomass by bacteria Clostridium saccharoperbutylacenaticum N1-4 produced various type of value added product such as organic acid and solvent. Approximately 0.27 and 0.90gL^-1 of organic acid, which corresponding to acetic and butyric acid were produced from the fermentation of Chlorella sp. and T. suecica biomass. Overall, this study suggests that Chlorella sp. and T. suecica are efficient microorganism that can be used for CO₂ biofixation and as a feedstock for chemical production.

An overview of biological processes and their potential for CO2 capture

The extensive amount of available information on global warming suggests that this issue has become prevalent worldwide. Majority of countries have issued laws and policies in response to this concern by requiring their industrial sectors to reduce greenhouse gas emissions, such as CO2. Thus, introducing new and more effective treatment methods, such as biological techniques, is crucial to control the emission of greenhouse gases. Many studies have demonstrated CO2 fixation using photo-bioreactors and raceway ponds, but a comprehensive review is yet to be published on biological CO2 fixation. A comprehensive review of CO2 fixation through biological process is presented in this paper as biological processes are ideal to control both organic and inorganic pollutants. This process can also cover the classification of methods, functional mechanisms, designs, and their operational parameters, which are crucial for efficient CO2 fixation. This review also suggests the bio-trickling filter process as an appropriate approach in CO2 fixation to assist in creating a pollution-free environment. Finally, this paper introduces optimum designs, growth rate models, and CO2 fixation of microalgae, functions, and operations in biological CO2 fixation.

Genetic Engineering: A Promising Tool to Engender Physiological, Biochemical, and Molecular Stress Resilience in Green Microalgae

As we march into the 21st century, the prevailing scenario of depleting energy resources, global warming and ever increasing issues of human health and food security will quadruple. In this context, genetic and metabolic engineering of green microalgae complete the quest toward a continuum of environmentally clean fuel and food production. Evolutionarily related, but unlike land plants, microalgae need nominal land or water, and are best described as unicellular autotrophs using light energy to fix atmospheric carbon dioxide (CO2) into algal biomass, mitigating fossil CO2 pollution in the process. Remarkably, a feature innate to most microalgae is synthesis and accumulation of lipids (60-65% of dry weight), carbohydrates and secondary metabolites like pigments and vitamins, especially when grown under abiotic stress conditions. Particularly fruitful, such an application of abiotic stress factors such as nitrogen starvation, salinity, heat shock, etc., can be used in a biorefinery concept for production of multiple valuable products. The focus of this mini-review underlies metabolic reorientation practices and tolerance mechanisms as applied to green microalgae under specific stress stimuli for a sustainable pollution-free future. Moreover, we entail current progress on genetic engineering as a promising tool to grasp adaptive processes for improving strains with potential biotechnological interests.

Potential of Monoraphidium minutum for carbon sequestration and lipid production in response to varying growth mode

Mixotrophic growth at flask level and, autotrophic-mixotrophic and autotrophic growth in photobioreactor by utilizing CO2/air/flue gas were checked for the isolated strain of Monoraphidium minutum from polluted habitat. Our study confirmed that it is a saturated fatty acid rich (30.92-68.94%) microalga with lower degree of unsaturation oil quality (42.06-103.99) making it potential biodiesel producing candidate. It showed encouraging biomass productivity (80.3-303.8mgl(-1)day(-1)) with higher total lipid (22.80-46.54%) under optimum glucose, fructose, microalgal biodiesel waste residue and sodium acetate fed mixotrophic conditions. The pH control by intermittent CO2, continuous illumination with 30% flue gas, and utilization of biodiesel glycerin were effective schemes to ameliorate either biomass productivity or % lipids or both of these parameters at photobioreactor scale (7.5L working volume). The modulation of environmental variables (pH control, CO2 and organic substrates concentration) could augment % saturated fatty acids, such as C16:0.

Desmodesmus sp. 3Dp86E-1-a novel symbiotic chlorophyte capable of growth on pure CO2

A novel chlorophyte Desmodesmus sp. 3Dp86E-1 isolated from a White Sea hydroid Dynamena pumila was cultivated at CO2 levels from atmospheric (the 'low-CO2' conditions) to pure carbon dioxide (the 5, 20, and 100 % CO2 conditions) under high (480 μE/(m(2) s) PAR) light. After 7 days of cultivation, the '100 % CO2' (but not 5 or 20 % CO2) cells possessed ca. four times higher chlorophyll content per dry weight (DW) unit than the low-CO2 culture. The rate of CO2 fixation under 100 % CO2 comprised ca. 1.5 L/day per L culture volume. After a lag period which depended on the CO2 level, biomass accumulation and volumetric fatty acid (FA) content of the Desmodesmus sp. 3Dp86E-1 bubbled with CO2-enriched gas mixtures increased and was comparable to that of the culture continuously bubbled with air. Under the low-to-moderate CO2 conditions, the FA percentage of the algal cells increased (to 40 % DW) whereas under high-CO2 conditions, FA percentage did not exceed 15 % DW. A strong increase in oleate (18:1) proportion of total FA at the expense of linolenate (18:3) was recorded in the '100 % CO2' cells. Electron microscopy and pulse-amplitude-modulated chlorophyll fluorescence investigation revealed no damage to or significant downregulation of the photosynthetic apparatus in '100 % CO2' cells grown at the high-PAR irradiance. Possible mechanisms of high-CO2 tolerance of Desmodesmus sp. 3Dp86E-1 are discussed in view of its symbiotic origin and possible application for CO2 biomitigation.

Improvement of the phosphoenolpyruvate carboxylase activity of Phaeodactylum tricornutum PEPCase 1 through protein engineering

In order to mitigate CO2 accumulation and decrease the rate of global warming and climate change, we previously presented a strategy for the development of an efficient CO2 capture and utilization system. The system employs two recombinant enzymes, carbonic anhydrase and phosphoenolpyruvate carboxylase, which were originated from microalgae. Although utilization of this integrated system would require a large quantity of high quality PEPCase protein, such quantities could be produced by increasing the solubility of the Phaeodactylum tricornutum PEPCase 1 (PtPEPCase 1) protein in the Escherichia coli heterologous expression system. We first expressed the putative mitochondria targeting peptide- and chloroplast transit peptide-truncated proteins of PtPEPCase 1, mPtPEPCase 1 and cPtPEPCase 1, respectively, in E. coli. After affinity chromatography, the amount of purified PEPCase protein from 500mL of E. coli culture was greatest for cPtPEPCase 1 (1.99mg), followed by mPtPEPCase 1 (0.82mg) and PtPEPCase 1 (0.61mg). Furthermore, the enzymatic activity of mPtPEPCase 1 and cPtPEPCase 1 showed approximately 1.6-fold (32.19 units/mg) and 3-fold (59.48 units/mg) increases, respectively. Therefore, cPtPEPCase 1 purified using the E. coli heterogeneous expression system could be a strong candidate for a platform technology to capture CO2 and produce value-added four-carbon platform chemicals.

Characterization of carbonic anhydrase II from Chlorella vulgaris in bio-CO2 capture

Carbonic anhydrase II (CA II) can catalyze the reversible hydration reaction of CO(2) at a maximum of 1.4 × 10(6) molecules of CO(2) per second. The crude intracellular enzyme extract containing CA II was derived from Chlorella vulgaris. A successful CO(2) capture experiment with the presence of calcium had been conducted on the premise that the temperature was conditioned at a scope of 30-40 °C, that the biocatalyst-nurtured algal growth period lasted 3 days, and that pH ranged from7.5 to 8.5. Ions of K(+), Na(+), Ca(2+), Co(2+), Cu(2+), Fe(3+), Mg(2+), Mn(2+), and Zn(2+) at 0.01, 0.1, and 0.5 M were found to exhibit no more than 30 % inhibition on the residual activity of the biocatalyst. It is reasonable to expect that calcification catalyzed by microalgae presents an alternative to geological carbon capture and sequestration through a chain of fundamental researches carried on under the guidance of sequestration technology.

Photosynthetic accumulation of carbon storage compounds under CO₂ enrichment by the thermophilic cyanobacterium Thermosynechococcus elongatus

The growth characteristics of Thermosynechococcus elongatus on elevated CO₂ were studied in a photobioreactor. Cultures were able to grow on up to 20% CO₂. The maximum productivity and CO₂ fixation rates were 0.09 ± 0.01 and 0.17 ± 0.01 mg ml⁻¹ day⁻¹, respectively, for cultures grown on 20% CO₂. Three major carbon pools--lipids, polyhydroxybutyrates (PHBs), and glycogen--were measured. These carbon stores accounted for 50% of the total biomass carbon in cultures grown on atmospheric CO₂ (no supplemental CO₂), but only accounted for 30% of the total biomass carbon in cultures grown on 5-20% CO₂. Lipid content was approximately 20% (w/w) under all experimental conditions, while PHB content reached 14.5% (w/w) in cultures grown on atmospheric CO₂ and decreased to approximately 2.0% (w/w) at 5-20% CO₂. Glycogen levels did not vary significantly and remained about 1.4% (w/w) under all test conditions. The maximum amount of CO₂ sequestered over the course of the nine-day chemostat experiment was 1.15 g l⁻¹ in cultures grown on 20% CO₂.

Regulation of the expression of H43/Fea1 by multi-signals

The composition of extracellular proteins is known to be drastically changed in the unicellular green alga Chlamydomonas reinhardtii when the cells are transferred from ambient CO(2) to elevated CO(2) conditions. We previously observed very high production of the H43/Fea1 protein under high-CO(2) (0.3-3% in air) conditions. In addition, H43/Fea1 gene expression was reported to be induced under iron-deficient and cadmium-excess conditions, but it remains unclear how gene expression is regulated by multiple signals. To elucidate the regulatory mechanism of H43/Fea1 expression, this study intended to identify a high-CO(2)-responsive cis-element in a wall-deficient strain C. reinhardtti CC-400. Cells incubated in the presence of acetate in the dark, namely heterotrophically generated high-CO(2) conditions, were used for inducing H43/Fea1 gene expression following our previous study (Hanawa et al., Plant Cell Physiol 48:299-309, 2007) in Fe-sufficient and Cd-deficient medium to prevent the generation of other signals. First, we constructed a reporter assay system using transformants constructed by introducing genes with series of 5'-deleted upstream sequences of H43/Fea1 that were fused to a coding sequence of the Ars for arylsulfatase2 reporter gene. Consequently, the high-CO(2)-responsive cis-element (HCRE) was found to be located at a -537/-370 upstream region from the transcriptional initiation site of H43/Fea1. However, it still remains possible that a -724/-537 upstream region may also have a significant role in activating gene expression regulated by high-CO(2). Remarkably, a -925/-370 upstream region could successfully activate the Ars reporter gene under heterotrophically generated high-CO(2) conditions even when the sequence containing two Fe-deficiency-responsive elements was completely deleted. These results clearly showed that H43/Fea1 expression is regulated by high-CO(2) signal independently via the HCRE that is located distantly from Fe-deficient-signal responsive element, indicating that H43/Fea1 is a multi-signal-regulated gene.

Inhibition of the growth of two blue-green algae species (Microsystis aruginosa and Anabaena spiroides) by acidification treatments using carbon dioxide

The effect of pH adjusted by aeration with carbon dioxide (CO(2)) on the growth of two species of blue-green algae, Microcystis aeruginosa and Anabaena spiroides, was investigated. Three conditions (pH 5.5, 6.0 and 6.5) were found to have significant inhibitory effects on the growth of the two algae species when acidification treatment was conducted during the logarithmic phase. Differences in the inhibition effect of acidification existed between the two species algae. The tolerance of M. aeruginosa to these conditions was also investigated. The results indicated that M. aeruginosa was inhibited significantly, but not dead at pH 6.5, whereas death occurred at pH 5.5 and 6.0. The greatest inhibitory effect of acidification treatment conducted during the stable breeding phase of M. aeruginosa occurred at pH 5.5, while no inhibitory effect was found at pH 6.5.

Auxiliary electron transport pathways in chloroplasts of microalgae

Microalgae are photosynthetic organisms which cover an extraordinary phylogenic diversity and have colonized extremely diverse habitats. Adaptation to contrasted environments in terms of light and nutrient's availabilities has been possible through a high flexibility of the photosynthetic machinery. Indeed, optimal functioning of photosynthesis in changing environments requires a fine tuning between the conversion of light energy by photosystems and its use by metabolic reaction, a particularly important parameter being the balance between phosphorylating (ATP) and reducing (NADPH) power supplies. In addition to the main route of electrons operating during oxygenic photosynthesis, called linear electron flow or Z scheme, auxiliary routes of electron transfer in interaction with the main pathway have been described. These reactions which include non-photochemical reduction of intersystem electron carriers, cyclic electron flow around PSI, oxidation by molecular O(2) of the PQ pool or of the PSI electron acceptors, participate in the flexibility of photosynthesis by avoiding over-reduction of electron carriers and modulating the NADPH/ATP ratio depending on the metabolic demand. Forward or reverse genetic approaches performed in model organisms such as Arabidopsis thaliana for higher plants, Chlamydomonas reinhardtii for green algae and Synechocystis for cyanobacteria allowed identifying molecular components involved in these auxiliary electron transport pathways, including Ndh-1, Ndh-2, PGR5, PGRL1, PTOX and flavodiiron proteins. In this article, we discuss the diversity of auxiliary routes of electron transport in microalgae, with particular focus in the presence of these components in the microalgal genomes recently sequenced. We discuss how these auxiliary mechanisms of electron transport may have contributed to the adaptation of microalgal photosynthesis to diverse and changing environments.

Biomass production potential of a wastewater alga Chlorella vulgaris ARC 1 under elevated levels of CO₂and temperature

The growth response of Chlorella vulgaris was studied under varying concentrations of carbon dioxide (ranging from 0.036 to 20%) and temperature (30, 40 and 50°C). The highest chlorophyll concentration (11 μg mL^–1) and biomass (210 μg mL^–1), which were 60 and 20 times more than that of C. vulgaris at ambient CO₂ (0.036%), were recorded at 6% CO₂ level. At 16% CO₂ level, the concentrations of chlorophyll and biomass values were comparable to those at ambient CO₂ but further increases in the CO₂ level decreased both of them. Results showed that the optimum temperature for biomass production was 30°C under elevated CO₂ (6%). Although increases in temperature above 30°C resulted in concomitant decrease in growth response, their adverse effects were significantly subdued at elevated CO₂. There were also differential responses of the alga, assessed in terms of NaH¹⁴CO₃ uptake and carbonic anhydrase activity, to increases in temperature at elevated CO₂. The results indicated that Chlorella vulgaris grew better at elevated CO₂ level at 30°C, albeit with lesser efficiencies at higher temperatures.

Photosynthetic carbon assimilation in the coccolithophorid Emiliania huxleyi (Haptophyta): Evidence for the predominant operation of the c3 cycle and the contribution of {beta}-carboxylases to the active anaplerotic reaction

The coccolithophorid Emiliania huxleyi (Haptophyta) is a representative and unique marine phytoplankton species that fixes inorganic carbon by photosynthesis and calci-fication. We examined the initial process of photosynthetic carbon assimilation by analyses of metabolites, enzymes and genes. When the cells were incubated with a radioactive substrate (2.3 mM NaH(14)CO(3)) for 10 s under illumination, 70% of the (14)C was incorporated into the 80% methanol-soluble fraction. Eighty-five and 15% of (14)C in the soluble fraction was incorporated into phosphate esters (P-esters), including the C(3) cycle intermediates and a C(4) compound, aspartate, respectively. A pulse-chase experiment showed that (14)C in P-esters was mainly transferred into lipids, while [(14)C]aspartate, [(14)C]alanine and [(14)C]glutamate levels remained almost constant. These results indicate that the C(3) cycle functions as the initial pathway of carbon assimilation and that beta-carboxylation contributes to the production of amino acids in subsequent metabolism. Transcriptional analysis of beta-carboxylases such as pyruvate carboxylase (PYC), phosphoenolpyruvate carboxylase (PEPC) and phosphoenolpyruvate carboxykinase (PEPCK) revealed that PYC and PEPC transcripts were greatly increased under illumination, whereas the PEPCK transcript decreased remarkably. PEPC activity was higher in light-grown cells than in dark-adapted cells. PYC activity was detected in isolated chloroplasts of light-grown cells. According to analysis of their deduced N-terminal sequence, PYC and PEPC are predicted to be located in the chloroplasts and mitochondria, respectively. These results suggest that E. huxleyi possesses unique carbon assimila-tion mechanisms in which beta-carboxylation by both PYC and PEPC plays important roles in different organelles.

Bioenergetic changes in the microalgal photosynthetic apparatus by extremely high CO2 concentrations induce an intense biomass production

Unicellular green alga Chlorella minutissima, grown under extreme carbon dioxide concentrations (0.036-100%), natural temperature and light intensities (Mediterranean conditions), strongly increase the microalgal biomass through photochemical and non-photochemical changes in the photosynthetic apparatus. Especially, CO(2) concentrations up to 10% enhance the density of active reaction centers (RC/CS(o)), decrease the antenna size per active reaction center (ABS/RC), decrease the dissipation energy (DI(o)/RC) and enhance the quantum yield of primary photochemistry (F(v)/F(m)). Higher CO(2) concentrations (20-25%) combine the above-mentioned photochemical changes with enhanced non-photochemical quenching of surplus energy, which leads to an enhanced steady-state fraction of 'open' (oxidized) PSII reaction centers (q(p)), and minimize the excitation pressure of PSII (1 - q(p)) under very high light intensities (approximately 1700 micromol m(-2) s(-1) maximal value), avoiding the photoinhibition and leading to an enormous biomass production (approximately 2500%). In conclusion, these extreme CO(2) concentrations - about 1000 times higher than the ambient one - can be easily metabolized from the unicellular green alga to biomass and can be used, on a local scale at least, for the future development of microalgal photobioreactors for the mitigation of the factory-produced carbon dioxide.