Bio-mitigation of CO(2), calcite formation and simultaneous biodiesel precursors production using Chlorella sp

Reforming CO2 bio-mitigation utilizing Bacillus cereus from hypersaline realms in pilot-scale bubble column bioreactor

The bubble column reactor of 10 and 20 L capacity was designed to bio-mitigate 10% CO2 (g) with 90% air utilizing thermophilic bacteria (Bacillus cereus SSLMC2). The maximum biomass yield during the growth phase was obtained as 9.14 and 10.78 g L-1 for 10 and 20 L capacity, respectively. The maximum removal efficiency for CO2 (g) was obtained as 56% and 85% for the 10 and 20 L reactors, respectively. The FT-IR and GC-MS examination of the extracellular and intracellular samples identified value-added products such as carboxylic acid, fatty alcohols, and hydrocarbons produced during the process. The total carbon balance for CO2 utilization in different forms confirmed that B. cereus SSLMC2 utilized 1646.54 g C in 10 L and 1587 g of C in 20 L reactor out of 1696.13 g of total carbon feed. The techno-economic assessment established that the capital investment required was $286.21 and $289.08 per reactor run of 11 days and $0.167 and $0.187 per gram of carbon treated for 10 and 20 L reactors, respectively. The possible mechanism pathways for bio-mitigating CO2 (g) by B. cereus SSLMC2 were also presented utilizing the energy reactions. Hence, the work presents the novelty of utilizing thermophilic bacteria and a bubble column bioreactor for CO2 (g) bio-mitigation.

Integrated Approach for Carbon Sequestration and Wastewater Treatment Using Algal–Bacterial Consortia: Opportunities and Challenges

Increasing concentrations of carbon dioxide (CO2), one of the important greenhouse gases, due to combustion of fossil fuels, particularly burning coal, have become the major cause for global warming. As a consequence, many research programs on CO2 management (capture, storage, and sequestration) are being highlighted. Biological sequestration of CO2 by algae is gaining importance, as it makes use of the photosynthetic capability of these aquatic species to efficiently capture CO2 emitted from various industries and converting it into algal biomass as well as a wide range of metabolites such as polysaccharides, amino acids, fatty acids, pigments, and vitamins. In addition, their ability to thrive in rugged conditions such as seawater, contaminated lakes, and even in certain industrial wastewaters containing high organic and inorganic nutrients loads, has attracted the attention of researchers to integrate carbon capture and wastewater treatment. Algae offer a simple solution to tertiary treatments due to their nutrient removal efficiency, particularly inorganic nitrogen and phosphorus uptake. The algal–bacterial energy nexus is an important strategy capable of removing pollutants from wastewater in a synergistic manner. This review article highlights the mechanism involved in biological fixation of CO2 by microalgae, their cultivation systems, factors influencing algal cultivation in wastewater and CO2 uptake, the effect of co-cultivation of algae and bacteria in wastewater treatment systems, and challenges and opportunities.

Optimised spectral effects of programmable LED arrays (PLA)s on bioelectricity generation from algal-biophotovoltaic devices

The biophotovoltaic cell (BPV) is deemed to be a potent green energy device as it demonstrates the generation of renewable energy from microalgae; however, inadequate electron generation from microalgae is a significant impediment for functional employment of these cells. The photosynthetic process is not only affected by the temperature, CO₂ concentration and light intensity but also the spectrum of light. Thus, a detailed understanding of the influences of light spectrum is essential. Accordingly, we developed spectrally optimized light using programmable LED arrays (PLA)s to study the effect on algae growth and bioelectricity generation. Chlorella is a green microalga and contains chlorophyll-a (chl-a), which is the major light harvesting pigment that absorbs light in the blue and red spectrum. In this study, Chlorella is grown under a PLA which can optimally simulate the absorption spectrum of the pigments in Chlorella. This experiment investigated the growth, photosynthetic performance and bioelectricity generation of Chlorella when exposed to an optimally-tuned light spectrum. The algal BPV performed better under PLA with a peak power output of 0.581 mW m⁻² for immobilized BPV device on day 8, which is an increase of 188% compared to operation under a conventional white LED light source. The photosynthetic performance, as measured using pulse amplitude modulation (PAM) fluorometry, showed that the optimized spectrum from the PLA gave an increase of 72% in the rETRmax value (190.5 μmol electrons m⁻² s⁻¹), compared with the conventional white light source. Highest algal biomass (1100 mg L⁻¹) was achieved in the immobilized system on day eight, which translates to a carbon fixation of 550 mg carbon L⁻¹. When artificial light is used for the BPV system, it should be optimized with the light spectrum and intensity best suited to the absorption capability of the pigments in the cells. Optimum artificial light source with algal BPV device can be integrated into a power management system for low power application (eg. environment sensor for indoor agriculture system).

Upgrading of microalgal consortia with CO2 from fermentation of wheat straw for the phycoremediation of domestic wastewater

Algae have been considered as a best feedstock for combating CO2. In the present study, two mixed microalgal cultures i.e. MAC1 and MAC2 were evaluated in batch mode with an extraneous supply of CO2 from the fermentation of wheat straw. Both the mixed cultures displayed promising CO2 sequestration potentials of 287 and 263 mg L-1d-1, respectively. The removal efficiencies in terms of ammonium, phosphate, chemical oxygen demand, and nitrate were found to be 87%, 78%, 68% and 65%, respectively. Enriching the tolerance of the microalgal consortia to CO2 supply and wastewater as the nutrient source significantly enhanced the lipid production for both the microalgae consortia. Lipid contents of MAC1 and MAC2 were observed to be 12.29 & 11.37%, respectively while the biomass yield from both the consortia was 0.36 g L-1. Total chlorophyll and protein contents of MAC1 and MAC2 were 14.27 & 12.28 µgmL-1 and 0.13 & 0.15 mgmL-1, respectively. Both the consortia found to have significant potential for CO2 sequestration, wastewater remediation and biofuel production.

High density long-term cultivation of Chlorella vulgaris SAG 211-12 in a novel microgravity-capable membrane raceway photobioreactor for future bioregenerative life support in SPACE

Hybrid life support systems are of great interest for future far-distant space exploration missions to planetary surfaces, e.g. Mars, planned until 2050. By synergistically combining physicochemical and biotechnological algae-based subsystems, an essential step towards the closure of the carbon loop in environmental control and life support systems (ECLSS) shall be accomplished, offering a wide beneficial potential for ECLSS through the utilization of oxygenic photosynthesis: O₂ and potential human food can be formed in-situ from CO₂ and water. The wild type green alga Chlorella vulgaris strain SAG 211-12 was selected as model microorganism due to its photoautotrophic growth, high biomass yield, cultivation flexibility and long-term cultivation robustness. The current study presents for the first time a stable xenic long-term processing of microalgae in a novel microgravity capable membrane raceway photobioreactor for 188 days with the focus on algal growth kinetics and gas evolution. In particular, culture homogeneity and viability were monitored and evaluated during the whole cultivation process due to their putative crucial impact on long-term functionality and efficiency of a closed cultivation system. Based on a specially designed cyclic batch cultivation process for SAG 211-12, a successive biomass growth up to a maximum of 12.2 g l^-1 with a max. global volumetric productivity of 1.3 g l^-1 d^-1 was reached within the closed loop system. The photosynthetic capacity was assessed to a global molar photosynthetic quotient of 0.31. Furthermore, cultivation parameters for a change from batch to continuous processing at high biomass densities and proliferation rates are introduced. The presented µgPBR miniature plant and the developed high throughput cultivation process are planned to be tested under real space conditions within the PBR@LSR project (microgravity and cosmic radiation) aboard the International Space Station with an operation period of up to 180 days to investigate the impact on long-term system stability.

Modification and improvement of microalgae strains for strengthening CO2 fixation from coal-fired flue gas in power plants

Biological CO₂ capture using microalgae is a promising new method for reducing CO₂ emission of coal-fired flue gas. The strain of microalgae used in this process plays a vital role in determining the rate of CO₂ fixation and characteristics of biomass production. High requirements are put forward for algae strains due to high CO₂ concentration and diverse pollutants in flue gas. CO₂ can directly diffuse into the cytoplasm of cells by extra- and intracellular CO₂ osmotic pressure under high CO₂ concentrations. The flue gas pollutants, such as SO_x, NO_x and fly ashes, have negative effects on the growth of microalgae. This work reviewed the state-of-the-art advances on microalgae strains used for CO₂ fixation, focusing on the modification and improvement of strains that are used for coal-fired flue gas. Methods such as genetic engineering, random mutagenesis, and adaptive evolution have the potential to facilitate photosynthesis, improve growth rate and reduce CO₂ emission.

Enhanced biomass and lipid production of Neochloris oleoabundans under high light conditions by anisotropic nature of light-splitting CaCO3 crystal

The aim of this work was to study the anisotropic effect of crystalline CaCO₃ nanoparticles (CN)-driven multiple refraction/scattering from the CN-coated agglomerated cells on the rate of photosynthesis and the product yield under high light conditions in the freshwater microalgae Neochloris oleoabundans. The CN-coating via biomineralization significantly improved the biomass and lipid production of N. oleoabundans during second stage of autotrophic induction by sustaining relatively high rate of photosynthesis at high irradiance using the multiple-splitting effect of the anisotropic polymorphism. The CN were successfully produced, adsorbed and grown on the external cells under conditions of mild alkalinity (pH 7.5-8.0), mild CaCl₂ concentration (0.05 M) and under nitrogen starvation with strong light (400 µE m^-2 s^-1). Consequently, lipid content and productivity of N. oleoabundans cells cultured with 0.05 M CaCl₂ increased by 18.4% and 31.5%, respectively, compared to the cells cultured with 0.05 M CaCl₂ and acetazolamide to inhibit calcification.

Enhancing Hydrogen Production from Chlorella sp. Biomass by Pre-Hydrolysis with Simultaneous Saccharification and Fermentation (PSSF)

Simultaneous saccharification and fermentation (SSF) and pre-hydrolysis with SSF (PSSF) were used to produce hydrogen from the biomass of Chlorella sp. SSF was conducted using an enzyme mixture consisting of 80 filter paper unit (FPU) g-biomass−1 of cellulase, 92 U g-biomass−1 of amylase, and 120 U g-biomass−1 of glucoamylase at 35 °C for 108 h. This yielded 170 mL-H2 g-volatile-solids−1 (VS), with a productivity of 1.6 mL-H2 g-VS−1 h−1. Pre-hydrolyzing the biomass at 50 °C for 12 h resulted in the production of 1.8 g/L of reducing sugars, leading to a hydrogen yield (HY) of 172 mL-H2 g-VS−1. Using PSSF, the fermentation time was shortened by 36 h in which a productivity of 2.4 mL-H2 g-VS−1 h−1 was attained. To the best of our knowledge, the present study is the first report on the use of SSF and PSSF for hydrogen production from microalgal biomass, and the HY obtained in the study is by far the highest yield reported. Our results indicate that PSSF is a promising process for hydrogen production from microalgal biomass.

Carbon-dioxide biofixation and phycoremediation of municipal wastewater using Chlorella vulgaris and Scenedesmus obliquus

The pure cultures of microalgae Chlorella vulgaris ATCC 13482 and Scenedesmus obliquus FACHB 417 were grown in municipal wastewater in 7-L airlift bubble column photobioreactor supplied with 5% CO₂/air (v/v). Batch experiments were conducted at 25 °C with 14-h light/10-h dark cycle for a period of 10 days. The CO₂ capture efficiencies for both the microalgae were monitored in terms of their respective biomass productivities, carbon contents, and CO₂ consumption rates. In the present study, the initial concentration of ammonia (43.7 mg L^-1) was decreased to 2.9 and 3.7 mg L^-1 by C. vulgaris and S. obliquus, respectively. And, the initial concentration of phosphate (18.5 mg L^-1) was decreased to 1.1 and 1.6 mg L^-1 by C. vulgaris and S. obliquus, respectively. CO₂ biofixation rates by C. vulgaris and S. obliquus, cultivated in municipal wastewater, were calculated to be 140.91 and 129.82 mg L^-1 day^-1, respectively. The findings from the present study highlight the use of microalgae for wastewater treatment along with CO₂ uptake and biomass utilization for pilot scale production of biodiesel, biogas, feed supplements for animals, etc., thus minimizing the production costs.

Prospects, recent advancements and challenges of different wastewater streams for microalgal cultivation

Microalgae are recognized as one of the most powerful biotechnology platforms for many value added products including biofuels, bioactive compounds, animal and aquaculture feed etc. However, large scale production of microalgal biomass poses challenges due to the requirements of large amounts of water and nutrients for cultivation. Using wastewater for microalgal cultivation has emerged as a potential cost effective strategy for large scale microalgal biomass production. This approach also offers an efficient means to remove nutrients and metals from wastewater making wastewater treatment sustainable and energy efficient. Therefore, much research has been conducted in the recent years on utilizing various wastewater streams for microalgae cultivation. This review identifies and discusses the opportunities and challenges of different wastewater streams for microalgal cultivation. Many alternative routes for microalgal cultivation have been proposed to tackle some of the challenges that occur during microalgal cultivation in wastewater such as nutrient deficiency, substrate inhibition, toxicity etc. Scope and challenges of microalgal biomass grown on wastewater for various applications are also discussed along with the biorefinery approach.

CO2 utilization in the production of biomass and biocompounds by three different microalgae

The atmospheric CO₂ increase is considered the main cause of global warming. Microalgae are photosynthetic microorganisms that can help in CO₂ mitigation and at the same time produce value-added compounds. In this study, Scenedesmus obliquus, Chlorella vulgaris, and Chlorella protothecoides were cultivated under 0.035 (air), 5 and 10% (v/v) of CO₂ concentrations in air to evaluate the performance of the microalgae in terms of kinetic growth parameters, theoretical CO₂ biofixation rate, and biomass composition. Among the microalgae studied, S. obliquus presented the highest values of specific growth rate (μ = 1.28 d^-1), maximum productivities (P _max = 0.28 g L^-1d^-1), and theoretical CO₂ biofixation rates (0.56 g L^-1d^-1) at 10% CO₂. The highest oil content was found at 5% CO₂, and the fatty acid profile was not influenced by the concentration of CO₂ in the inflow gas mixture and was in compliance with EN 14214, being suitable for biodiesel purposes. The impact of the CO₂ on S. obliquus cells' viability/cell membrane integrity evaluated by the in-line flow cytometry is quite innovative and fast, and revealed that 86.4% of the cells were damaged/permeabilized in cultures without the addition of CO₂.

Optimization of carbon dioxide supply in raceway reactors: Influence of carbon dioxide molar fraction and gas flow rate

Influence of CO2 composition and gas flow rate to control pH in a pilot-scale raceway producing Scenedesmus sp. was studied. Light and temperature determined the biomass productivity whereas neither the CO2 molar fraction nor the gas flow rate used influenced it; because pH was always controlled and carbon limitation did not take place. The CO2 molar fraction and the gas flow rate influenced carbon loss in the system. At low CO2 molar fraction (2-6%) or gas flow rate (75-100l·min(-1)) the carbon efficiency in the sump was higher than 95%, 85% of the injected carbon being transformed into biomass. Conversely, at high CO2 molar fraction (14%) or gas flow rate (150l·min(-1)) the carbon efficiency in the sump was lower than 67%, 32% of the carbon being fixed as biomass. Analysis here reported allows the pH control to be optimized and production costs to be reduced by optimizing CO2 efficiency.

Modelling of oxygen-evolving-complex ionization dynamics for energy-efficient production of microalgal biomass, pigment and lipid with carbon capture: an engineering vision for a biorefinery

Development of an algal growth kinetics model, incorporating oxygen-evolving-complex ionization dynamics, for sustainable production of algal biomass, lipid, and chlorophyll (with associated carbon dioxide capture) in an algal biorefinery.

Performance evaluation of a green process for microalgal CO2 sequestration in closed photobioreactor using flue gas generated in-situ

In the present study, carbon-dioxide capture from in situ generated flue gas was carried out using Chlorella sp. in bubble column photobioreactors to develop a cost effective process for concomitant carbon sequestration and biomass production. Firstly, a comparative analysis of CO2 sequestration with varying concentrations of CO2 in air-CO2 and air-flue gas mixtures was performed. Chlorella sp. was found to be tolerant to 5% CO2 concentration. Subsequently, inhibitory effect of pure flue gas was minimized using various strategies like use of high initial cell density and photobioreactors in series. The final biofixation efficiency was improved by 54% using the adopted strategies. Further, sequestered microalgal biomass was analyzed for various biochemical constituents for their use in food, feed or biofuel applications.

Improving high carbon dioxide tolerance and carbon dioxide fixation capability of Chlorella sp. by adaptive laboratory evolution

CO2 capture by microalgae is a promising method to reduce greenhouse gas emissions. It is critical to construct a highly efficient way to obtain a microalgal strain tolerant to high CO2 concentrations with high CO2 fixation capability. In this study, two evolved Chlorella sp. strains, AE10 and AE20 were obtained after 31 cycles of adaptive laboratory evolution (ALE) under 10% and 20% CO2, respectively. Both of them grew rapidly in 30% CO2 and the maximal biomass concentration of AE10 was 3.68±0.08g/L, which was 1.22 and 2.94 times to those of AE20 and original strain, respectively. The chlorophyll contents of AE10 and AE20 were significantly higher than those of the original one under 1-30% CO2. The influences of ALE process on biochemical compositions of Chlorella cells were also investigated. This study proved that ALE was an effective approach to improve high CO2 tolerance of Chlorella sp.

Estimation of carbon dioxide sequestration potential of microalgae grown in a batch photobioreactor

The carbon dioxide (CO2) sequestration potential of two microalgae, Chlorella pyrenoidosa and Scenedesmus abundans was evaluated in a tubular batch photobioreactor with provision for continuous flow of 10% CO2 enriched air through the headspace. CO2 sequestration and biomass growth was affected by gas flow rate over the range 20-60ml/min and 40ml/min was found to maximize algal growth and CO2 sequestration. Moles of CO2 sequestered over 20h at a gas flow rate of 40ml/min was estimated using a novel rapid screening approach as 0.096 and 0.036, respectively, for C. pyrenoidosa and S. abundans. At this gas flow rate the maximum growth rate was 4.9mgL(-1)h(-1) and 2.5mgL(-1)h(-1) for C. pyrenoidosa and S. abundans, respectively. The CO2 sequestration and growth rate were comparable at height/diameter ratio of 8 and 16.

Lipid production of microalga Ankistrodesmus falcatus increased by nutrient and light starvation in a two-stage cultivation process

The aim of this work was to study the stimulation of lipid production on the microalga Ankistrodesmus falcatus by varying cultivation conditions during the stationary phase. The effect of three factors (presence and absence of nitrogen, phosphorus, and light) has been tested once the cultures reached the stationary phase with the aim to increase the value of the biomass for further applications. Lipid content, elemental composition, Nile red fluorescence evolution, and calorific value of microalgal biomass were studied as well as biomass growth. Biomass presented a lipid content of 36.54 % at the end of the first stage, while at the end of the second stage, the experiments with the absence of phosphorus increased their lipid content until 45.94 and 44.55 %, the first with nitrogen and light presence and the second with absence of all factors. The combination of phosphorus absence and nitrogen and light presence achieved the highest lipid productivity (20.27 mg/L/day). The two-stage strategy to culture microalgae is a feasible option to increase the economic or energetic value of biomass.

Evaluation of carbon dioxide mass transfer in raceway reactors for microalgae culture using flue gases

Mass transfer of CO2 from flue gas was quantified in a 100m(2) raceway. The carbonation sump was operated with and without a baffle at different liquid/gas ratios, with the latter having the greatest influence on CO2 recovery from the flue gas. A rate of mass transfer sufficient to meet the demands of an actively growing algal culture was best achieved by maintaining pH at ∼8. Full optimisation of the process required both pH control and selection of the best liquid/gas flow ratio. A carbon transfer rate of 10gCmin(-1) supporting an algal productivity of 17gm(-2)day(-1) was achieved with only 4% direct loss of CO2 in the sump. 66% of the carbon was incorporated into biomass, while 6% was lost by outgassing and the remainder as dissolved carbon in the liquid phase. Use of a sump baffle required additional power without significantly improving carbon mass transfer.

Carbonic anhydrases in industrial applications

Carbonic anhydrases (CAs) catalyze a fundamental reaction: the reversible hydration and dehydration of carbon dioxide (CO2) and bicarbonate ([Formula: see text]), respectively. Current methods for CO2 capture and sequestration are harsh, expensive, and require prohibitively large energy inputs, effectively negating the purpose of removing CO2 from the atmosphere. Due to CA's activity on CO2 there is increasing interest in using CAs for industrial applications such as carbon sequestration and biofuel production. A lot of work in the last decade has focused on immobilizing CA onto various supports for incorporation into CO2 scrubbing applications or devices. Although the proof of principle has been validated, current CAs being tested do not withstand the harsh industrial conditions. The advent of large-scale genome sequencing projects has resulted in several emerging efforts seeking out novel CAs from a variety of microorganisms, including bacteria, micro-, and macro-algae. CAs are also being investigated for their use in medical applications, such drug delivery systems and artificial lungs. This review also looks at possible downstream uses of captured and sequestered CO2, from using it to enhance oil recovery to incorporating it into useful and financially viable products.

Biology and Industrial Applications of Chlorella: Advances and Prospects

Chlorella represents a group of eukaryotic green microalgae that has been receiving increasing scientific and commercial interest. It possesses high photosynthetic ability and is capable of growing robustly under mixotrophic and heterotrophic conditions as well. Chlorella has long been considered as a source of protein and is now industrially produced for human food and animal feed. Chlorella is also rich in oil, an ideal feedstock for biofuels. The exploration of biofuel production by Chlorella is underway. Chlorella has the ability to fix carbon dioxide efficiently and to remove nutrients of nitrogen and phosphorous, making it a good candidate for greenhouse gas biomitigation and wastewater bioremediation. In addition, Chlorella shows potential as an alternative expression host for recombinant protein production, though challenges remain to be addressed. Currently, omics analyses of certain Chlorella strains are being performed, which will help to unravel the biological implications of Chlorella and facilitate the future exploration of industrial applications.

Dynamic microstructures and fractal characterization of cell wall disruption for microwave irradiation-assisted lipid extraction from wet microalgae

To extract lipids from wet microalgae through cell disruption, the effects of microwave treatment on the dynamic cell wall microstructures were investigated. The fractal dimension of raw, untreated microalgal cells was 1.46. The disruption level of microalgal cell walls was enhanced when microwave treatment temperature increased from 80 to 120°C, resulting in an increase in microalgal cell fractal dimension from 1.61 to 1.91. The cell wall thickness and pore diameters in cell walls increased from 0.11 to 0.59 μm and from 0.005 to 0.18 μm, respectively, when microwave treatment time increased from 0 to 20 min. The outer pectin layers of cell walls gradually detached and the porosity of inner cellulose layers increased when microwave treatment time increased to 26 min. The initial point of disruption appeared at the maximum curvature (approximately 1.01×10(7) m(-1)) of cell walls. Numbers of short-chain and saturated lipids increased because of microwave electromagnetic effect.

Structural and catalytic characterization of a thermally stable and acid-stable variant of human carbonic anhydrase II containing an engineered disulfide bond

The carbonic anhydrases (CAs) are a family of mostly zinc metalloenzymes that catalyze the reversible hydration of CO2 to bicarbonate and a proton. Recently, there has been industrial interest in utilizing CAs as biocatalysts for carbon sequestration and biofuel production. The conditions used in these processes, however, result in high temperatures and acidic pH. This unfavorable environment results in rapid destabilization and loss of catalytic activity in CAs, ultimately resulting in cost-inefficient high-maintenance operation of the system. In order to negate these detrimental industrial conditions, cysteines at residues 23 (Ala23Cys) and 203 (Leu203Cys) were engineered into a wild-type variant of human CA II (HCAII) containing the mutation Cys206Ser. The X-ray crystallographic structure of the disulfide-containing HCAII (dsHCAII) was solved to 1.77 Å resolution and revealed that successful oxidation of the cysteine bond was achieved while also retaining desirable active-site geometry. Kinetic studies utilizing the measurement of (18)O-labeled CO2 by mass spectrometry revealed that dsHCAII retained high catalytic efficiency, and differential scanning calorimetry showed acid stability and thermal stability that was enhanced by up to 14 K compared with native HCAII. Together, these studies have shown that dsHCAII has properties that could be used in an industrial setting to help to lower costs and improve the overall reaction efficiency.

Carbonic Anhydrase: An Efficient Enzyme with Possible Global Implications

As the global atmospheric emissions of carbon dioxide (CO2) and other greenhouse gases continue to grow to record-setting levels, so do the demands for an efficient and inexpensive carbon sequestration system. Concurrently, the first-world dependence on crude oil and natural gas provokes concerns for long-term availability and emphasizes the need for alternative fuel sources. At the forefront of both of these research areas are a family of enzymes known as the carbonic anhydrases (CAs), which reversibly catalyze the hydration of CO2into bicarbonate. CAs are among the fastest enzymes known, which have a maximum catalytic efficiency approaching the diffusion limit of 108 M−1s−1. As such, CAs are being utilized in various industrial and research settings to help lower CO2atmospheric emissions and promote biofuel production. This review will highlight some of the recent accomplishments in these areas along with a discussion on their current limitations.

Enhancement of lipid accumulation in Scenedesmus obliquus by Optimizing CO2 and Fe3+ levels for biodiesel production

The effects of cultivation of Scenedesmus obliquus in nutrient medium supplemented with 0.03%, 3, 9% and 12% CO(2) or 2.5-20 mg L(-1) of Fe(3+) on dry weight of biomass (DW), total lipid accumulation (TL contents) and total lipid productivity (TLP) were evaluated under indoor conditions. The accumulation of TL and TLP showed an increasing trend with increasing of CO(2) or Fe(3+) levels. In cultures with 12% CO(2) or 20mg/L Fe(3+), maximum TL contents of 33.14% and 28.12%, respectively were obtained. These lipids displayed a fatty acid profile which is suitable for biodiesel production as the most abundant compounds were oleic (32.19-34.44%), palmitic (29.54-25.12%) and stearic (12.26-16.58% of total FAMEs) acids. The properties of biodiesel obtained from S. obliquus, were the same with those specification for biodiesel standards including ASTM D 6751 (American Society for Testing Material) and the European Standard En 14214. Thus, S. obliquus biomass could be used as suitable feedstock for biodiesel production.

Kinetic and structural characterization of thermostabilized mutants of human carbonic anhydrase II

Carbonic anhydrases (CAs) are ubiquitous enzymes that catalyze the reversible hydration/dehydration of carbon dioxide/bicarbonate. As such, there is enormous industrial interest in using CA as a bio-catalyst for carbon sequestration and biofuel production. However, to ensure cost-effective use of the enzyme under harsh industrial conditions, studies were initiated to produce variants with enhanced thermostability while retaining high solubility and catalytic activity. Kinetic and structural studies were conducted to determine the structural and functional effects of these mutations. X-ray crystallography revealed that a gain in surface hydrogen bonding contributes to stability while retaining proper active site geometry and electrostatics to sustain catalytic efficiency. The kinetic profiles determined under a variety of conditions show that the surface mutations did not negatively impact the carbon dioxide hydration or proton transfer activity of the enzyme. Together these results show that it is possible to enhance the thermal stability of human carbonic anhydrase II by specific replacements of surface hydrophobic residues of the enzyme. In addition, combining these stabilizing mutations with strategic active site changes have resulted in thermostable mutants with desirable kinetic properties.

Optimizing net energy gain in algal cultivation for biodiesel production

An approach based on energy gain was utilized to optimize algal cultivation in bubble columns. Net energy gain was estimated considering the energy input for mixing and providing carbon dioxide, and the energy that can be generated from the lipids extracted from the algal biomass. Energy input for sparging was minimized based on the gas-to-culture volume ratio and energy output from lipid production was maximized based on nitrate and CO(2) levels. Sparging at a gas-to-culture volume ratio of 0.18 min(-1) with CO(2)-enrichment of 0.5% and initial nitrate concentration of 1mM was optimal for improving net energy gain with Nannochloropsis salina. Sparging with CO(2)-enriched air of 0.5% along with nitrogen starvation resulted in 50% more lipid productivity than sparging with ambient air.

Effects of inorganic carbon concentration on carbon formation, nitrate utilization, biomass and oil accumulation of Nannochloropsis oculata CS 179

This investigation examined the effects of the inorganic carbon concentration (4, 0.8 and 0 g/L NaHCO(3)) on the carbon formation, nitrate utilization, growth and fatty acids compositions of Nannochloropsis oculata. The dissolved inorganic carbon (DIC) concentration in the three treatments decreased sharply during the first 6 days, and the percentage of dissolved organic carbon (DOC) (% of total organic carbon (TOC)) decreased with the depletion of the DIC. The NO(3)(-) assimilation of the algae was correlated with the DIC concentration. The algae in the highest DIC treatment had the highest specific grow rate (0.0843 d(-1)) (P<0.0001), and their biomass and fatty acid methyl esters (FAME) productivity were 84.00 and 9.69 mg/L/d, respectively (P<0.0001). Contents of C16 and C18 series (% of FAME) were high and the C16:0 increased with the decrease of C18:1 during the cultivation. The iodine value (IV) of the algae was low at the low DIC media.