Impacts of CO2 concentration on growth, lipid accumulation, and carbon-concentrating-mechanism-related gene expression in oleaginous Chlorella

Advancements in Nano-Enhanced microalgae bioprocessing

Microalgae are promising sources of valuable compounds: carotenoids, polyunsaturated fatty acids, lipids, etc. To overcome the feasibility challenge due to low yield and attain commercial potential, researchers merge technologies to enhance algal bioprocess. In this context, nanomaterials are attractive for enhancing microalgal bioprocessing, from cultivation to downstream extraction. Nanomaterials enhance biomass and product yields (mainly lipid and carotenoids) through improved nutrient uptake and stress tolerance during cultivation. They also provide mechanistic insights from recent studies. They also revolutionize harvesting via nano-induced sedimentation, flocculation, and flotation. Downstream processing benefits from nanomaterials, improving extraction and purification. Special attention is given to cost-effective extraction, showcasing nanomaterial integration, and providing a comparative account. The review also profiles nanomaterial types, including metallic nanoparticles, magnetic nanomaterials, carbon-based nanomaterials, silica nanoparticles, polymers, and functionalized nanomaterials. Challenges and future trends are discussed, emphasizing nanomaterials' role in advancing sustainable and efficient microalgal bioprocessing, unlocking their potential for bio-based industries.

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.

Effect of the N-hexanoyl-L-homoserine Lactone on the Carbon Fixation Capacity of the Algae-Bacteria System

Algae-bacteria systems are used widely in wastewater treatment. N-hexanoyl-L-homoserine lactone (AHL) plays an important role in algal-bacteria communication. However, little study has been conducted on the ability of AHLs to regulate algal metabolism and the carbon fixation ability, especially in algae-bacteria system. In this study, we used the Microcystis aeruginosa + Staphylococcus ureilyticus strain as a algae-bacteria system. The results showed that 10 ng/L C₆-HSL effectively increased the chlorophyll-a (Chl-a) concentration and carbon fixation enzyme activities in the algae-bacteria group and algae group, in which Chl-a, carbonic anhydrase activity, and Rubisco enzyme increased by 40% and 21%, 56.4% and 137.65%, and 66.6% and 10.2%, respectively, in the algae-bacteria group and algae group, respectively. The carbon dioxide concentration mechanism (CCM) model showed that C₆-HSL increased the carbon fixation rate of the algae-bacteria group by increasing the CO₂ transport rate in the water and the intracellular CO₂ concentration. Furthermore, the addition of C₆-HSL promoted the synthesis and secretion of the organic matter of algae, which provided biogenic substances for bacteria in the system. This influenced the metabolic pathways and products of bacteria and finally fed back to the algae. This study provided a strategy to enhance the carbon fixation rate of algae-bacteria consortium based on quorum sensing.

Development of a Strategy for Enhancing the Biomass Growth and Lipid Accumulation of Chlorella sp. UJ-3 Using Magnetic Fe3O4 Nanoparticles

In this study, magnetic Fe₃O₄ nanoparticles (NPs) were used as an effective enhancer to increase the biomass and total lipid production of Chlorella sp. UJ-3. It was found that the biomass of algal cells increased significantly when they were exposed to low concentrations of Fe₃O₄ NPs (20 mg/L), while the best total lipid content of algal cells was achieved when they were exposed to high concentrations of Fe₃O₄ NPs (100 mg/L). Therefore, we established a strategy to promote the growth and lipid accumulation of microalgae by initially exposing the algal cells to low concentrations of Fe₃O₄ NPs and then treating them with an increased concentration of Fe₃O₄ NPs after 12 days of culture. For this strategy, the biomass and total lipid production of algal cells increased by 50% and 108.7%, respectively, compared to the untreated control. The increase in lipid production and change in the fatty acid composition of Chlorella cells were found to help them to cope with the increased number of reactive oxygen species produced due to oxidative stress in alga cells after the addition of Fe₃O₄ NPs. This study provided a highly efficient way to improve the lipid production of microalgae using nanoparticles.

A comparative assessment of growth, pigment and enhanced lipid production by two toxic freshwater cyanobacteria Anabaena circinalis FSS 124 and Cylindrospermopsis raciborskii FSS 127 under various combinations of nitrogen and phosphorous inputs

Nitrogen and phosphorous are important nutritional regulators for the growth of cyanobacteria, thereby having a significant impact in bloom formation by toxic species. Usage of toxic cyanobacteria for increasing valuable metabolite production by nutrient manipulation is still unexplored. Hence, the current work is aimed to estimate and compare growth, pigment, and increased lipid production coupled with the identification of fatty acids between two toxic strains-Anabaena circinalis FSS 124 and Cylindrospermopsis raciborskii FSS 127 under various combinations of these two nutrients. Low level of nitrogen and phosphorous enhanced lipid content in both strains (˃ 20% and 30% respectively) and C. raciborskii, respectively. Lipid productivity in low phosphorous concentration (P0.5) was achieved significantly high in C. raciborskii. Similarly, a substantial amount of carotenoids was obtained at reduced nitrogen and phosphorous in C. raciborskii accompanied by lessened growth and Chl a concentration. Unlikely, enough biomass (˃ 2 g L^-1) was produced at high nutrient levels in both species. Comparative statistical significance (p < 0.05) was found between two species regarding biomass production, chlorophyll concentration, lipid content, and productivity and between these factors in each species under both nutrient variations. FAME of Cylindrospermopsis is composed of saturated fatty acids (˃ 50%) and MUFA (˃ 25%) while Anabaena contains PUFA (˃ 21%) additionally. However, the study highlights C. raciborskii as potential lipid and carotenoid producer at nutrient stress and finds a novel way to utilize these cyanobacterial biomasses, which cause immense environmental hazards and life threats.

Integrating Transcriptomics and Metabolomics to Characterize Metabolic Regulation to Elevated CO2 in Chlamydomonas Reinhardtii

With atmospheric CO₂ increasing, a large amount of CO₂ is absorbed by oceans and lakes, which changes the carbonate system and affects the survival of aquatic plants, especially microalgae. The main aim of our study was to explore the responses of Chlamydomonas reinhardtii (Chlorophyceae) to elevated CO₂ by combined transcriptome and metabolome analysis under three different scenarios: control (CK, 400 ppm), short-term elevated CO₂ (ST, 1000 ppm), and long-term elevated CO₂ (LT, 1000 ppm). The transcriptomic data showed moderate changes between ST and CK. However, metabolic analysis indicated that fatty acids (FAs) and partial amino acids (AAs) were increased under ST. There was a global downregulation of genes involved in photosynthesis, glycolysis, lipid metabolism, and nitrogen metabolism but increase in the TCA cycle and β-oxidation under LT. Integrated transcriptome and metabolome analyses demonstrated that the nutritional constituents (FAs, AAs) under LT were poor compared with CK, and most genes and metabolites involved in C and N metabolism were significantly downregulated. However, the growth and photosynthesis of cells under LT increased significantly. Thus, C. reinhardtii could form a specific adaptive evolution to elevated CO₂, affecting future biogeochemical cycles.

Storage of starch and lipids in microalgae: Biosynthesis and manipulation by nutrients

Microalgae accumulate starch and lipid as storage metabolites under nutrient depletion, which can be used as sustainable feedstock for biorefinery. Omics analysis coupled with enzymatic and genetic verifications uncovered a partial picture of pathways and important enzymes or regulators related to starch and lipid biosynthesis as well as the carbon partitioning between them under nutrient depletion conditions. Depletion of macronutrients (N, P, and S) resulted in considerable enhancement of starch and/or lipid content in microalgae, but the accompanying declined photosynthesis hampered the achievements of high concentrations. This review summarized the current knowledge on the pathways and the committed steps as well as their carbon allocation involved in starch and lipid biosynthesis, and focused on the manipulation of different nutrients and the alleviation of oxidative stress for enhanced storage metabolites production. The biological and engineering approaches to cope with the conflict between biomass production and storage metabolites accumulation are proposed.

Leveraging algal omics to reveal potential targets for augmenting TAG accumulation

Ongoing global efforts to commercialize microalgal biofuels have expedited the use of multi-omics techniques to gain insights into lipid biosynthetic pathways. Functional genomics analyses have recently been employed to complement existing sequence-level omics studies, shedding light on the dynamics of lipid synthesis and its interplay with other cellular metabolic pathways, thus revealing possible targets for metabolic engineering. Here, we review the current status of algal omics studies to reveal potential targets to augment TAG accumulation in various microalgae. This review specifically aims to examine and catalog systems level data related to stress-induced TAG accumulation in oleaginous microalgae and inform future metabolic engineering strategies to develop strains with enhanced bioproductivity, which could pave a path for sustainable green energy.

High-value biomass from microalgae production platforms: strategies and progress based on carbon metabolism and energy conversion

Microalgae are capable of producing sustainable bioproducts and biofuels by using carbon dioxide or other carbon substances in various cultivation modes. It is of great significance to exploit microalgae for the economical viability of biofuels and the revenues from high-value bioproducts. However, the industrial performance of microalgae is still challenged with potential conflict between cost of microalgae cultivation and revenues from them, which is mainly ascribed to the lack of comprehensive understanding of carbon metabolism and energy conversion. In this review, we provide an overview of the recent advances in carbon and energy fluxes of light-dependent reaction, Calvin-Benson-Bassham cycle, tricarboxylic acid cycle, glycolysis pathway and processes of product biosynthesis in microalgae, with focus on the increased photosynthetic and carbon efficiencies. Recent strategies for the enhanced production of bioproducts and biofuels from microalgae are discussed in detail. Approaches to alter microbial physiology by controlling light, nutrient and other environmental conditions have the advantages of increasing biomass concentration and product yield through the efficient carbon conversion. Engineering strategies by regulating carbon partitioning and energy route are capable of improving the efficiencies of photosynthesis and carbon conversion, which consequently realize high-value biomass. The coordination of carbon and energy fluxes is emerging as the potential strategy to increase efficiency of carbon fixation and product biosynthesis. To achieve more desirable high-value products, coordination of multi-stage cultivation with engineering and stress-based strategies occupies significant positions in a long term.

Competition between cyanobacteria and green algae at low versus elevated CO2: who will win, and why?

Traditionally, it has often been hypothesized that cyanobacteria are superior competitors at low CO2 and high pH in comparison with eukaryotic algae, owing to their effective CO2-concentrating mechanism (CCM). However, recent work indicates that green algae can also have a sophisticated CCM tuned to low CO2 levels. Conversely, cyanobacteria with the high-flux bicarbonate uptake system BicA appear well adapted to high inorganic carbon concentrations. To investigate these ideas we studied competition between three species of green algae and a bicA strain of the harmful cyanobacterium Microcystis aeruginosa at low (100 ppm) and high (2000 ppm) CO2. Two of the green algae were competitively superior to the cyanobacterium at low CO2, whereas the cyanobacterium increased its competitive ability with respect to the green algae at high CO2. The experiments were supported by a resource competition model linking the population dynamics of the phytoplankton species with dynamic changes in carbon speciation, pH and light. Our results show (i) that competition between phytoplankton species at different CO2 levels can be predicted from species traits in monoculture, (ii) that green algae can be strong competitors under CO2-depleted conditions, and (iii) that bloom-forming cyanobacteria with high-flux bicarbonate uptake systems will benefit from elevated CO2 concentrations.

Enhancing growth of Chlamydomonas reinhardtii and nutrient removal in diluted primary piggery wastewater by elevated CO2 supply

The coupling of primary piggery wastewater as a culture medium with elevated CO₂ aeration is thought to be an economically feasible option for the cultivation of microalgae. However, little information is available regarding the photosynthetic characteristics of microalgae and nutrient removal from wastewater at different CO₂ concentrations. It was found that elevated CO₂ aeration provided sustained growth at CO₂ concentrations ranging from 5% to 15% and performed best with 5% CO₂ aeration in primary piggery wastewater for Chlamydomonas reinhardtii growth. Photosynthesis, respiration, and nutrient uptake (total nitrogen and total phosphorus) were stimulated in response to CO₂ enrichment, thus increasing nutrient uptake in primary piggery wastewater, particularly total nitrogen and total phosphorus. A study of carbon-concentrating mechanism-related gene expression revealed that the levels of mRNAs, such as CAH1, LCIB and HLA3, were significantly downregulated. This represents a possible method for the reconciliation of CO₂-stimulated growth with mixotrophic cultivation of C. reinhardtii in diluted primary piggery wastewater.

A Holistic Approach to Managing Microalgae for Biofuel Applications

Microalgae contribute up to 60% of the oxygen content in the Earth’s atmosphere by absorbing carbon dioxide and releasing oxygen during photosynthesis. Microalgae are abundantly available in the natural environment, thanks to their ability to survive and grow rapidly under harsh and inhospitable conditions. Microalgal cultivation is environmentally friendly because the microalgal biomass can be utilized for the productions of biofuels, food and feed supplements, pharmaceuticals, nutraceuticals, and cosmetics. The cultivation of microalgal also can complement approaches like carbon dioxide sequestration and bioremediation of wastewaters, thereby addressing the serious environmental concerns. This review focuses on the factors affecting microalgal cultures, techniques adapted to obtain high-density microalgal cultures in photobioreactors, and the conversion of microalgal biomass into biofuels. The applications of microalgae in carbon dioxide sequestration and phycoremediation of wastewater are also discussed.

Transcriptome and key genes expression related to carbon fixation pathways in Chlorella PY-ZU1 cells and their growth under high concentrations of CO2

BACKGROUND

The biomass yield of Chlorella PY-ZU1 drastically increased when cultivated under high CO₂ condition compared with that cultivated under air condition. However, less attention has been given to the microalgae photosynthetic mechanisms response to different CO₂ concentrations. The genetic reasons for the higher growth rate, CO₂ fixation rate, and photosynthetic efficiency of microalgal cells under higher CO₂ concentration have not been clearly defined yet.

RESULTS

In this study, the Illumina sequencing and de novo transcriptome assembly of Chlorella PY-ZU1 cells cultivated under 15% CO₂ were performed and compared with those of cells grown under air. It was found that carbonic anhydrase (CAs, enzyme for interconversion of bicarbonate to CO₂) dramatically decreased to near 0 in 15% CO₂-grown cells, which indicated that CO₂ molecules directly permeated into cells under high CO₂ stress without CO₂-concentrating mechanism. Extrapolating from the growth conditions and quantitative Real-Time PCR of CCM-related genes, the K_m (CO₂) (the minimum intracellular CO₂ concentration that rubisco required) of Chlorella PY-ZU1 might be in the range of 80-192 μM. More adenosine triphosphates was saved for carbon fixation-related pathways. The transcript abundance of rubisco (the most important enzyme of CO₂ fixation reaction) was 16.3 times higher in 15% CO₂-grown cells than that under air. Besides, the transcript abundances of most key genes involved in carbon fixation pathways were also enhanced in 15% CO₂-grown cells.

CONCLUSIONS

Carbon fixation and nitrogen metabolism are the two most important metabolisms in the photosynthetic cells. These genes related to the two most metabolisms with significantly differential expressions were beneficial for microalgal growth (2.85 g L^-1) under 15% CO₂ concentration. Considering the micro and macro growth phenomena of Chlorella PY-ZU1 under different concentrations of CO₂ (0.04-60%), CO₂ transport pathways responses to different CO₂ (0.04-60%) concentrations was reconstructed.

Degradation of disperse blue 2BLN by oleaginous C. sorokiniana XJK

In this study, an oil-producing freshwater microalgae Chlorella sorokiniana XJK was identified and used for the degradation of disperse blue 2BLN.

Experimental and theoretical study on dissolution of a single mixed gas bubble in a microalgae suspension

Visualization experiments using a promoted bubble grafting method were carried out and the non-equilibrium theory at the gas–liquid interface was adopted to predict the dissolution and transmission process of CO₂ gas.