A comprehensive review on carbon source effect of microalgae lipid accumulation for biofuel production

Functional use of carbon dioxide for the sustainable valorization of orange peel in the pyrolysis process

Although biomass is carbon-neutral, its use as a primary feedstock faces challenges arising from inconsistent supply chains. Therefore, it becomes crucial to explore alternatives with reliable availability. This study proposes a strategic approach for the thermochemical valorization of food processing waste, which is abundantly generated at single sites within large-scale processing plants. As a model biomass waste from the food industry, orange peel waste was particularly chosen considering its substantial consumption. To impart sustainability to the pyrolysis system, CO2, a key greenhouse gas, was introduced. As such, this study highlights elucidating the functionality of CO2 as a reactive feedstock. Specifically, CO2 has the potential to react with volatile pyrolysates evolved from orange peel waste, leading to CO formation at ≥490 °C. The formation of chemical constituents, encompassing acids, ketones, furans, phenols, and aromatics, simultaneously decreased by 15.1 area% in the presence of CO2. To activate the efficacy of CO2 at the broader temperature spectrum, supplementary measures, such as an additional heating element (700 °C) and a nickel-based catalyst (Ni/Al2O3), were implemented. These configurations promote thermal cracking of the volatiles and their reaction kinetics with CO2, representing an opportunity for enhanced carbon utilization in the form of CO. Finally, the integrated process of CO2-assisted catalytic pyrolysis and water-gas shift reaction was proposed. A potential revenue when maximizing the productivity of H2 was estimated as 2.62 billion USD, equivalent to 1.11 times higher than the results from the inert (N2) environment. Therefore, utilizing CO2 in the pyrolysis system creates a promising approach for enhancing the sustainability of the thermochemical valorization platform while maximizing carbon utilization in the form of CO.

Inorganic carbon limitation decreases ammonium removal and N2O production in the algae-nitrifying bacteria symbiosis system

Ammonium removal by a symbiosis system of algae (Chlorella vulgaris) and nitrifying bacteria was evaluated in a long-term photo-sequencing batch reactor under varying influent inorganic carbon (IC) concentrations (15, 10, 5 and 2.5 mmol L-1) and different nitrogen loading rate (NLR) conditions (270 and 540 mg-N L-1 d-1). The IC/N ratios provided were 2.33, 1.56, 0.78 and 0.39, respectively, for an influent NH4+-N concentration of 90 mg-N L-1 (6.43 mmol L-1). The results confirmed that both ammonium removal and N2O production were positively related with IC concentration. Satisfactory ammonium removal efficiencies (>98 %) and rates (29-34 mg-N gVSS-1 h-1) were achieved regardless of NLR levels under sufficient IC of 10 and 15 mmol L-1, while insufficient IC at 2.5 mmol L-1 led to the lowest ammonium removal rates of 0 mg-N gVSS-1 h-1. The ammonia oxidation process by ammonia oxidizing bacteria (AOB) played a predominant role over the algae assimilation process in ammonium removal. Long-time IC deficiency also resulted in the decrease in biomass and pigments of algae and nitrifying bacteria. IC limitation led to the decreasing N2O production, probably due to its negative effect on ammonia oxidation by AOB. The optimal IC concentration was determined to be 10 mmol L-1 (i.e., IC/N of 1.56, alkalinity of 500 mg CaCO3 L-1) in the algae-bacteria symbiosis reactor, corresponding to higher ammonia oxidation rate of ∼41 mg-N gVSS-1 h-1 and lower N2O emission factor of 0.13 %. This suggests regulating IC concentrations to achieve high ammonium removal and low carbon emission simultaneously in the algae-bacteria symbiosis wastewater treatment process.

Regulatory mechanisms of autophagy on DHA and carotenoid accumulation in Crypthecodinium sp. SUN

BACKGROUND

Autophagy is a crucial process of cellular self-destruction and component reutilization that can affect the accumulation of total fatty acids (TFAs) and carotenoids in microalgae. The regulatory effects of autophagy process in a docosahexaenoic acid (DHA) and carotenoids simultaneously producing microalga, Crypthecodinium sp. SUN, has not been studied. Thus, the autophagy inhibitor (3-methyladenine (MA)) and activator (rapamycin) were used to regulate autophagy in Crypthecodinium sp. SUN.

RESULTS

The inhibition of autophagy by 3-MA was verified by transmission electron microscopy, with fewer autophagy vacuoles observed. Besides, 3-MA reduced the glucose absorption and intracellular acetyl-CoA level, which resulting in the decrease of TFA and DHA levels by 15.83 and 26.73% respectively; Surprisingly, 3-MA increased intracellular reactive oxygen species level but decreased the carotenoids level. Comparative transcriptome analysis showed that the downregulation of the glycolysis, pentose phosphate pathway and tricarboxylic acid cycle may underlie the decrease of acetyl-CoA, NADPH and ATP supply for fatty acid biosynthesis; the downregulation of PSY and HMGCR may underlie the decreased carotenoids level. In addition, the class I PI3K-AKT signaling pathway may be crucial for the regulation of carbon and energy metabolism. At last, rapamycin was used to activate autophagy, which significantly enhanced the cell growth and TFA level and eventually resulted in 1.70-fold increase in DHA content.

CONCLUSIONS

Our findings indicate the mechanisms of autophagy in Crypthecodinium sp. SUN and highlight a way to manipulate cell metabolism by regulating autophagy. Overall, this study provides valuable insights to guide further research on autophagy-regulated TFA and carotenoids accumulation in Crypthecodinium sp. SUN.

Mechanisms of the novel pesticide sodium dodecyl benzene sulfonate in the mitigation of protozoan ciliated pathogens during microalgal cultivation

Protozoan ciliates represent a common biological contaminant during microalgae cultivation, which will lead to a decline in microalgae productivity. This study investigated the effectiveness of sodium dodecyl benzene sulfonate (SDBS) in controlling ciliate populations within microalgae cultures. SDBS concentrations of 160 mg/L and 100 mg/L were found to effectively manage the representative species of ciliates contamination by Euplotes vannus and Uronema marinum during the cultivation of Synechococcus and Chlorella, and the growth vitality of microalgae has been restored. Additionally, SDBS at these concentrations reduced oxidative stress resistance and induced membrane damage to remove biological pollutants by modulating enzyme activity, affecting lipid, energy, amino acid metabolism pathways, and processes such as translation and protein folding. This research provides insights into the mechanisms through which SDBS effectively combats protozoan ciliates during the microalgal cultivation. This contributes to reduce biological pollution, ensure the overall productivity and healthy and sustainable management of microalgae ecosystems.

Effect of acetochlor on the symbiotic relationship between microalgae and bacteria

In this study, two strains of symbiotic bacteria (SOB-1 and SOB-2) were isolated from Scenedesmus obliquus, and various algal-bacterial mutualistic systems were established under acetochlor (ACT) stress conditions. Following exposure to varying ACT concentrations from 2.0 to 25.0 μg/L, the capacity for co-cultured bacteria to degrade ACT was enhanced in 7 days by up to 226.9% (SOB-1) and 193.0% (SOB-2), compared with axenic algae, although bacteria exposed to higher ACT concentrations exacerbated algal metabolic stress, oxidative states, apoptosis and cellular lysis. ACT reduced carbohydrates in the phycosphere by up to 31.5%; compensatory nutrient plunder and structural damage by bacteria were the potential exploitation pathways determined based on the inhibition of bacterial infection using a glucanase inhibitor. The ACT-induced reduction in algal antimicrobial substances, including fatty acids and phenolics (by up to 58.1% and 56.6%, respectively), also facilitated bacterial exploitation of algae. ACT-dependent interspecific interaction coefficients between algae and bacteria generated from long-term symbiosis cultures implied that bacteria moved from mutualism (0 and 2.0 μg/L ACT) to exploitation (7.9 and 25.0 μg/L ACT). The population dynamic model under incremental ACT-concentration scenarios inferred that theoretical systematic extinction may occur in algal-bacterial systems earlier than in axenic algae. These outcomes provide interspecific insights into the distortion of algal-bacterial reciprocity due to the ecotoxicological effects of ACT.

Regulation effects of indoleacetic acid on lipid production and nutrient removal of Chlorella pyrenoidosa in seawater-containing wastewater

The utilization of seawater supplemented with wastewater nutrients for microalgae cultivation represents a promising and cost-effective approach that combines the benefits of wastewater treatment and microalgal resource recovery. However, the high salt content in seawater poses a significant challenge, hindering microalgal growth and reducing the removal of nitrogen and phosphorus on a large scale. The phytohormone indoleacetic acid (IAA) was used in this study to enhance stress resistance and lipid production of Chlorella pyrenoidosa grown in seawater-wastewater medium. Compared to the control groups involving regular wastewater and seawater-containing wastewater without IAA, Chlorella pyrenoidosa cultivated in the seawater-containing wastewater supplemented with IAA exhibited remarkable outcomes. Specifically, microalgae in IAA-enhanced seawater-containing wastewater achieved the highest lipid productivity (22.67 mg L-1 d-1) along with impressive nitrogen (99.3 %) and phosphorus (97.3 %) removal rates. Moreover, their cell sedimentation ratio reached 76.6 %, indicating enhanced settling properties. Additionally, the physiological mechanism changes after exposure to seawater stress and IAA were revealed based on the changes in antioxidant enzymes, endogenous hormones, and fatty acid saturation. Furthermore, the transcriptomic analysis elucidated the molecular mechanisms underlying microalgal lipid synthesis and their response to antioxidant stress when exposed to seawater. The supplementation of IAA under seawater stress stimulated energy metabolism and the antioxidant response in microalgal cells, effectively mitigating the adverse effects of seawater stress and promoting overall algal lipid productivity. Overall, this study unveiled the potential of exogenous plant hormones, particularly IAA, in enhancing stress resistance and lipid productivity of microalgae grown in seawater-wastewater medium, which significantly contributed towards the efficient use of seawater resources for microalgae cultivation and biofuel production.

Plastic bio-mitigation by Pseudomonas mendocina ABF786 and simultaneous conversion of its CO2 byproduct to microalgal biodiesel

Bio-mitigation of plastics by microorganisms generates carbon dioxide (CO2) that can be utilized for algal biomass generation. Pseudomonas mendocina ABF786, reportedly the most efficient plastic-degrading bacteria, was screened using the modified most probable number technique. This study highlights the use of an integrative prototype for the production of microalgal biomass (Chlorella vulgaris) in combination with bio-mitigation of plastics, which serves a dual purpose: (i) increased plastic-degradation capability by microorganisms (53%-85% increase in plastic weight loss) due to removal of CO2 feedback inhibition and (ii) increased algal biomass generation (200%-237%) due to supply of extra CO2 from plastic degradation to the algal cultivation flask. Whole-genome sequencing and functional annotation confirmed that all the genes involved in the mineralization of plastic to CO2 are present within the genome of P. mendocina ABF786. Using two or more microbial cultures for remediation may increase the process efficiency.

The growth and lipid accumulation of Scenedesmus quadricauda under nitrogen starvation stress during xylose mixotrophic/heterotrophic cultivation

In order to conquer the block of high cost and low yields which limit to realize the commercialization of microalgal biodiesel, the mixotrophic and heterotrophic cultivation of Scenedesmus quadricauda FACHB-1297 fed on xylose was separately studied employing six forms of media: phosphorus sufficient, phosphorus restricted, and phosphorus starvation were combined with nitrogen sufficient and nitrogen starvation conditions. The maximum lipid content (about 41% of dry weight) was obtained on the 5th day (heterotrophic cultivation) and 8th day (mixotrophic cultivation) under the nitrogen starved and phosphorus sufficient (N0&P) conditions, which was about twofold in comparison to the final lipid content on the sufficient nitrogen condition (control). Under mixotrophic and heterotrophic modes, the highest lipid production was achieved in the N0&P trial, with the value of 274.96 mg/L and 193.77 mg/L, respectively. Xylose utilization rate of 30-96% under heterotrophic modes was apparently higher than that of 20-50% in mixotrophic modes. In contrast, phosphorus uptake rate of 100% under mixotrophic cultivation was significantly more than that of 60-90% in heterotrophic cultivation. Furthermore, under the condition of heterotrophic cultivation using xylose as a carbon source, the phosphorus had a positive impact on microalgae cell synthesis and the lipid content enhanced with the augmentation in phosphorus concentrations. We suggested that sufficient phosphorus should be supplied for obtaining higher microalgal lipid production in the lack of nitrogen under xylose heterotrophic/mixotrophic condition. This was a highly effective way to obtain efficient microalgae lipid production.

Microalgae Biofuel for a Heavy-Duty Transport Sector within Planetary Boundaries

In this contribution, we study the extent to which 68 scenarios for microalgae biofuels could help the heavy-duty transport sector operate within planetary boundaries. The proposed scenarios are built considering a range of alternative configurations based on three types of fuel production processes (i.e., transesterification, hydrodeoxygenation, and hydrothermal liquefaction), different carbon sources (such as natural gas power plants and direct air capture), byproduct treatments, and two electricity mixes. Our results reveal that microalgae biofuels could significantly reduce the environmental and human health impacts of the business-as-usual (fossil-based) heavy-duty transport sector. Moreover, relative to standard biofuels that show large land-use requirements, we find that microalgae biofuels also decrease the damage on biosphere integrity substantially. Notably, pathways resorting to hydrodeoxygenation of microalgae oil and direct air capture and carbon storage could reduce the current impact induced globally on climate change by the heavy transport by 77%, while attaining six-fold reductions in biosphere integrity impacts, both relative to conventional biofuels.

Variability in Macro- and Micronutrients of 15 Rarely Researched Microalgae

Microalgae have enormous potential for human nutrition, yet the European Commission has authorized the consumption of only eleven species. Strains of fifteen rarely researched microalgae from two kingdoms were screened regarding their nutritional profile and value for human health in two cultivation phases. Contents of protein, fiber, lipids, fatty acids, minerals, trace elements and heavy metals were determined. In the growth phase, microalgae accumulated more arginine, histidine, ornithine, pure and crude protein, Mg, Mn, Fe and Zn and less Ni, Mo and I₂ compared to the stationary phase. Higher contents of total fat, C14:0, C14:1_n5, C16:1_n7, C20:4_n6, C20:5_n3 and also As were observed in microalgae from the chromista kingdom in comparison to microalgae from the plantae kingdom (p < 0.05). Conversely, the latter had higher contents of C20:0, C20:1_n9 and C18:3_n3 as well as Ca and Pb (p < 0.05). More precisely, Chrysotila carterae appeared to have great potential for human nutrition because of its high nutrient contents such as fibers, carotenoids, C20:6_n3, Mg, Ca, Mn, Fe, Se, Zn, Ni, Mo and I₂. In summary, microalgae may contribute to a large variety of nutrients, yet the contents differ between kingdoms, cultivation phases and also species.

Evidence of the drying technique's impact on the biomass quality of Tetraselmis subcordiformis (Chlorophyceae)

Rapid drying, cost-effective and safe, will increase the viability of using microalgae for several bio-industrial applications. In this study, five different drying techniques of microalgal biomass were investigated. These include freeze drying, oven drying, air drying, sun drying, and microwave drying. Morphology, metabolite content, FAME profiling, chlorophyll content, total organic carbon, and total nitrogen were analyzed. Results showed that the freeze-drying technique preserves the highest amounts of chlorophyll, proteins, and lipids. Oven drying underperformed as it retained the lowest amount of chlorophyll, protein, and lipid content. More importantly, FAME profiling results showed that air drying was the best technique in maintaining the highest amount of polyunsaturated fatty acids and more specifically docosahexaenoic acid (DHA). Furthermore, this process requires the least capital and energy needs. The findings from this study confirmed that the drying technique affects the microalga biomass quality.

The Potential of Algae in the Nutricosmetic Sector

Seaweeds or algae are marine autotrophic organisms. They produce nutrients (e.g., proteins, carbohydrates, etc.) essential for the survival of living organisms as they participate in biochemical processes and non-nutritive molecules (such as dietary fibers and secondary metabolites), which can improve their physiological functions. Seaweed polysaccharides, fatty acids, peptides, terpenoids, pigments, and polyphenols have biological properties that can be used to develop food supplements and nutricosmetic products as they can act as antibacterial, antiviral, antioxidant, and anti-inflammatory compounds. This review examines the (primary and secondary) metabolites produced by algae, the most recent evidence of their effect on human health conditions, with particular attention to what concerns the skin and hair's well-being. It also evaluates the industrial potential of recovering these metabolites from biomass produced by algae used to clean wastewater. The results demonstrate that algae can be considered a natural source of bioactive molecules for well-being formulations. The primary and secondary metabolites' upcycling can be an exciting opportunity to safeguard the planet (promoting a circular economy) and, at the same time, obtain low-cost bioactive molecules for the food, cosmetic, and pharmaceutical industries from low-cost, raw, and renewable materials. Today's lack of methodologies for recovering bioactive molecules in large-scale processes limits practical realization.

Lipid production characteristics of a newly isolated microalga Asterarcys quadricellulare R-56 as biodiesel feedstock

In this study, a new microalgal strain, Asterarcys quadricellulare R-56, was isolated for biomass and lipid production. The effects of carbon and nitrogen sources and initial pH on the cell growth and lipid accumulation of strain R-56 were investigated. At 10 g L^-1 glucose, 0.6 g L^-1 sodium nitrate, and pH 7, the highest biomass of 4.18 g L^-1 and lipid content of 43.66% were obtained. Microalgae had a broad pH tolerance in the range of 5-11, and the pH of the culture medium was close to neutral at the end of cultivation. The maximum contents of chlorophyll, carbohydrate, and protein under the recommended culture conditions were 19.47 mg mL^-1, 21.80%, and 29.94%, respectively. Palmitic and palmitoleic acid contents in strain R-56 accounted for as high as 83.73% of total fatty acids. This study suggested that strain R-56 was a promising lipid producer for high-quality biodiesel production.

Sodium Bicarbonate (NaHCO3) Increases Growth, Protein and Photosynthetic Pigments Production and Alters Carbohydrate Production of Spirulina platensis

Improving the biochemical status of Spirulina platensis will enhance the functional properties of this microalgae. The present study investigated the effects of adding NaHCO₃ to the culture medium on the growth rate and biochemical composition, particularly the coproduction of proteins, carbohydrates, and photosynthetic pigments of S. platensis. Spirulina platensis was grown in different NaHCO₃ concentrations (0-16 g L^-1). NaHCO₃ positively affected the biomass production. The growth of S. platensis and biochemical compound content increased with an increase in the NaHCO₃ concentration. The microalgae biomass grown on NaHCO₃ also contained higher amounts of protein (64.20 ± 4.18% w w^-1) and photosynthetic pigments (phycocyanin and chlorophyll a, b, and total). Protein productivity was especially enhanced by approximately 6-25% (from 0.006 ± 0.0030 to 0.025 ± 0.0031 mg L^-1 day^-1) with the addition of NaHCO₃ compared to the control. In contrast, the content of carbohydrates and antioxidant compounds (phenolic, polyphenol oxidase, and peroxidase activities) decreased with culture age and an increase in the NaHCO₃ concentration. These results suggest that S. platensis uses NaHCO₃ as a carbon source for photosynthesis, biomass production, and acts as a metabolic energy carrier toward the synthesis of proteins and photosynthetic pigments, which are more energy-consuming metabolites than carbohydrates. The addition of NaHCO₃ to the culture media is a potentially useful strategy toward improving the protein and photosynthetic pigment productivity of S. platensis.

The effects of acetate and glucose on carbon fixation and carbon utilization in mixotrophy of Haematococcus pluvialis

The culture method using sodium acetate and glucose, widely used as organic carbon sources in the mixotrophy of Haematococcus pluvialis, was compared with its autotrophy. In the 12-day culture, mixotrophy using sodium acetate and glucose increased by 40.4% and 77.1%, respectively, compared to autotrophy, but the mechanisms for the increasing biomass were different. The analysis of the mechanism was divided into autotrophic and heterotrophic metabolism. The mixotrophy with glucose increased the biomass by directly supplying the substrate and ATP to the TCA cycle while inhibiting photosynthesis. Gene expressions related to glycolysis and carbon fixation pathway were confirmed in autotrophy and mixotrophy with glucose and acetate. The metabolism predicted in the mixotrophy with acetate and glucose was proposed via autotrophic and heterotrophic metabolism analysis. The mechanism of Haematococcus pluvialis under mixotrophic conditions with high CO₂ concentration was confirmed through this study.

Role of microalgae in achieving sustainable development goals and circular economy

In 2015, the United Nations General Assembly (UNGA) set out 17 Sustainable Development Goals (SDGs) to be achieved by 2030. These goals highlight key objectives that must be addressed. Each target focuses on a unique perspective crucial to meeting these goals. Social, political, and economic issues are addressed to comprehensively review the main issues combating climate change and creating sustainable and environmentally friendly industries, jobs, and communities. Several mechanisms that involve judicious use of biological entities are among instruments that are being explored to achieve the targets of SDGs. Microalgae have an increasing interest in various sectors, including; renewable energy, food, environmental management, water purification, and the production of chemicals such as biofertilizers, cosmetics, and healthcare products. The significance of microalgae also arises from their tendency to consume CO2, which is the main greenhouse gas and the major contributor to the climate change. This work discusses the roles of microalgae in achieving the various SDGs. Moreover, this work elaborates on the contribution of microalgae to the circular economy. It was found that the microalgae contribute to all the 17th SDGs, where they directly contribute to 9th of the SDGs and indirectly contribute to the rest. The major contribution of the Microalgae is clear in SDG-6 "Clean water and sanitation", SDG-7 "Affordable and clean energy", and SDG-13 "Climate action". Furthermore, it was found that Microalgae have a significant contribution to the circular economy.

Microalgal remediation and valorisation of polluted wastewaters for zero-carbon circular bioeconomy

Overexploitation of natural resources to meet human needs has considerably impacted CO₂ emissions, contributing to global warming and severe climatic change. This review furnishes an understanding of the sources, brutality, and effects of CO₂ emissions and compelling requirements for metamorphosis from a linear to a circular bioeconomy. A detailed emphasis on microalgae, its types, properties, and cultivation are explained with significance in attaining a zero-carbon circular bioeconomy. Microalgal treatment of a variety of wastewaters with the conversion of generated biomass into value-added products such as bio-energy and pharmaceuticals, along with agricultural products is elaborated. Challenges encountered in large-scale implementation of microalgal technologies for low-carbon circular bioeconomy are discussed along with solutions and future perceptions. Emphasis on the suitability of microalgae in wastewater treatment and its conversion into alternate low-carbon footprint bio-energies and value-added products enforcing a zero-carbon circular bioeconomy is the major focus of this review.

Bioremediation of Crude Oil by Haematococcus Pluvialis: A Preliminary Study

Nowadays, oil pollution is one of the main environmental problems. The current methods for recovering spills mainly involve chemical agents, but scientific research has focused on more natural and less harmful techniques for the environment, including a consortium of bacteria and microalgae to clean up water contaminated by hydrocarbons. The purpose of this preliminary study was to evaluate the ability of a microalga belonging to Chlorophyceae to grow in the presence of crude oil and remove the principal contaminants. H. pluvialis, which is usually used for nutraceutical purposes, thanks to the production of astaxanthin, was able to grow in anaerobic conditions, varying its metabolism from autotrophic to heterotrophic, exploiting the carbon present in the solution deriving from the presence of 1% of crude oil. Furthermore, the results of bioremediation showed a relevant reduction in chemical pollutants such as nitrate, fluoride, sulfate, and phosphate. The most important aspect of the study was the reduction after 160 days in the hydrocarbon concentration inside not only the culture medium (−32%) but also the algal biomass (−80.25%), demonstrating an optimized degradation rather than a simple absorption inside the alga.

Microalgae-driven swine wastewater biotreatment: Nutrient recovery, key microbial community and current challenges

As a promising technology, the microalgae-driven strategy can achieve environmentally sustainable and economically viable swine wastewater treatment. Currently, most microalgae-based research focuses on remediation improvement and biomass accumulation, while information on the removal mechanisms and dominant microorganisms is emerging but still limited. In this review, the major removal mechanisms of pollutants and pathogenic bacteria are systematically discussed. In addition, the bacterial and microalgal community during the swine wastewater treatment process are summarized. In general, Blastomonas, Flavobacterium, Skermanella, Calothrix and Sedimentibacter exhibit a high relative abundance. In contrast to the bacterial community, the microalgal community does not change much during swine wastewater treatment. Additionally, the effects of various parameters (characteristics of swine wastewater and cultivation conditions) on microalgal growth and current challenges in the microalgae-driven biotreatment process are comprehensively introduced. This review stresses the need to integrate bacterial and microalgal ecology information into the conventional design of full-scale swine wastewater treatment systems and operations. Herein, future research needs are also proposed, which will facilitate the development and operation of a more efficient microalgae-based swine wastewater treatment process.

Enhancement of black and odorous water treatment coupled with accelerated lipid production by microalgae exposed to 12C6+ heavy-ion beam irradiation

In this study, Auxenochlorella protothecoides (AP-CK) was selected due to its reported high growth potential in sterilized black and odorous water (SBOW). In order to improve the resource utilization level of microalgae for wastewater treatment, AP-CK was mutated using ¹²C⁶⁺ heavy-ion beam irradiation, and a high lipid-containing mutant (AP-34#) was isolated and further evaluated to treat original black and odorous water (OBOW). Compared with the wild type, the maximum removal rates of COD, NH₄⁺-N and TP of the mutant increased by 8.12 ± 0.33%, 10.43 ± 0.54% and 11.97 ± 0.16%, respectively, while maximum dissolved oxygen content increased from 0 to 4.36 ± 0.25 mg/L. Besides, the mutant lipid yield increased by 115.87 ± 3.22% over the wild type in OBOW. The fatty acid profile of AP-34# grown in SBOW and OBOW showed higher proportion of saturated fatty acids (C16:0 and C18:0) and valuable polyunsaturated fatty acids (mainly C20:5n3 and C22:6n3) which are more suitable for biodiesel production and value-added products, respectively. This work provides a new perspective on improving the characteristics of microalgae and an innovative approach for resource-based microalgae wastewater treatment through bioremediation of black and odorous water.

Reducing culture medium nitrogen supply coupled with replenishing carbon nutrient simultaneously enhances the biomass and lipid production of Chlamydomonas reinhardtii

Chlamydomonas reinhardtii is a model strain to explore algal lipid metabolism mechanism, and exhibits great potentials in large-scale production of lipids. Completely lacking nitrogen is an efficient strategy to trigger the lipid synthesis in microalgal cells, while it always leads to the obvious reduction in the biomass. To illustrate the optimal culture substrate carbon (C) and nitrogen (N) levels to simultaneously stimulate the growth and lipid production of C. reinhardtii, cells were cultivated under altered C and N concentrations. Results showed that replenishing 6 g/L sodium acetate (NaAc) could increase 1.50 and 1.53 times biomass and lipid productivity compared with 0 g/L NaAc treatment (the control), but total lipid content slightly decreased. Reducing 75% of basic medium (TAP) N level (0 g/L NaAc + 0.09 g/L NH₄Cl treatment) could promote 21.57% total lipid content in comparison with the control (containing 0.38 g/L NH₄Cl), but decrease 44.45% biomass and 34.15% lipid productivity. The result of the central composite design (CCD) experiment suggested the optimum total lipid content together with higher biomass and lipid productivity could be obtained under the condition of 4.12 g/L NaAc and 0.20 g/L NH₄Cl. They reached 32.14%, 1.68 g/L and 108.21 mg/L/d, and increased by 36.77%, 93.10% and 1.75 times compared with the control, respectively. It suggests moderately increasing C supply and decreasing N levels could synchronously improve the biomass and lipid content of C. reinhardtii.

Physiological insights into enhanced lipid accumulation and temperature tolerance by Tetraselmis suecica ultraviolet mutants

High outdoor temperatures significantly inhibit the growth and lipid production of the industrially promising marine microalga Tetraselmis suecica, which is viewed as a potential feedstock for high-value bioproducts and biofuels. To overcome this limitation, T. suecica was subjected to ultraviolet irradiation to generate mutants capable of being productive at higher temperatures. The top two high-lipid mutants UV-25 and UV-31 isolated at 25 °C and 31 °C, respectively, were compared to the wild type (WT) to delineate physiological alterations and shed light on the mutants' increased biomass and lipid productivity. At 25 °C, UV-25 and UV-31 exhibited lipid productivity of 36.12 and 31.33 mg/L day, which were 1.4- and 1.2-fold higher than WT, respectively. This increase in lipid biosynthesis correlated well with increased carotenoid content in UV-25 (2.2-fold) and UV-31 (3.6-fold), indicating an improved capacity to quench reactive oxygen species. At 31 °C, the growth and lipid accumulation of UV-31 remained high, signifying adaptation to higher temperatures. This is attributed to a well-coordinated modulation of the mutant's cellular metabolism through an increase in galactose and phosphatidylglycerol levels, as well as in protein, all of which contributed to its performance at elevated temperatures. The study successfully established a UV mutagenesis strategy for producing superior- performing microalgae strains with industrially desired traits, paving the way for future outdoor cultivation deployment.

Screening of symbiotic Streptomyces spp. and optimization of microalgal growth in a microalgae-actinomycetes co-culture system

Microalgal biodiesel as a substitute for fossil energy has attracted extensive attention. However, the high cost of microalgae cultivation limits the industrial production of microalgal biodiesel. The co-culture system may offer a means to increase microalgae's biomass production. In this study, Streptomyces strains were selected to construct and optimize co-culture systems with Monoraphidium sp. HDMA-11 and the algal cell biomass, lipid content, phycocyanin content, starch content, and fatty acid composition were determined. The results showed that Streptomyces nojiriensis significantly promoted Monoraphidium sp. HDMA-11 growth and a co-culture system were established. Orthogonal experiments showed that the Monoraphidium sp. HDMA-11 biomass was further increased when the initial culture pH was 7.5, the inoculation time of Streptomyces strain supernatants was 36 h, the volume ratio of microalgal actinomycetes was 1:1, and no additional acetic acid was added. Under these conditions, compared with monocultured Monoraphidium sp. HDMA-11, the cell biomass and lipid productivity of the co-culture system increased by 525.8 and 155.1%, respectively. These results suggest that S. nojiriensis supernatant potentially enhances microalgae biomass and may represent a new method to improve microalgae growth.

Mechanisms of Sodium-Acetate-Induced DHA Accumulation in a DHA-Producing Microalga, Crypthecodinium sp. SUN

Docosahexaenoic acid (DHA) is an omega-3 polyunsaturated fatty acid (PUFA) that is critical for the intelligence and visual development of infants. Crypthecodinium is the first microalga approved by the Food and Drug Administration for DHA production, but its relatively high intracellular starch content restricts fatty acid accumulation. In this study, different carbon sources, including glucose (G), sodium acetate (S) and mixed carbon (M), were used to investigate the regulatory mechanisms of intracellular organic carbon distribution in Crypthecodinium sp. SUN. Results show that glucose favored cell growth and starch accumulation. Sodium acetate limited glucose utilization and starch accumulation but caused a significant increase in total fatty acid (TFA) accumulation and the DHA percentage. Thus, the DHA content in the S group was highest among three groups and reached a maximum (10.65% of DW) at 96 h that was 2.92-fold and 2.24-fold of that in the G and M groups, respectively. Comparative transcriptome analysis showed that rather than the expression of key genes in fatty acids biosynthesis, increased intracellular acetyl-CoA content appeared to be the key regulatory factor for TFA accumulation. Additionally, metabolome analysis showed that the accumulated DHA-rich metabolites of lipid biosynthesis might be the reason for the higher TFA content and DHA percentage of the S group. The present study provides valuable insights to guide further research in DHA production.

Global market and economic analysis of microalgae technology: Status and perspectives

Microalgae have been a promising alternative source of high-value compounds to replace the non-sustainable fossil fuels resource. The recent research development of algae-based bioproducts has remarkable impact various industries section for its renewability, efficiency, and environmentally friendly crops over those synthetic-made product. However, by utilizing microalgae biomass toward their full potential is still limited due to lack of research funding, social acceptability and challenges in policy implementation. This present review highlights the various microalgae biotechnology with consideration of economical aspect for the global potential of algae market, comparison between the microalgae market in Malaysia and international countries. In addition, the cultivation technologies and feasibility of microalgae biomass production globally, followed by insightful challenges and future development of microalgae industry are mentioned. The current study will contribute to the understanding of upstream and downstream of microalgae processing along with technical economical understandings for the successful commercialisation of microalgae products.

Low-frequency ultrasound and nitrogen limitation induced enhancement in biomass production and lipid accumulation of Tetradesmus obliquus FACHB-12

The two-stage cultivation strategy was optimized in this study to simultaneously promote the growth and lipid accumulation of Tetradesmus obliquus. Results showed that the optimal dual-stress conditions were nitrogen concentration at 25 mg N·L^-1 and low-frequency ultrasound at 200 Watt, 1 min, and 8 h interval. The biomass and lipid content of Tetradesmus obliquus were increased by 32.1% and 44.5%, respectively, comparing to the control, and the lipid productivity reached 86.97 mg^-1·L^-1·d^-1 at the end of the cultivation period. The protein and photosynthetic pigment contents of microalgae decreased by 22.4% and 14.0% under dual stress comparing to the control environment. In addition, dual stress cultivation of microalgae presented higher level of antioxidant capacity to balance to oxidation level in microalgal cells. This study provides a new insight for microalgae growth and lipid accumulation with dual stress stimulation.

A review on biodiesel production from microalgae: Influencing parameters and recent advanced technologies

Microalgae are the important part of carbon cycle in the nature, and they could utilize the carbon resource in water and soil efficiently. The abilities of microalgae to mitigate CO₂ emission and produce oil with a high productivity have been proven. Hence, this third-generation biodiesel should be popularized. This review firstly introduce the basic characteristics and application fields of microalgae. Then, the influencing parameters and recent advanced technologies for the microalgae biodiesel production have been discussed. In influencing parameters for biodiesel production section, the factors of microalgae cultivation, lipid accumulation, microalgae harvesting, and lipid extraction have been summarized. In recent advanced technologies for biodiesel production section, the microalgae cultivation systems, lipid induction technologies, microalgae harvesting technologies, and lipid extraction technologies have been reviewed. This review aims to provide useful information to help future development of efficient and commercially viable technology for microalgae-based biodiesel production.

Granular indigenous microalgal-bacterial consortium for wastewater treatment: Establishment strategy, functional microorganism, nutrient removal, and influencing factor

Granular indigenous microalgal-bacterial consortium (G-IMBC) system integrates the advantages of the MBC and granular activated sludge technologies, also with superior microalgal wastewater adaptation capacity. In this review, the concept of IMBC was firstly described, followed by its establishment and acclimation strategies. Characteristics and advantages of G-IMBC system compared to other IMBC systems (i.e., attached and floc IMBC systems) were then introduced. Moreover, the involved functional microorganisms and their interactions, as well as nutrient removal mechanisms were systematically and critically reviewed. Finally, the influencing factors including wastewater characteristics and operation factors were discussed. This study aims to provide a comprehensive up-to-date summary of the G-IMBC system for sustainable wastewater treatment.

Regulation of Reactive Oxygen Species Promotes Growth and Carotenoid Production Under Autotrophic Conditions in Rhodobacter sphaeroides

Industrial demand for capture and utilization using microorganisms to reduce CO₂, a major cause of global warming, is significantly increasing. Rhodobacter sphaeroides is a suitable strain for the process of converting CO₂ into high-value materials because it can accept CO₂ and has various metabolic pathways. However, it has been mainly studied for heterotrophic growth that uses sugars and organic acids as carbon sources, not autotrophic growth. Here, we report that the regulation of reactive oxygen species is critical for growth when using CO₂ as a sole carbon source in R. sphaeroides. In general, the growth rate is much slower under autotrophic conditions compared to heterotrophic conditions. To improve this, we performed random mutagenesis using N-methyl-N’-nitro-N-nitrosoguanidine (NTG). As a result, we selected the YR-1 strain with a maximum specific growth rate (μ) 1.44 day^–1 in the early growth phase, which has a 110% faster growth rate compared to the wild-type. Based on the transcriptome analysis, it was confirmed that the growth was more sensitive to reactive oxygen species under autotrophic conditions. In the YR-1 mutant, the endogenous contents of H₂O₂ levels and oxidative damage were reduced by 33.3 and 42.7% in the cells, respectively. Furthermore, we measured that concentrations of carotenoids, which are important antioxidants. The total carotenoid is produced 9.63 g/L in the YR-1 mutant, suggesting that the production is 1.7-fold higher than wild-type. Taken together, our observations indicate that controlling ROS promotes cell growth and carotenoid production under autotrophic conditions.

The Effect of Trophic Modes on Biomass and Lipid Production of Five Microalgal Strains

Five microalgae strains, namely Isochrysis galbana, Microchloropsis gaditana, Scenedesmus obliquus, Nannochloropsis oculata and Tetraselmis suecica, were selected as potential candidates for polyunsaturated fatty acids’ production, evaluating biomass productivity and their capacity to accumulate high lipid contents under different trophic modes. Microalgae strains were cultivated in the presence of 1% glucose using mixotrophic and heterotrophic conditions, while autotrophic cultures served as control experiments. The results demonstrate that S. obliquus performed the highest biomass productivity that reached 0.13 and 0.14 g L−1 d−1 under mixotrophic and heterotrophic conditions, respectively. I. galbana and S. obliquus utilized elevated contents of glucose in mixotrophy, removing 55.9% and 95.6% of the initial concentration of the carbohydrate, respectively, while glucose consumption by the aforementioned strains also remained high under heterotrophic cultivation. The production of lipids was maximal for I. galbana in mixotrophy and S. obliquus in heterotrophy, performing lipid productivities of 24.85 and 22.77 mg L−1 d−1, respectively. The most abundant saturated acid detected for all microalgae strains evaluated was palmitic acid (C16:0), while oleic and linolenic acids (C18:1n9c/C18:3n3) comprised the most abundant unsaturated fatty acids. I. galbana performed the highest linoleic acid (C18:2n6c) content under heterotrophic nutrition, which reached 87.9 mg g−1 of ash-free dry weight. Among the microalgae strains compared, the biomass and lipid production monitored for I. galbana and S. obliquus confirm that both strains could serve as efficient bioproducers for application in algal biorefineries.