Improved lipid productivity of Scenedesmus obliquus with high nutrient removal efficiency by mixotrophic cultivation in actual municipal wastewater

Enhanced biomass production and harvesting efficiency of Chlamydomonas reinhardtii under high-ammonium conditions by powdered oyster shell

Chlamydomonas reinhardtii prefers ammonium (NH4+) as a nitrogen source, but its late-stage growth under high-NH4+ concentrations (0.5 ∼ 1 g/L) is retarded due to medium acidification. In this study, oyster shell powders were shown to increase the tolerance of C. reinhardtii to NH4+ supplementation at 0.7 g/L in TAP medium in 1-L bubble-column bioreactors, resulting in a 22.9 % increase in biomass production, 62.1 % rise in unsaturated fatty acid accumulation, and 19.2 % improvement in harvesting efficiency. Powdered oyster shell mitigated medium acidification (pH 7.2-7.8) and provided dissolved inorganic carbon up to 8.02 × 103 μmol/L, facilitating a 76.3 % NH4+ consumption, release of up to 189 mg/L of Ca2+, a 42.1 % reduction in ζ-potential and 27.7 % increase in flocculation activity of microalgae cells. This study highlights a promising approach to utilize powdered oyster shell as a liming agent, supplement carbon source, and bio-flocculant for enhancing biomass production and microalgae harvesting in NH4+-rich environments.

Sub-pilot scale sequential microalgal consortium-based cultivation for treatment of municipal wastewater and biomass production

Municipal wastewater (MWW) was treated by a sequential pilot microalgal cultivation process. The cultivation was performed inside a specifically designed low-cost photobioreactor (PBR) system. A microalgal consortium 2:1 was developed using Tetraselmis indica (TS) and Picochlorum sp. (PC) in the first stage and PC:TS (2:1) in the second stage and the nutrient removal efficiency and biomass production and biomolecules production was evaluated and also compared with monoculture in a two-stage sequential cultivation system. This study also investigated the effect of seasonal variations on microalgae growth and MWW treatment. The results showed that mixed microalgal consortium (TS:PC) had higher nutrient removal efficiency, with chemical oxygen demand (COD), total phosphate (TP), and total nitrate (TN) removal efficiencies of 78.50, 84.49, and 84.20%, respectively, and produced a biomass of 2.50 g/L with lipid content of 37.36% in the first stage of cultivation under indoor conditions. In the second stage of indoor cultivation, the PC:TS consortium demonstrated maximum COD, TP, and TN removal efficiencies of 92.49, 94.24, and 94.16%, respectively. It also produced a biomass of 2.65 g/L with a lipid content of 40.67%. Among all the seasonal variations, mass flow analysis indicated that the combination of mixed consortium-based two-stage sequential process during the winter season favored maximum nutrient removal efficiency of TN i.e. 88.54% (84.12 mg/L) and TP i.e., 90.18% (43.29 mg/L), respectively. It also enhanced total biomass production of 49.10 g in 20-L medium, which includes lipid yield ∼15.68 g compared to monoculture i.e., 82.06% (78.70 mg/L) and 82.87% (40.26 mg/L) removal of TN and TP, respectively, and produced biomass 43.60 g with 11.90 g of lipids.

Dialysis bag-microalgae photobioreactor: Novel strategy for enhanced bioresource production and wastewater purification

Cultivating microalgae in wastewater offers various advantages, but it still faces limitations such as bacteria and other impurities in wastewater affecting the growth and purity of microalgae, difficulty in microalgae harvesting, and extracellular products of microalgae affecting effluent quality. In this study, a novel dialysis bag-microalgae photobioreactor (Db-PBR) was developed to achieve wastewater purification and purer bioresource recovery by culturing microalgae in a dialysis bag. The dialysis bag in the Db-PBR effectively captured the microalgae cells and promoted their lipid accumulation, leading to higher biomass (1.53 times of the control) and lipid production (2.50 times of the control). During the stable operation stage of Db-PBR, the average soluble microbial products (SMP) content outside the dialysis bag was 25.83 mg L-1, which was significantly lower than that inside the dialysis bag (185.63 mg L-1), indicating that the dialysis bag effectively intercepted the SMP secreted by microalgae. As a result, the concentration of dissolved organic carbon (DOC) in Db-PBR effluent was significantly lower than that of traditional photobioreactor. Furthermore, benefiting from the dialysis bag in the reactor effectively intercepted the microorganisms in wastewater, significantly improving the purity of the cultured microalgae biomass, which is beneficial for the development of high-value microalgae products.

Reconstruction of a Genome-Scale Metabolic Model of Scenedesmus obliquus and Its Application for Lipid Production under Three Trophic Modes

Green microalgae have emerged as beneficial feedstocks for biofuel production. A systems-level understanding of the biochemical network is needed to harness the microalgal metabolic capacity for bioproduction. Genome-scale metabolic modeling (GEM) showed immense potential in rational metabolic engineering, utilizing biochemical flux distribution analysis. Here, we report the first GEM for the green microalga, Scenedesmus obliquus (iAR632), a promising biodiesel feedstock with high lipid-storing capability. iAR632 comprises 1467 reactions, 734 metabolites, and 632 genes distributed among 7 compartments. The model was optimized under three different trophic modes of microalgal cultivation, i.e., autotrophy, mixotrophy, and heterotrophy. The robustness of the reconstructed network was confirmed by analyzing its sensitivity to the biomass components. Pathway-level flux profiles were analyzed, and significant flux space expansion was noticed majorly in reactions associated with lipid biosynthesis. In agreement with the experimental observation, iAR632 predicted about 3.8-fold increased biomass and almost 4-fold higher lipid under mixotrophy than the other trophic modes. Thus, the assessment of the condition-specific metabolic flux distribution of iAR632 suggested that mixotrophy is the preferred cultivation condition for improved microalgal growth and lipid production. Overall, the reconstructed GEM and subsequent analyses will provide a systematic framework for developing model-driven strategies to improve microalgal bioproduction.

Advancements of microalgal upstream technologies: Bioengineering and application aspects in the paradigm of circular bioeconomy

Sustainable energy transition has brought the attention towards microalgae utilization as potential feedstock due to its tremendous capabilities over its predecessors for generating more energy with reduced carbon footprint. However, the commercialization of microalgae feedstock remains debatable due to the various factors and considerations taken into scaling-up the conventional microalgal upstream processes. This review provides a state-of-the-art assessment over the recent developments of available and existing microalgal upstream cultivation systems catered for maximum biomass production. The key growth parameters and main cultivation modes necessary for optimized microalgal growth conditions along with the fundamental aspects were also reviewed and evaluated comprehensively. In addition, the advancements and strategies towards potential scale-up of the microalgal cultivation technologies were highlighted to provide insights for further development into the upstream processes aimed at sustainable circular bioeconomy.

Growth of Scenedesmus obliquus on anaerobic soybean wastewater using different wasted organics for high biomass production and nutrients recycling

The microalgae culture in mixing sewage with different characteristics may significantly improve biomass production and nutrients recycling efficiency. In this study, three waste organic wastewater including molasses, alcohol and glycerol wastewater were mixed with anaerobic soybean wastewater as mediums for microalgae culture. The optimal mixture of molasses, alcohol and glycerol wastewater was at an initial carbon-nitrogen ratio of 7:1, 5:1 and 10:1, improving biomass production by 60.4%, 31.3% and 68.7%, respectively. The removal efficiencies of organics, ammonia nitrogen and phosphorus at optimal mixture were 54.8-62.4%, 79.5-99.1% and 49.3-61.5%, and the removal rates increased by 340-630%, 27.5-66.3% and 36.3-70.2% compared to the blank culture. In addition, the culture in mixed wastewater increased lipids contrast by 0.7-1.3 times, while achieving higher saturation in fatty acids. The results suggested that microalgae culture using mixed wastewater was a strategy for high biomass production and nutrients recycling efficiency.

A comprehensive review on microbial lipid production from wastes: research updates and tendencies

Microbial lipids have recently attracted attention as an intriguing alternative for the biodiesel and oleochemical industries to achieve sustainable energy generation. However, large-scale lipid production remains limited due to the high processing costs. As multiple variables affect lipid synthesis, an up-to-date overview that will benefit researchers studying microbial lipids is necessary. In this review, the most studied keywords from bibliometric studies are first reviewed. Based on the results, the hot topics in the field were identified to be associated with microbiology studies that aim to enhance lipid synthesis and reduce production costs, focusing on the biological and metabolic engineering involved. The research updates and tendencies of microbial lipids were then analyzed in depth. In particular, feedstock and associated microbes, as well as feedstock and corresponding products, were analyzed in detail. Strategies for lipid biomass enhancement were also discussed, including feedstock adoption, value-added product synthesis, selection of oleaginous microbes, cultivation mode optimization, and metabolic engineering strategies. Finally, the environmental implications of microbial lipid production and possible research directions were presented.

High efficiency lipid production, biochar yield and chlorophyll a content of chlorella sp. microalgae exposed on sea water and TiO2 nanoparticles

This study explores the challenges facing microalgae biofuel production, specifically low lipid content and difficulties with algal cell harvesting. The purpose of the research is to investigate the effect of seawater content and nanoparticle concentration on freshwater microalgae growth and biofuel production. The principal results of the study show that increasing the proportion of seawater and nanoparticles enhances the lipid content and cell diameter of microalgae, while excessive concentrations of nanoparticles and low seawater content lead to reduced microalgae growth. Furthermore, an optimal cell diameter was identified at a nanoparticle concentration of 150 mg/L. The study also reveals that increasing seawater content can decrease zeta potential and increase chlorophyll a content due to the concentration of dissolved organic matter. Increasing the seawater content from 0% to 25% decreased zeta potential by 1% owing to the instability and aggregation of the cells. Chlorophyll a for the 0% seawater was 0.55 which is increased to 1.32 only due to the increase in the seawater content. This significant increase is due to the concentration of dissolved organic matter in seawater. Additionally, the presence of seawater positively affects microalgae metabolic activity and biochar yield. The findings of this study offer valuable insights into the potential for optimizing microalgae biofuel production. The use of seawater and nanoparticles has shown promise in enhancing microalgae growth and biofuel yield, and the results of this study underscore the scientific value of exploring the role of seawater and nanoparticles in microalgae biofuel production. Further research in this area has the potential to significantly contribute to the development of sustainable energy solutions.

Integrated culture and harvest systems for improved microalgal biomass production and wastewater treatment

Microalgae cultivation in wastewater has received much attention as an environmentally sustainable approach. However, commercial application of this technique is challenging due to the low biomass output and high harvesting costs. Recently, integrated culture and harvest systems including microalgae biofilm, membrane photobioreactor, microalgae-fungi co-culture, microalgae-activated sludge co-culture, and microalgae auto-flocculation have been explored for efficiently coupling microalgal biomass production with wastewater purification. In such systems, the cultivation of microalgae and the separation of algal cells from wastewater are performed in the same reactor, enabling microalgae grown in the cultivation system to reach higher concentration, thus greatly improving the efficiency of biomass production and wastewater purification. Additionally, the design of such innovative systems also allows for microalgae cells to be harvested more efficiently. This review summarizes the mechanisms, characteristics, applications, and development trends of the various integrated systems and discusses their potential for broad applications, which worth further research.

Efficient coupling of sulfadiazine removal with microalgae lipid production in a membrane photobioreactor

This study explored the feasibility of a coupled system for antibiotic removal and biofuel production through microalgae cultivation. Initial, batch culture experiments demonstrated that sulfadiazine (SDZ) had an inhibitory effect on Chlorella sp. G-9, and 100.0 mg L-1 SDZ completely inhibited its growth. In order to improve SDZ removal efficiency by microalgae, three membrane photobioreactors (MPBRs) with different hydraulic retention times (HRTs) were established for continuous microalgae cultivation. The efficient coupling of SDZ removal and microalgal lipid production was achieved through the gradual increment of influent SDZ concentration from 0 to 100.0 mg L-1. The reduction in SDZ ranged between 57.8 and 89.7%, 54.7-91.7%, and 54.6-93.5% for the MPBRs with HRT of 4 d, 2 d, and 1 d, respectively. Chlorella sp. Was found to tolerate higher concentrations of SDZ in the MPBR system, and the resulting stress from high concentrations of SDZ effectively increased the lipid content of microalgae for potential biodiesel production. With the increase of influent SDZ concentration from 0 to 100.0 mg L-1, the lipid content of microalgae increased by 43.5%. Chlorophyll content, superoxide dismutase activity, and malondialdehyde content of microalgae were also evaluated to explore the mechanism of microalgae tolerance to SDZ stress in MPBR.

Improving the growth of Spirulina in CO2 absorption and microalgae conversion (CAMC) system through mixotrophic cultivation: Reveal of metabolomics

Mixotrophic cultivation was proposed to enhance the biomass and carbon sequestration efficiency of Spirulina in CO₂ absorption and microalgae conversion (CAMC) system, and the underlying metabolic mechanism was also explored. The result showed that mixotrophic enhanced the performance of CAMC system, the maximum biomass, total carbon conversion capacity and efficiency was obtained at 0.5 g/L acetate group, which was 60.47 %, 63.06 % and 59.77 % higher than control. Adding 0.5 g/L acetate enhanced the activities of Rubisco and Acetyl-CoA, arrived at 89.59 U/g and 5.16 nmol/g, respectively. Metabolomics analyses suggested that mixotrophic changed metabolic flux and affected intracellular composition. Mixotrophic up-regulated Calvin cycle, glycolysis, and tricarboxylic acid (TCA) cycle, induced more carbon fluxes into central carbon metabolism for the growth of Spirulina. These results suggested that mixotrophic could supply effective energy and carbon skeleton for rapid growth of Spirulina, and provided a theoretical basis for large-scale application of CAMC system.

Anaerobic digestates grown oleaginous microalgae for pollutants removal and lipids production

Anaerobic digestates were potential mediums for cultivating oleaginous microalgae, but their various components brought uncertainties for aglal growth and lipids production. In this study, three microalgae strains were tested to grow on four typical anaerobic digestates. The results showed that anaerobic food wastewater was an optimal medium for C. pyrenoidosa and S. obliquus culture (N. oleoabundanst cannot survive), achieving the highest biomass (2.15-2.32 g L^-1) and lipids production (20.6-32.5 mg L^-1·d^-1). In contrast, three microalgae strains could grow suboptimally in anaerobic municipal (0.79-0.95 g L^-1) and toilet (0.92-1.40 g L^-1) wastewater, but showed poor performances in anaerobic swine wastewater. The growth of microalgae removed 40.9-63.4% of TOC, 83.7-96.3% of NH₄⁺-N and 70.3-89.4% of TP in the three ADs. In addition, it was unfortunately found that the lipids content and saturation degree in fatty acids significantly decreased in ADs with sufficient nutrients. It suggests that some measures should be taken to balance biomass, lipids production and quality for cultivating microalgae in anaerobic digestates.

Promoting effect of plant hormone gibberellin on co-metabolism of sulfamethoxazole by microalgae Chlorella pyrenoidosa

In this study, sodium acetate (NaAC) as a co-substrate effectively promoted the metabolism of sulfamethoxazole (SMX) by microalgae Chlorella pyrenoidosa. In the cultivation supplied with 5.0 and 10.0 g L^-1 NaAC, 51.1% and 61.2% SMX was removed, respectively. On this basis, the improvement effect of plant hormone gibberellin (GA₃) on SMX removal by 5 g L^-1 NaAC supplied as co-substrate was further investigated. The results showed that biodegradation played decisive role in the removal of SMX. As a plant hormone, GA₃ effectively improved the co-metabolic removal efficiency of SMX by C. pyrenoidosa. Especially when GA₃ dosage reached 10.0 and 50.0 mg L^-1, C. pyrenoidosa showed a very high SMX removal rate of 83.5% and 95.3%, respectively. Transcriptome analysis showed that GA₃ promoted the removal of SMX by C. pyrenoidosa was the result of the combined action of exogenous and endogenous plant hormones.

Improving sedimentation and lipid production of microalgae in the photobioreactor using saline wastewater

Saline wastewater was used in this study to culture freshwater microalgae Chlorella pyrenoidosa in sequencing batch photobioreactor to improve the sedimentation and lipid production of algal cells. Influent salinity of 0.5% or above effectively promoted the sedimentation of microalgae in the settling stage of photobioreactor, and greatly reduced the algal biomass in effluent. The mechanism of the saline wastewater in improving the sedimentation of microalgae included decreasing zeta potential, increasing cell particle size and promoting extracellular polymeric substances synthesis, which varied with influent salinity. Saline wastewater also promoted the lipid accumulation in microalgae. Lipid content of microalgae increased with increasing influent salinity. However, the growth of microalgae was greatly inhibited at the influent salinity of 2.0% and 3.0%. Therefore, the PBR with influent salinity of 1.0% achieved the highest productivity of microalgae lipid. The saturation of fatty acids of microalgae gradually increased with increasing influent salinity.

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.

Application of Microalgal Stress Responses in Industrial Microalgal Production Systems

Adaptive laboratory evolution (ALE) has been widely utilized as a tool for developing new biological and phenotypic functions to explore strain improvement for microalgal production. Specifically, ALE has been utilized to evolve strains to better adapt to defined conditions. It has become a new solution to improve the performance of strains in microalgae biotechnology. This review mainly summarizes the key results from recent microalgal ALE studies in industrial production. ALE designed for improving cell growth rate, product yield, environmental tolerance and wastewater treatment is discussed to exploit microalgae in various applications. Further development of ALE is proposed, to provide theoretical support for producing the high value-added products from microalgal production.