Bicarbonate supplementation enhanced biofuel production potential as well as nutritional stress mitigation in the microalgae Scenedesmus sp. CCNM 1077

Recovery of volatile ethanol gas via microalgal-bacterial consortium: Ethanol-to-acetate conversion pathway boosts lipid production

The pharmaceutical industry, an essential sector of the global economy, heavily relies on ethanol solvents, which leads to significant volatile organic compounds (VOCs) emissions. As a sustainable treatment method aligning with carbon reduction goals, this study proposed and demonstrated a synergistic approach of using microalgae (Chlorella sorokiniana FACHB-24) and acetic acid bacteria (Acetobacter pasteurianus CICC 20056) to recover ethanol into value-added products (algal lipids). In the innovative co-culture, A. pasteurianus oxidizes ethanol to acetic acid, which is fed to algae for lipid production. This method increased biomass and lipid yield by 21.29% and 150.16% (p < 0.05), respectively, compared to microalgae directly using ethanol. Some operational parameters including ethanol concentration, bacterial-algal biomass ratio, pH value, and light intensity made influence on lipid production. Under the optimal conditions (1.0% v/v ethanol concentration, 1:10 bacterial-algal biomass ratio, pH 6.5, and 5000 lux light intensity), the maximal biomass and lipid yields were 572.5 mg L-1 and 161.1 mg L-1 (26.7% lipid content), respectively. In the harvested lipid from microalgae, C16 - C18 fatty acids made up 98.22% of the total fatty acid methyl esters content. In proteomic comparison of the single culture and co-culture, the conversion of ethanol to acetate by A. pasteurianus provides C. sorokiniana with a more efficient acetyl-CoA source by bypassing energy-intensive glycolysis and directly enhancing lipid synthesis. This study provides a solution to increasing the lipid production from ethanol gas as a sustainable VOCs management of pharmaceutical industry.

Deciphering the promotion and inhibition of bicarbonate fertilization on microalgal activity and nutrient uptake from wastewater

Microalgal bioremediation is a promising alternative for biological wastewater treatment but constrained by low microalgal activities. Here, bicarbonate fertilization was introduced to enhance microalgal wastewater treatment, with systematic investigations of its biphasic dose-dependent effects on microalgal activity and nutrient uptake. The results showed that moderate inorganic carbon (MIC, 0.05 M) group significantly improved the biomass production, NH4+-N removal, and PO43--P removal by 76.0%, 21.3%, and 11.9%, respectively; whereas high inorganic carbon (HIC, 0.1 M) group inhibited them by 11.0%, 4.48%, and 52.7%, respectively, compared with low inorganic carbon (LIC, 0.005 M) group. Mechanistic analyses suggested that LIC group encountered high alkalinity, exacerbated carbon/trace element limitation, and attenuated extracellular polymeric substances (EPS) barriers and antioxidant systems; while HIC group increased salinity stresses, triggered morphological defense, and diminished light harvesting and phycospheric mass transfer, restricting microalgal activity and nutrient uptake. In contrast, MIC group relieved carbon limitation, accelerated photosynthetic electron transfer, and sustained intracellular redox homeostasis, underpinning the highest biomass production and nutrient removal. These findings could facilitate the practical application of bicarbonate fertilization in microalgal wastewater treatment.

Metagenomic Insights into Pollutants in Biorefinery and Dairy Wastewater: rDNA Dominance and Electricity Generation in Double Chamber Microbial Fuel Cells

This study evaluates the potential of biorefinery and dairy wastewater as substrates for electricity generation in double chamber Microbial Fuel Cells (DCMFC), focusing on their microbial taxonomy and electrochemical viability. Taxonomic analysis using 16S/18S rDNA-targeted DGGE and high-throughput sequencing identified Proteobacteria as dominant in biorefinery biomass, followed by Firmicutes and Bacteriodota. In dairy biomass, Lactobacillus (77.36%) and Clostridium (15.70%) were most prevalent. Biorefinery wastewater exhibited the highest bioelectrochemical viability due to its superior electrical conductivity and salinity, achieving a voltage yield of 65 mV, compared to 75.2 mV from mixed substrates and 1.7 mV from dairy wastewater. Elevated phosphate levels in dairy wastewater inhibited bioelectrochemical processes. This study recommends Biorefinery wastewater as the most suitable purely organic substrate for efficient bioelectricity generation and scaling up of MFCs, emphasising the importance of substrate selection for optimal energy output for practical and commercial viability.

Recovery mechanism of a microalgal species, Chlorella sp. from toxicity of doxylamine: Physiological and biochemical changes, and transcriptomics

Ubiquitous distribution of pharmaceutical contaminants in environment has caused unexpected adverse effects on ecological organisms; however, how microorganisms recover from their toxicities remains largely unknown. In this study, we comprehensively investigated the effect of a representative pollutant, doxylamine (DOX) on a freshwater microalgal species, Chlorella sp. by analyzing the growth patterns, biochemical changes (total chlorophyll, carotenoid, carbohydrate, protein, and antioxidant enzymes), and transcriptomics. We found toxicity of DOX on Chlorella sp. was mainly caused by disrupting synthesis of ribosomes in nucleolus, and r/t RNA binding and processing. Intriguingly, additional bicarbonate enhanced the toxicity of DOX with decreasing the half-maximum effective concentrations from 15.34 mg L-1 to 4.63 mg L-1, which can be caused by inhibiting fatty acid oxidation and amino acid metabolism. Microalgal cells can recover from this stress via upregulating antioxidant enzymatic activities to neutralize oxidative stresses, and photosynthetic pathways and nitrogen metabolism to supply more energies and cellular signaling molecules. This study extended our understanding on how microalgae can recover from chemical toxicity, and also emphasized the effect of environmental factors on the toxicity of these contaminants on aquatic microorganisms.

Bicarbonate concentration influences carbon utilization rates and biochemical profiles of freshwater and marine microalgae

Selecting the optimal microalgal strain for carbon capture and biomass production is crucial for ensuring the commercial viability of microalgae-based biorefinery processes. This study aimed to evaluate the impact of varying bicarbonate concentrations on the growth rates, inorganic carbon (IC) utilization, and biochemical composition of three freshwater and two marine microalgal species. Parachlorella kessleri, Vischeria cf. stellata, and Porphyridium purpureum achieved the highest carbon removal efficiency (>85%) and biomass production at 6 g L-1 sodium bicarbonate (NaHCO3), while Phaeodactylum tricornutum showed optimal performance at 1 g L-1 NaHCO3. The growth and carbon removal rate of Scenedesmus quadricauda increased with increasing NaHCO3 concentrations, although its highest carbon removal efficiency (∼70%) was lower than the other species. Varying NaHCO3 levels significantly impacted the biochemical composition of P. kessleri, S. quadricauda, and P. purpureum but did not affect the composition of the remaining species. The fatty acid profiles of the microalgae were dominated by C16 and C18 fatty acids, with P. purpureum and P. tricornutum yielding relatively high polyunsaturated fatty acid content ranging between 14% and 30%. Furthermore, bicarbonate concentration had a species-specific effect on the fatty acid and chlorophyll-a content. This study demonstrates the potential of bicarbonate as an effective IC source for microalgal cultivation, highlighting its ability to select microalgal species for various applications based on their carbon capture efficiency and biochemical composition.

A novel integrated approach employing Desertifilum tharense BERC-3 for efficient wastewater valorization and recycling for developing peri-urban algae farming system

Peri-urban environments are significant reservoirs of wastewater, and releasing this untreated wastewater from these resources poses severe environmental and ecological threats. Wastewater mitigation through sustainable approaches is an emerging area of interest. Algae offers a promising strategy for carbon-neutral valorization and recycling of urban wastewater. Aiming to provide a proof-of-concept for complete valorization and recycling of urban wastewater in a peri-urban environment in a closed loop system, a newly isolated biocrust-forming cyanobacterium Desertifilum tharense BERC-3 was evaluated. Here, the highest growth and lipids productivity were achieved in urban wastewater compared to BG11 and synthetic wastewater. D. tharense BERC-3 showed 60-95% resource recovery efficiency and decreased total dissolved solids, chemical oxygen demand, biological oxygen demand, nitrate nitrogen, ammonia nitrogen and total phosphorus contents of the water by 60.37%, 81.11%, 82.75%, 87.91%, 85.13%, 85.41%, 95.87%, respectively, making it fit for agriculture as per WHO's safety limits. Soil supplementation with 2% wastewater-cultivated algae as a soil amender, along with its irrigation with post-treated wastewater, improved the nitrogen content and microbial activity of the soil by 0.3-2.0-fold and 0.5-fold, respectively. Besides, the availability of phosphorus was also improved by 1.66-fold. The complete bioprocessing pipeline offered a complete biomass utilization. This study demonstrated the first proof-of-concept of integrating resource recovery and resource recycling using cyanobacteria to develop a peri-urban algae farming system. This can lead to establishing wastewater-driven algae cultivation systems as novel enterprises for rural migrants moving to urban areas.

Characterization of exogenous lactate addition on the growth, photosynthetic performance, and biochemical composition of four bait microalgae strains

AIMS

To quickly obtain the biomass of bait microalgae with high value-added products, researchers have examined the influence of biochemical and environmental factors on the growth rates and biochemical composition of microalgae. Previous studies have shown that lactate plays an important role in metabolic regulation in Phaeodactylum tricornutum. In this study, we investigated the effect of exogenous lactate on the growth rates, photosynthetic efficiency, and biochemical composition of four commonly used bait microalgae in aquaculture.

METHODS AND RESULTS

The optical density of the algal cultures at specific time points, YII, Fv/Fm, and the total lipid, protein, soluble sugar, insoluble sugar, chlorophyll a, and carotenoid content of P. tricornutum, Isochrysis galbana (I. galbana), Chaetoceros muelleri, and Cylindrotheca fusiformis were determined. In I. galbana, the growth rate was enhanced with the addition of lactate, even though higher concentrations of lactate were associated with a decrease in YII and Fv/Fm. In general, the total lipid content of these microalgal strains increased gradually in a concentration-dependent manner over the range of lactate concentrations. In addition, higher concentrations of lactate also induced significant changes in the total soluble and insoluble sugar levels in all microalgal strains. However, chlorophyll a and carotenoid contents increased at lower but decreased at higher concentrations of lactate in all microalgal strains. The total protein content was significantly elevated at all concentrations of lactate in P. tricornutum, whereas there were no significant differences in that of C. fusiformis.

CONCLUSIONS

Lactate effective influences in the growth, metabolism, and synthesis of important biochemical components in the four microalgal strains under investigation.

Optimizing inorganic carbon and salinity for enhanced biomass and pigment production in Colaconema formosanum: Implications for sustainable carbon sequestration and stress responses

This study investigates a cultivation strategy for the macroalga Colaconema formosanum by determining optimal inorganic carbon concentration and salinity for maximizing biomass and photosynthetic pigment production while also facilitating carbon sequestration. The response surface method was used with a central composite design (CCD-RSM) to determine the optimal conditions. Results showed that adding 1.2 g/L of carbon increased the specific growth rate to 18%-19% per day. The maximum amount of pigment, including phycobiliprotein and chlorophyll, was achieved by adjusting both carbon content and salinity. This strategy enables mass pigment production and offers an eco-friendly approach to carbon sequestration while reducing culture period. This study also sheds light on algal mechanisms against enriched inorganic carbon and salinity content, contributing to an enhanced understanding of these vital processes.

Recent Advances in Marine Microalgae Production: Highlighting Human Health Products from Microalgae in View of the Coronavirus Pandemic (COVID-19)

Blue biotechnology can greatly help solve some of the most serious social problems due to its wide biodiversity, which includes marine environments. Microalgae are important resources for human needs as an alternative to terrestrial plants because of their rich biodiversity, rapid growth, and product contributions in many fields. The production scheme for microalgae biomass mainly consists of two processes: (I) the Build-Up process and (II) the Pull-Down process. The Build-Up process consists of (1) the super strain concept and (2) cultivation aspects. The Pull-Down process includes (1) harvesting and (2) drying algal biomass. In some cases, such as the manufacture of algal products, the (3) extraction of bioactive compounds is included. Microalgae have a wide range of commercial applications, such as in aquaculture, biofertilizer, bioenergy, pharmaceuticals, and functional foods, which have several industrial and academic applications around the world. The efficiency and success of biomedical products derived from microalgal biomass or its metabolites mainly depend on the technologies used in the cultivation, harvesting, drying, and extraction of microalgae bioactive molecules. The current review focuses on recent advanced technologies that enhance microalgae biomass within microalgae production schemes. Moreover, the current work highlights marine drugs and human health products derived from microalgae that can improve human immunity and reduce viral activities, especially COVID-19.

Enhanced Colaconema formosanum biomass and phycoerythrin yield after manipulating inorganic carbon, irradiance, and photoperiod

Due to an increasing CO₂ concentration leading to global warming, the techniques as carbon capture utilization and storage are currently critical issues. This study aimed to investigate a cultivation strategy using optimal inorganic carbon level, irradiance, and photoperiod for producing the highest biomass and photosynthesis pigment contents (chlorophyll and phycobiliprotein) in the macroalga Colaconema formosanum. The results revealed that adding 1 g L^-1 carbon increases phycoerythrin ratio by 12.52-13.74% and decreases allophycocyanin by 10.4-9.57%. Optimal conditions can increase algal growth by 60%, providing 5-6 mg g^-1 total phycobiliprotein and 650-680 µg g^-1 total chlorophyll. The results in this study illustrate the sensitivity of photosynthesis pigment after treatment with carbon, and suggest a hypothesis explaining the mechanism. The results also provide a feasible use of carbon for high-value large-scale production of pigment in the macroalgae industry.

Physiological and Biochemical Responses of Bicarbonate Supplementation on Biomass and Lipid Content of Green Algae Scenedesmus sp. BHU1 Isolated From Wastewater for Renewable Biofuel Feedstock

In the present study, different microalgae were isolated from wastewater environment and evaluated for higher growth and lipid accumulation. The growth adaptability of all the isolated microalgae were tested for carbon source with supplementation of sodium bicarbonate in BG-11 N⁺ medium. Further based on the uptake rate of sodium bicarbonate and growth behavior, microalgal strains were selected for biofuel feedstock. During the study, growth parameters of all the isolates were screened after supplementation with various carbon sources, in which strain Scenedesmus sp. BHU1 was found highly effective among all. The efficacy of Scenedesmus sp. BHU1 strain under different sodium bicarbonate (4–20 mM) concentration, in which higher growth 1.4 times greater than control was observed at the concentration 12 mM sodium bicarbonate. In addition, total chlorophyll content (Chl-a + Chl-b), chlorophyll fluorescence (Fv/Fm, Y(II), ETR max, and NPQmax), and biomass productivity were found to be 11.514 μg/ml, 0.673, 0.675, and 31.167 μmol electrons m⁻² s⁻¹, 1.399, 59.167 mg/L/day, respectively, at the 12 mM sodium bicarbonate. However, under optimum sodium bicarbonate supplementation, 56.920% carbohydrate and 34.693% lipid content were accumulated, which showed potential of sodium bicarbonate supplementation in renewable biofuel feedstock by using Scenedesmus sp. BHU1 strain.

Improving carbohydrate accumulation in Chlamydomonas debaryana induced by sulfur starvation using response surface methodology

Most methods that promote carbohydrate production negatively affect cell growth and microalgal biomass production. This study explores, in a two-stage cultivation strategy, in Chlamydomonas debaryana the optimization of certain culture conditions for high carbohydrate production without loss of biomass. In the first stage, the interaction between sodium bicarbonate supplementation, aeration, and different growth periods was optimized using the response surface methodology (RMS). The 3-factor Box-Behnken design (BBD) was applied, and a second-order polynomial regression analysis was used to analyze the experimental data. The results showed that 0.45 g L^-1 of sodium bicarbonate combined with a good aerated agitation (0.6 L min^-1) and a cultivation period of 18 days are optimal to produce 5.02 g L^-1 of biomass containing 43% of carbohydrates.Under these optimized growth conditions, accumulation of carbohydrates was studied using different modes of nutritional stress. The results indicated that carbohydrate content was improved and the maximum accumulation (about 60% of the dry weight) was recorded under sulfur starvation with only a 14% reduction in biomass as compared to control. This study showed promising results as to biomass production and carbohydrate yield by the microalgae C. debaryana in view of production of third-generation biofuels.

Optimization Analysis to Evaluate the Relationships between Different Ion Concentrations and Prymnesium parvum Growth Rate

The purpose of this study was to evaluate the optimum environmental condition required for reaching the maximum growth rate of P. parvum. Eight ions (Na+, K+, CO32−, HCO3−, Ca2+, Mg2+, Cl−, and SO42−) were divided into two groups with a uniform design of 4 factors and 10 levels. The results showed a rising trend in growth rate with increasing ion concentrations. However, concentrations that exceeded the threshold led to a slowdown in the growth rate. Therefore, adequate supply of ion concentrations promoted growth of P. parvum, whereas excessively abundant or deficient ion concentrations inhibited its growth rate. Specifically, the order of impact of the first four ion factors on the growth rate was Na+ > HCO3− > K+ > CO32−. The growth rate of P. parvum reached the maximum theoretical 0.999 when the concentrations of Na+, K+, CO32−, and HCO3− ions were 397.98, 11.60, 3.37, and 33.31 mg/L, respectively. This theoretical growth maximum was inferred from the experimental results obtained in this study. For other ion factors, SO42− had the most influence on the growth rate of P. parvum, followed by Mg2+, Ca2+, and Cl− ions. The growth rate of P. parvum reached the maximum theoretical value of 0.945 when the concentrations of Ca2+, Mg2+, Cl−, and SO42− ions were 11.52, 32.95, 326.29, and 377.31 mg/L, respectively. The findings presented in this study add to our understanding of the growth conditions of P. parvum and provide a theoretical basis for dealing with the water bloom it produces in order to control and utilize it.

Removal of Nutrients and Pesticides from Agricultural Runoff Using Microalgae and Cyanobacteria

The use of pesticides in agriculture has ensured the production of different crops. However, pesticides have become an emerging public health problem for Latin American countries due to their excessive use, inadequate application, toxic characteristics, and minimal residue control. The current project evaluates the ability of two strains of algae (Chlorella and Scenedesmus sp.) and one cyanobacteria (Hapalosyphon sp.) to remove excess pesticides and other nutrients present in runoff water from rice production. Different concentrations of wastewater and carbon sources (Na2CO3 and NaHCO3) were evaluated. According to the results, all three strains can be grown in wastewater without dilution (100%), with a biomass concentration comparable to a synthetic medium. All three strains significantly reduced the concentration of NO3 and PO4 (95 and 85%, respectively), with no difference between Na2CO3 or NaHCO3. Finally, Chlorella sp. obtained the highest removal efficiency of the pesticide (Chlorpyrifos), followed by Scenedesmus and Hapalosyphon sp. (100, 75, and 50%, respectively). This work shows that it is possible to use this type of waste as an alternative source of nutrients to obtain biomass and metabolites of interest, such as lipids and carbohydrates, to produce biofuels.

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

Energy is a major driving force for the economic development. Due to the scarcity of fossil fuels and negative impact on the environment, it is important to develop renewable and sustainable energy sources for humankind. Microalgae as the primary feedstock for biodiesel has shown great application potential. However, lipid yield from microalgae is limited by the upstream cost, which restrain the realization of large-scale biofuel production. The modification of lipid-rich microalgae cell has become the focus over the last few decades to improve the lipid content and productivity of microalgae. Carbon is a vital nutrient that regulates the growth and metabolism of microalgae. Different carbon sources are assimilated by microalgae cells via different pathways. Inorganic carbon sources are mainly used through the CO₂-concentrating mechanisms (CCMs), while organic carbon sources are absorbed by microalgae mainly through the Pentose Phosphate (PPP) Pathway and the Embden-Meyerhof-Pranas (EMP) pathway. Therefore, the addition of carbon source has a significant impact on the production of microalgae biomass and lipid accumulation. In this paper, mechanisms of lipid synthesis and carbon uptake of microalgae were introduced, and the effects of different carbon conditions (types, concentrations, and addition methods) on lipid accumulation in microalgal biomass production and biodiesel production were comprehensively discussed. This review also highlights the recent advances in microalgae lipid cultivation with large-scale commercialization and the development prospects of biodiesel production. Current challenges and constructive suggestions are proposed on cost-benefit concerns in large-scale production of microalgae biodiesel.

Algal treatment of membrane rejects: a unique approach towards zero liquid discharge

A major concern in membrane-based water purification system is generation of huge concentrate stream and wastage of water. A typical Reverse osmosis (RO) or Nanofiltration (NF) system generates 20-25% reject containing high amount of dissolved salts and other contaminants. Contrary to popular belief, this reject water cannot be used without removing the contaminants or cannot be discharged anywhere. Main goal of this project is to find a cheapest and green way for treatment of RO/NF reject. Algal evaporation technique was explored in laboratory scale, to find its suitability for treatment of chloride-rich membrane reject in actual scenario and based on the results obtained, a pilot plant of 48KL was established on Hooghly Met Coke division (HMC), Tata Steel. Particular species of microalgae was selected, to take up minerals from reject water. There are several types of bacteria and symbiotic algae associated with selected micro algae survive in high TDS. A unique slope roof system, connected with algae growth tank, helps in efficient evaporation of water ensuring a Zero discharge. A markedly improved performance was achieved when algal evaporation followed solar evaporation. A total evaporation of 11 L/m²/day was observed, which was almost five times faster than Solar evaporation.

Microalgae as Contributors to Produce Biopolymers

Biopolymers are very favorable materials produced by living organisms, with interesting properties such as biodegradability, renewability, and biocompatibility. Biopolymers have been recently considered to compete with fossil-based polymeric materials, which rase several environmental concerns. Biobased plastics are receiving growing interest for many applications including electronics, medical devices, food packaging, and energy. Biopolymers can be produced from biological sources such as plants, animals, agricultural wastes, and microbes. Studies suggest that microalgae and cyanobacteria are two of the promising sources of polyhydroxyalkanoates (PHAs), cellulose, carbohydrates (particularly starch), and proteins, as the major components of microalgae (and of certain cyanobacteria) for producing bioplastics. This review aims to summarize the potential of microalgal PHAs, polysaccharides, and proteins for bioplastic production. The findings of this review give insight into current knowledge and future direction in microalgal-based bioplastic production considering a circular economy approach. The current review is divided into three main topics, namely (i) the analysis of the main types and properties of bioplastic monomers, blends, and composites; (ii) the cultivation process to optimize the microalgae growth and accumulation of important biobased compounds to produce bioplastics; and (iii) a critical analysis of the future perspectives on the field.

Improving the feasibility of aquaculture feed by using microalgae

The aquaculture industry is an efficient edible protein producer and grows faster than any other food sector. Therefore, it requires enormous amounts of fish feed. Fish feed directly affects the quality of produced fish, potential health benefits, and cost. Fish meal (FM), fis oil (FO), and plant-based supplements, predominantly used in fish feed, face challenges of low availability, low nutritional value, and high cost. The cost associated with aquaculture feed represents 40-75% of aquaculture production cost and one of the key market drivers for the thriving aquaculture industry. Microalgae are a primary producer in aquatic food chains. Microalgae are expanding continuously in renewable energy, pharmaceutical pigment, wastewater treatment, food, and feed industries. Major components of microalgal biomass are proteins with essential amino acids, lipids with polyunsaturated fatty acids (PUFA), carbohydrates, pigments, and other bioactive compounds. Thus, microalgae can be used as an essential, viable, and alternative feed ingredient in aquaculture feed. In recent times, live algae culture, whole algae, and lipid-extracted algae (LEA) have been tested in fish feed for growth, physiological activity, and nutritional value. The present review discusses the potential application of microalgae in aquaculture feed, its mode of application, nutritional value, and possible replacement of conventional feed ingredients, and disadvantages of plant-based feed. The review also focuses on integrated processes such as algae cultivation in aquaculture wastewater, aquaponics systems, challenges, and future prospects of using microalgae in the aquafeed industry.

Impact of wastewater cultivation on pollutant removal, biomass production, metabolite biosynthesis, and carbon dioxide fixation of newly isolated cyanobacteria in a multiproduct biorefinery paradigm

The impact of wastewater cultivation was studied on pollutant removal, biomass production, and biosynthesis of high-value metabolites by newly isolated cyanobacteria namely Acaryochloris marina BERC03, Oscillatoria sp. BERC04, and Pleurocapsa sp. BERC06. During cultivation in urabn wastewater, its pH used to adjust from pH 8.0 to 11, offering contamination-free cultivation, and flotation-based easy harvesting. Besides, wastewater cultivation improved biomass production by 1.3-fold when compared to control along with 3.54-4.2 gL^-1 of CO₂ fixation, concomitantly removing suspended organic matter, total nitrogen, and phosphorus by 100%, 53%, and 88%, respectively. Biomass accumulated 26-36% carbohydrates, 15-28% proteins, 38-43% lipids, and 6.3-9.5% phycobilins, where phycobilin yield was improved by 1.6-fold when compared to control. Lipids extracted from the pigment-free biomass were trans-esterified to biodiesel where pigment extraction showed no negative impact on quality of the biodiesel. These strains demonstrated the potential to become feedstock of an integrated biorefinery using urban wastewater as low-cost growth media.

Homoacetogenesis and solventogenesis from H2/CO2 by granular sludge at 25, 37 and 55 °C

CO₂ fermentation is a promising process to produce biofuels like ethanol. It can be integrated in third generation biofuel production processes to substitute traditional sugar fermentation when supplied with cheap electron donors, e.g. hydrogen derived from wind energy or as surplus gas in electrolysis. In this study, granular sludge from an industrial wastewater treatment plant was tested as inoculum for ethanol production from H₂/CO₂ via non-phototropic fermentation at submesophilic (25 °C), mesophilic (37 °C) and thermophilic (55 °C) conditions. The highest ethanol concentration (17.11 mM) was obtained at 25 °C and was 5-fold higher than at 37 °C (3.36 mM), which was attributed to the fact that the undissociated acid (non-ionized acetic acid) accumulation rate constant (0.145 h^-1) was 1.39 fold higher than at 25 °C (0.104 h^-1). Methane was mainly produced at 55 °C, while neither acetic acid nor ethanol were formed. Ethanol production was linked to acetic acid production with the highest ethanol to acetic acid ratio of 0.514 at 25 °C. The carbon recovery was 115.7%, 131.2% and 117.1%, while the electron balance was almost closed (97.1%, 110.1% and 109.1%) at 25 °C, 37 °C and 55 °C, respectively. The addition of bicarbonate inhibited ethanol production both at 25 °C and 37 °C. Clostridium sp. were the prevalent species at both 25 and 37 °C at the end of the incubation, which possibly contributed to the ethanol production.

Biodegradation of Doxylamine From Wastewater by a Green Microalga, Scenedesmus obliquus

Pharmaceutical contaminants (PCs) have been recognized as emerging contaminants causing unexpected consequences to environment and humans. There is an urgent need for development of efficient technologies to treat these PCs from water. The current study has investigated the removal capacity of a green microalgal species, Scenedesmus obliquus, for doxylamine, chemical oxygen demand (COD), and nutrients from real wastewater. Results have indicated that S. obliquus can grow well in the doxylamine-polluted wastewater with the achievement of 56, 78.5, 100, and 89% removal of doxylamine, COD, total nitrogen (TN), and total phosphorus (TP). Addition of 2 g L^-1 bicarbonate enhanced the removal of doxylamine up to 63% and slightly inhibited the removal of COD. Decreased carbohydrate (28-26%) and increased protein content (30-33%) of the harvested biomass have been observed after cultivation in the wastewater. The current study has shown the feasibility of using microalgae-based biotechnologies for PC-contaminated wastewater.

Seawater supplemented with bicarbonate for efficient marine microalgae production in floating photobioreactor on ocean: A case study of Chlorella sp

Cultivation of microalgae on ocean provides a promising way to produce massive biomass without utilizing limited land space, and using seawater as culture medium can avoid consumption of valuable fresh water. Bicarbonate is proved as a better approach for carbon supply in microalgae cultivation, but Ca²⁺ and Mg²⁺ in seawater is subjected to precipitate with carbonate derived from it. In this study, cultivation with this medium for a marine Chlorella sp. resulted in productivity of 0.470 g L^-1 day^-1, despite of continual precipitation caused by increased pH due to bicarbonate consumption. Actually, this precipitation is favorable, since it can work as a flocculation harvesting method for microalgae. The highest flocculation efficiency of 98.9 ± 0.0% was observed in cultures with 7.0 g L^-1 NaHCO₃, which was higher than that of cultures without bicarbonate (44.1 ± 0.2%). Additionally, the spent medium after flocculation supported better growth (1.60 ± 0.0 g L^-1) than the fresh medium (1.26 ± 0.0 g L^-1). Outdoor cultivation with floating photobioreactor on ocean resulted in the productivity of 0.190 g L^-1 day^-1, which was higher than that in land-based culture systems. The floating system also benefited from better temperature control with range from 20.6 to 37.2 °C, due to solar heating and surrounding water cooling. These results showed feasibility of efficient microalgae biomass production with fully utilizing of ocean resources, including culture medium preparation and temperature control with seawater, as well as wave energy for mixing, holding great potential to produce massive biomass to support sustainable development of human society.

The use of response surface methodology for improving fatty acid methyl ester profile of Scenedesmus vacuolatus

The present study has been designed to optimise certain important process parameters for Scenedesmus vacuolatus to achieve efficient carbon dioxide extenuation as well as suitable fatty acid profile in context to improve biodiesel properties. The effect of varying sodium bicarbonate concentration was evaluated in single and multicomponent system such as nitrate, phosphate, inoculum size to observe interactive effects on algae biomass production, carbon dioxide (CO₂) removal efficiency and fatty acid methyl ester (FAME) profile. Maximum biomass productivity of 117.0 ± 7.7 mg/L/day with 3 g/L of sodium bicarbonate was obtained i.e. approximately 2 folds higher than the control. Under multicomponent exposure, maximum biomass of 1701.5 ± 88.8 mg/L and maximum chlorophyll concentration of 15.3 ± 6.4 mg/L were achieved on 14th day at 3 g/L sodium nitrate, 0.1 g/L dipotassium hydrogen phosphate, 2 g/L of sodium bicarbonate and initial cell density of 0.3 (N₃P_0.1B₂OD_0.3). FAME content of 46.1 mg/g of biomass was obtained at this combination which is approximately 3 folds higher than the FAME content obtained under nitrogen and phosphate deprivation (16.6 mg/g at N₀P₀B₂OD_0.3). Confocal microscopy images confirmed the results with enhanced lipid droplet accumulation at high bicarbonate concentration as compared with the control. This interactive study concluded the variability in FAME profile along with the exposure to varying nutrient concentrations.

Assessing the robust growth and lipid-accumulating characteristics of Scenedesmus sp. for biodiesel production

In the present investigation, different salts of nitrogen and carbon sources were tested for their potential to boost biomass and lipid content in Scenedesmus sp. IITRIND2. Among the nitrogen sources, ammonium bicarbonate/nitrate cultures showed maximum dry cell weight (DCW) of ~ 1.8 g/L and lipid yield (~ 40%) while the addition of C6 sugars (glucose and mannose) and sodium acetate enhanced the DCW (~ 3 g/L) and lipid accumulation (~ 40%) compared with disaccharides, C4 and C5 sugars. On evaluating the synergistic effects of the nitrogen and carbon sources, maximum DCW (3.66 g/L) was obtained in ammonium bicarbonate + sodium acetate cultures with a lipid yield of 37.15%. The fatty acid profile of the derived biodiesel was similar to that of plant oils. The results clearly established the robust capability of the novel microalga to efficiently adapt, sustain, and grow in different carbon and nitrogen sources along with high lipid productivity, making it a potential source for biodiesel production.

Rapid and Positive Effect of Bicarbonate Addition on Growth and Photosynthetic Efficiency of the Green Microalgae Chlorella Sorokiniana (Chlorophyta, Trebouxiophyceae)

Bicarbonate ions are the primary source of inorganic carbon for autotrophic organisms living in aquatic environments. In the present study, we evaluated the short-term (hours) effects of sodium bicarbonate (NaHCO3) addition on the growth and photosynthetic efficiency of the green algae Chlorella sorokiniana (211/8k). Bicarbonate was added to nonaxenic cultures at concentrations of 1, 2, and 3 g L−1 leading to a significant increase in biomass especially at the highest salt concentration (3 g L−1) and also showing a bactericidal and bacteriostatic effect that helped to keep a reduced microbial load in the algal culture. Furthermore, bicarbonate stimulated the increase in cellular content of chlorophyll a, improving the photosynthetic performance of cells. Since microalgae of genus Chlorella spp. show great industrial potential for the production of biofuels, nutraceuticals, cosmetics, health, and dietary supplements and the use of bicarbonate as a source of inorganic carbon led to short-term responses in Chlorella sorokiniana, this method represents a valid alternative not only to the insufflation of carbon dioxide for the intensive cultures but also for the production of potentially bioactive compounds in a short period.

Carbon Mass Balance in Arthrospira platensis Culture with Medium Recycle and High CO2 Supply

Medium recycling combined with CO2 recovery helps sustainable use of the alkaline medium in Arthrospira culture. However, high CO2 supply may cause inorganic carbon accumulation and pH reduction, which could result in low CO2 recovery and reduced algal growth. This study aimed to elucidate the effect of medium recycling and high CO2 supply through carbon mass balance analysis in Arthrospira culture. In all CO2 supply conditions, carbon supply was higher than Arthrospira carbon assimilation, which accounted for 30–58% of supply. However, CO2 recovery of nearly 100% and 63% for lower (0.20 and 0.39 gC L−1 d−1) and higher (0.59 gC L−1 d−1) CO2 supply rates were achieved, respectively, because of the high concentration of the alkaline agent. The excess carbon accumulated in the medium and ultimately escaped from the system in a form of dissolved inorganic carbon (DIC). Dissolved organic carbon (DOC) contributed to 16–24% of the total photosynthetically assimilated carbon, and the final concentration reached 260–367 mgC L−1, but there was no significant growth reduction caused by DIC and DOC accumulation. This study demonstrated the stability of the medium-recycling process even at high CO2 supply rates although a balanced supply is recommended for longer operations.

Application of chemicals for enhancing lipid production in microalgae-a short review

Microalgae have attracted great attention as a promising sustainable resource for biofuel production. In studies aiming to improve lipid accumulation, many key enzymes involved in lipid biosynthesis were identified and confirmed, but genetic engineering remains a challenge in most species of microalgae. In an alternative approach, various chemical modulators can be used to directly regulate the lipid biosynthesis pathway, with similar effects to gene overexpression and interference approaches, including improving the precursor supply and blocking competing pathways. The produced lipid can be protected from being converted into other metabolites by the chemicals such as lipase inhibitors. In addition, a few chemicals were also demonstrated to greatly influence cell growth and lipid accumulation by indirect regulation of the lipid biosynthesis pathway, such as increasing cell permeability or regulating oxidative stress. Thus, adding chemical modulators can be a useful alternative strategy for improving lipid accumulation in large-scale cultivation of microalgae.

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.

Application of an in situ CO2-bicarbonate system under nitrogen depletion to improve photosynthetic biomass and starch production and regulate amylose accumulation in a marine green microalga Tetraselmis subcordiformis

BACKGROUND

Microalgal starch is regarded as a promising alternative to crop-based starch for biorefinery such as the production of biofuels and bio-based chemicals. The single or separate use of inorganic carbon source, e.g., CO₂ and NaHCO₃, caused aberrant pH, which restricts the biomass and starch production. The present study applied an in situ CO₂-NaHCO₃ system to regulate photosynthetic biomass and starch production along with starch quality in a marine green microalga Tetraselmis subcordiformis under nitrogen-depletion (-N) and nitrogen-limitation (±N) conditions.

RESULTS

The CO₂ (2%)-NaHCO₃ (1 g L^-1) system stabilized the pH at 7.7 in the -N cultivation, under which the optimal biomass and starch accumulation were achieved. The biomass and starch productivity under -N were improved by 2.1-fold and 1.7-fold, respectively, with 1 g L^-1 NaHCO₃ addition compared with the one without NaHCO₃ addition. NaHCO₃ addition alleviated the high-dCO₂ inhibition caused by the single CO₂ aeration, and provided sufficient effective carbon source HCO₃ ^- for the maintenance of adequate photosynthetic efficiency and increase in photoprotection to facilitate the biomass and starch production. The amylose content was also increased by 44% under this CO₂-bicarbonate system compared to the single use of CO₂. The highest starch productivity of 0.73 g L^-1 day^-1 under -N cultivation and highest starch concentration of 4.14 g L^-1 under ±N cultivation were both achieved with the addition of 1 g L^-1 NaHCO₃. These levels were comparable to or exceeded the current achievements reported in studies. The addition of 5 g L^-1 NaHCO₃ under ±N cultivation led to a production of high-amylose starch (59.3% of total starch), which could be used as a source of functional food.

CONCLUSIONS

The in situ CO₂-NaHCO₃ system significantly improved the biomass and starch production in T. subcordiformis. It could also regulate the starch quality with varied relative amylose content under different cultivation modes for diverse downstream applications that could promote the economic feasibility of microalgal starch-based biofuel production. Adoption of this system in T. subcordiformis would facilitate the CO₂ mitigation couple with its starch-based biorefinery.

Fed-batch cultivation with CO2 and monoethanolamine: Influence on Chlorella fusca LEB 111 cultivation, carbon biofixation and biomolecules production

The aim of this study was to evaluate the interaction between the periodic addition of monoethanolamine (MEA) and CO₂ during the cultivation of Chlorella fusca LEB 111. For this purpose, MEA has been added in abiotic assays, followed by fed-batch cultures with that green alga and the absorbent. BG-11 medium shown a higher potential of CO₂ absorption with MEA addition, and the bicarbonate was the chemical species of carbon prevailing in the chemical equilibrium. The periodic addition of MEA did not reduce the kinetics of growth, promoted a higher accumulation of DIC (81.4 mg L^-1) in the medium and protein (44.0% w w^-1) and lipid (30.8% w w^-1) concentrations in the biomass of C. fusca LEB 111. Therefore, it was demonstrated that fed-batch culture with MEA increased CO₂ fixation and the biomolecule synthesis as proteins and lipids.

Potential of using sodium bicarbonate as external carbon source to cultivate microalga in non-sterile condition

In this study, a saline-alkaline tolerant microalgal strain was isolated and identified as Chlorella sp. LPF. This strain was able to grow at pH values up to 10 and at salinities up to 5%, and tolerated to 80 g L^-1 of sodium bicarbonate. The utilization of bicarbonate as carbon source significantly promoted microalgal growth and lipid production. In the non-sterile cultivation supplying with 80 g L^-1 of sodium bicarbonate, the microalgal growth had no difference with their growth in the sterile medium; however, the bacterial growth was suppressed and the cell number decreased to low levels after six days cultivation. This study gives an insight into the potential that using high concentration of sodium bicarbonate as external carbon source to cultivate microalga in non-sterile condition, and suggests a possibility of using bicarbonate as growth promoter and antibacterial agent for the microalgal outdoor cultivation.

Large-scale cultivation of Spirulina in a floating horizontal photobioreactor without aeration or an agitation device

A low-cost floating photobioreactor (PBR) without the use of aeration and/or an agitation device, in which carbon was supplied in the form of bicarbonate and only wave energy was utilized for mixing, was developed in our previous study. Scaling up is a common challenge in the practical application of PBRs and has not yet been demonstrated for this new design. To fill this gap, cultivation of Spirulina platensis was conducted in this study. The results demonstrated that S. platensis had the highest productivity at 0.3 mol L^-1 sodium bicarbonate, but the highest carbon utilization (104 ± 2.6%) was obtained at 0.1 mol L^-1. Culture of Spirulina aerated with pure oxygen resulted in only minor inhibition of growth, indicating that its productivity will not be significantly reduced even if dissolved oxygen is accumulated to a high level due to intermittent mixing resulting from the use of wave energy. In cultivation using a floating horizontal photobioreactor at the 1.0 m² scale, the highest biomass concentration of 2.24 ± 0.05 g L^-1 was obtained with a culture depth of 5.0 cm and the highest biomass productivity of 18.9 g m^-2 day^-1 was obtained with a depth of 10.0 cm. This PBR was scaled up to 10 m² (1000 L) with few challenges; biomass concentration and productivity during ocean testing were little different than those at the 1.0 m² (100 L) scale. However, the larger PBR had an apparent carbon utilization efficiency of 45.0 ± 2.8%, significantly higher than the 39.4 ± 0.9% obtained at the 1 m² scale. These results verified the ease of scaling up floating horizontal photobioreactors and showed their great potential in commercial applications.

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

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

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

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

Abiotic stresses as tools for metabolites in microalgae

Microalgae, due to various environmental stresses, constantly tune their cellular mechanisms to cope with them. The accumulation of the stress metabolites is closely related to the changes occurring in their metabolic pathways. The biosynthesis of metabolites can be triggered by a number of abiotic stresses like temperature, salinity, UV- radiation and nutrient deprivation. Although, microalgae have been considered as an alternative sustainable source for nutraceutical products like pigments and omega-3 polyunsaturated fatty acids (PUFAs) to cater the requirement of emerging human population but inadequate biomass generation makes the process economically impractical. The stress metabolism for carotenoid regulation in green algae is a 2-step metabolism. There are a few major stresses which can influence the formation of phycobiliprotein in cyanobacteria. This review would primarily focus on the cellular level changes under stress conditions and their corresponding effects on lipids (including omega-3 PUFAs), pigments and polymers.

Salinity induced oxidative stress alters the physiological responses and improves the biofuel potential of green microalgae Acutodesmus dimorphus

The main aim of the present study was to analyze salinity stress induced physiological and biochemical changes in a freshwater microalgae Acutodesmus dimorphus. During single-stage cultivation, the accumulations of lipids and carbohydrates increased with an increase in an initial salinity of the culture medium. The carbohydrate and lipid accumulations of 53.30±2.76% and 33.40±2.29%, respectively, were observed in 200mM NaCl added culture. During two-stage cultivation, salinity stress of 200mM was favorable for the growth up to 2days, as suggested by higher biomass, lower levels of oxidative stress biomarkers and no significant changes in the biochemical composition of the cells. Extending the stress to 3days significantly increased the lipid accumulation by 43% without affecting the biomass production. This study, thus, provides the strategy to improve the biofuel potential of A. dimorphus along with presenting the physiological adaptive mechanisms of a cell against salinity stress.