Microalgal Cells Research Articles

An analysis of the processes, kinetics and equilibrium of iron's biosorption on immobilized green microalgae

Metals including Cu, Cd, Ni, Pb, Zn, Hg, Fe and Cr can be found in significant concentrations in acid mine drainage effluent from previous gold mining operations. Even at very low levels, heavy metals and their compounds are very toxic, carcinogenic, mutagenic and teratogenic. Acid mine drainage discharge must therefore be properly treated before disposal. In this study, iron removal from acid mine drainage effluent using immobilized green microalgae is explored. The impact of pH on iron metal concentrations was studied at multiple pH values between 2 and 12 using 0.1 M HCl or NaOH solutions. At pH 6, the optimal value, 82% and 90% of Fe (II) were removed from Ca-alginate beads and immobilized Desmodesmus sp., respectively. It was shown that Fe (II) ions were biosorbed by immobilized microalgae cells in a pseudo-second-order manner. This study demonstrated how immobilized green microalgae showed potential to remove iron from acid mine drainage effluent.

Use of airfoils for enhancement of photosynthesis rate of microalgae in raceways

The lack of adequate vertical mixing is one of the factors limiting the productivity of open raceway microalgae reactors. The existence of large gradients of light involves the cells being mainly adapted to local irradiance instead of average irradiance, which would allow for maximizing the light utilization efficiency, thus maximizing the biomass productivity of microalgae cultures. To overcome this problem, different alternatives have been proposed, one of the more suitable being the utilization of airfoils to improve vertical mixing. In this work, numerical and experimental studies were performed to analyse the effect of the aerodynamic airfoils patented by the University of Seville (WO2020120818A1). The goal is to improve the photosynthetic efficiency, but also a better understanding of the light regime to which the microalgae cells are exposed in these systems and how to improve it. Computational Fluid Dynamics (CFD) was used to optimize the flow generated by the airfoils. A dynamic photosynthesis model of Rubio Camacho et al. (Biotechnol Bioeng 81:459–473, 2003) was used to estimate the photosynthesis rate as a function of the light regime to which the cells are exposed, including photo-adaptation and photo-inhibition phenomena, the results confirm that the use of airfoils improves the vertical mixing and the photosynthesis rate. The photosynthetic benefits were observed 10 m downstream of the airfoils, resulting in an increase in photosynthesis rate and productivity by up to 30%. These results confirm the benefits of an increase in mixing in microalgae cultures, especially when focusing on the movement of the cells between the different illuminated zones while maintaining low energy consumption and capital expenses.

Insights into the Morphology and Surface Properties of Microalgae at the Nanoscale by Atomic Force Microscopy (AFM): A Review

Atomic force microscopy (AFM) is a method that provides the nanometer-resolution three-dimensional imaging of living cells in their native state in their natural physiological environment. In addition, AFM’s sensitivity to measure interaction forces in the piconewton range enables researchers to probe surface properties, such as elasticity, viscoelasticity, hydrophobicity and adhesion. Despite the growing number of applications of AFM as a method to study biological systems, AFM is not yet an established technique for studying microalgae. Following a brief introduction to the basic principles and operation modes of AFM, this review highlights the major contributions of AFM in the field of microalgae research. A pioneering AFM study on microalgae was performed on diatoms, revealing the fine structural details of diatom frustule, without the need for sample modification. While, to date, diatoms are the most studied class of microalgae using AFM, it has also been used to study microalgae belonging to other classes. Besides using AFM for the morphological characterization of microalgae at the single cell level, AFM has also been used to study the surface properties of microalgal cells, with cell elasticity being most frequently studied one. Here, we also present our preliminary results on the viscoelastic properties of microalgae cell (Dunaliella tertiolecta), as the first microrheological study of microalgae. Overall, the studies presented show that AFM, with its multiparametric characterization, alone or in combination with other complementary techniques, can address many outstanding questions in the field of microalgae.

Step-wise SO2-feeding strategies for microalgae-based CO2 fixation from flue gas and bioenergy production

Aiming at eliminating the negative effect of SO2 on microalgae CO2 fixation caused by inappropriate SO2-feeding strategies, physiology-based step-wise SO2-feeding strategies were proposed to enhance the bioenergy production of microalgae. The step-wise SO2-feeding strategy (four-step gradient increase from 100 to 400 ppm followed by four-step gradient decrease from 400 to 100 ppm) successfully realized the gratifying proliferation of microalgae by maintaining a better activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), total adenosine triphosphatase (Total ATPase), superoxide dismutase (SOD), and catalase (CAT) enzyme. Moreover, the step-wise SO2-feeding strategy, which considered the microalgal tolerance to sulfur at different growth stages, reduced the accumulation rate of toxic S-compounds in the culture medium by 47.2% and ensured a favorable solution pH of around 7 without the addition of alkaline chemical reagents. Sulfur replenishment and utilization were well-matched (SO2 → HSO3-+SO32-→SO42-→intracellular ammonia acid) under a step-wise SO2-feeding strategy, contributing to a maximum microalgal biomass concentration of 3.26 g L-1, a photosynthetic efficiency of 10.89 %, and biomass energy of 18.97 kJ, which was 85.2%, 185%, and 390% higher than the microalgae cells cultivated under 300 ppm SO2, respectively. Moreover, an increment of 81.4% in CO2 utilization efficiency was also achieved. Overall, this study provided an effective method for achieving microalgal carbon capture from industrial flue gas with high SO2 concentrations.

Manipulation of glycogen and sucrose synthesis increases photosynthetic productivity in cyanobacteria.

Photosynthetic productivity is limited by low energy conversion efficiency in naturally evolved photosynthetic organisms, via multiple mechanisms that are not fully understood. Here we show evidence that extends recent findings that cyanobacteria use "futile" cycles in the synthesis and degradation of carbon compounds to dissipate ATP. Reduction of the glycogen cycle or the sucrose cycle in the model cyanobacterium Synechocystis 6803 led to redirection of cellular energy toward faster growth under simulated outdoor light conditions in photobioreactors that was accompanied by higher energy charge [concentration ratio of ATP/(ATP + ADP)]. Such manipulation of energy metabolism may have potential in engineering microalgal chassis cells to increase productivity of biomass or target metabolites.

Microalgae from Cold Environments and Their Possible Biotechnological Applications.

Cold environments include deep ocean, alpine, and polar areas. Even if the cold conditions are harsh and extreme for certain habitats, various species have been adapted to survive in them. Microalgae are among the most abundant microbial communities which have adapted to live in low light, low temperature, and ice coverage conditions typical of cold environments by activating different stress-responsive strategies. These species have been shown to have bioactivities with possible exploitation capabilities for human applications. Even if they are less explored compared to species living in more accessible sites, various activities have been highlighted, such as antioxidant and anticancer activities. This review is focused on summarizing these bioactivities and discussing the possible exploitation of cold-adapted microalgae. Thanks to the possibility of mass cultivating algae in controlled photobioreactors, eco-sustainable exploitation is in fact possible by sampling a few microalgal cells without impacting the environment.

Reclamation of Nutrient Solution from Membrane-Based Microalgal Harvesting Processes for Cultivation of Vegetables in Hydroponic Systems

This study investigates membrane-based microalgal harvesting processes for simultaneously concentrating microalgal cells and reclaiming liquid nutrients for cultivating vegetables in hydroponic systems. During both gravity-driven (with microfiltration and ultrafiltration membranes) and crossflow (with microfiltration membranes) filtration, it was found that cake layer fouling was predominant, regardless of microalgal solution properties, membrane type, and filtration condition. Although the gravity-driven membrane process offered lower operation cost, the water productivity was limited due to its lower fluxes (<1 L/m2h). During crossflow microfiltration of the microalgal solutions, increasing periodic water (50 °C) flushing frequency helped in alleviating both irreversible fouling and cake fouling development, especially with an extending filtration cycle. Under the same water productivity condition, optimal periodic water flushing (50 °C, 5 min duration per 15 min filtration) displayed more effectiveness in cake fouling control compared to periodic ultrasonication (60 s per 15 min filtration). As the collected permeate contained phosphorus and nitrogen, it was utilized as a potential cultivation solution to grow lettuce and basil in hydroponic systems. Both plants cultivated with the permeate grew slower than those with the commercial fertilizer solution, possibly associated with the lack of some essential microelements and possible uptake of certain heavy metals. The targeted hazard quotient (THQ) values of all examined heavy metals in the edible part (stem and leaves) of basil and lettuce were 0.79 and 1.58, respectively, and comparable to those with the fertilizer solution (0.81 for basil and 1.35 for lettuce). This implies that the basil cultivated with the reclaimed microalgal nutrient solution did not pose a potential adverse health effect on human consumption.

Microalgae fractionation and pyrolysis of extracted microalgae biopolymers

In order to maximize the economic return from microalgae production, the feasibility of thermochemically converting microalgae by-products which have had one or more biochemical components extracted (lipids and/or protein) needs to be considered. The extracted microalgae are waste stemming from microalgae biorefineries where biodiesel and/or valuable proteins/peptides have been produced. The biorefinery scheme of maximized microalgae utilization based on isolation of lipids or lipids and proteins combined, and the thermochemical conversion of the remaining fractions are discussed in this study. The biopolymer components of Nannochloropsis gaditana were extracted to form a lipid fraction, a protein fraction and a carbohydrate-rich residue. The lipid extraction efficiency reached 76.9 wt.%, however, the efficiency of protein extraction was 42.2 wt.% due to the inability of the extraction process to remove all protein. The surface functional groups and thermochemical characteristics of extracted components were investigated by FTIR and py-GC/MS, respectively. The FTIR results suggested that the extraction process disrupted the microalgae cell walls and released intracellular components. Lipids are recommended to be extracted for biofuel production by transesterification. For specific high protein-containing microalgae, protein extraction has to be considered to obtain high value proteins and derived chemicals. After the extraction of high value products, the carbohydrate-rich residue which is considered of low value, can be used as a fast pyrolysis feedstock instead of the whole microalgae. The pyrolysis chemical pathways as a function of temperature were schematized in order to better understand the reaction mechanisms in microalgae pyrolysis. The formation of alkanes and alkenes at higher temperatures were stemming from the long-chain alcohols and carboxylic acids. The formation pathways of N-heterocyclic compounds include Maillard reactions, cracking of amino acids and the cyclization of amines and amides.

Toward a better understanding of microalgal photosynthesis in medium polluted with microplastics: a study of the radiative properties of microplastic particles.

Due to the wide presence of microplastics in water, the interaction between microplastic particles and microalgae cells in medium merits the attention of researchers. Microplastic particles can impact the original transmission of light radiation in water bodies since the refractive index of microplastics is different from that of water bodies. Accordingly, the accumulation of microplastics in water bodies will certainly impact microalgal photosynthesis. Therefore, experimental measurements and theoretical studies characterizing the radiative properties of the interaction between light and microplastic particles are highly significant. The extinction and absorption coefficient/cross-section of polyethylene terephthalate and polypropylene were experimentally measured using transmission and integrating methods in the spectral range of 200-1,100nm. The absorption cross-section of PET shows remarkable absorption peaks in the vicinity of 326nm, 700nm, 711nm, 767nm, 823nm, 913nm, and 1,046nm. The absorption cross-section of PP has distinctive absorption peaks near 334nm, 703nm, and 1,016nm. The measured scattering albedo of the microplastic particles is above 0.7, indicating that both microplastics are scattering dominant media. Based on the results of this work, an in-depth understanding of the interaction between microalgal photosynthesis and microplastic particles in the medium will be obtained.

Revealing mechanism and influence of microalgae cells' periodical auto-agglomeration induced by high concentration of carbon dioxide

The efficient cultivation of microalgae using CO2 from flue gas can be a win-win situation for both environmental protection and energy accessibility. In general, 10–20% of CO2 in flue gas would decrease pH and inhibit microalgae growth. However, Chlorella sorokiniana MB-1 under 15% CO2 showed a periodical auto-agglomeration, which promoted microalgae growth on the contrary in this study. The maximum biomass concentration of 3.27 g L−1 was higher than that cultivated with an optimal CO2 concentration. The pH decreased to 6.04 after the mixed gas with 15% CO2 (v/v) was bubbled into medium for 0.5 h, which resulted in auto-agglomeration to protect microalgae from acidification and keep a high specific growth rate of 0.03 h−1. Then the pH recovered to 7 during stabilization phase, auto-agglomeration ratio was up to 100% because of lamellar extracellular polymeric substances. Therefore, the interesting periodical agglomeration both enhanced growth and simplified harvesting.

The effect of low temperature and nitrogen starvation on the morphological and physiological characteristics of two strains of green microalgae of the genus &lt;i&gt;Lobosphaera&lt;/i&gt; sp.

The effect of nitrogen starvation and, for the first time, low temperature, as well as their simultaneouseffect, on the physiology and ultrastructure of cells of microalgae of the genus Lobosphaera (Chlorophyta, Trebouxiophyceae) was studied. Nitrogen deficiency in both strains led to a decrease in the content ofchlorophyll by three times and an increase in the proportion of carotenoids by two times. A decrease in thecontent of both chlorophyll and carotenoids was observed at +10C. The simultaneous effect of two factorsresulted in a threefold decrease in the chlorophyll content in NAMSU 924/2 and a sixfold decrease inNAMSU (CALU) 1497; the proportion of carotenoids in both strains decreased by 1.52 times. Data onultrastructural changes in cells of microalgae of the genus Lobosphaera under the influence of stress factorshave been obtained. A similar nature of the response in both strains to stress conditions was noted. Nitrogendeficiency led to the accumulation of numerous lipid droplets in the cytoplasm of cells along the cell wall. Long-term incubation on a nitrogen-free medium led to the filling of the entire volume of cells with lipiddroplets, disassembly of the membrane system of chloroplasts, that reduction in sizeand being locatedbetween densely lying lipid droplets. At low temperatures, the number of thylakoids decreased, while theinterthylakoid space and the size of chloroplasts increased. With simultaneous exposure to nitrogen starvation and low temperature, numerous lipid droplets accumulated, the number of thylakoids decreased, the interthylakoid space and the size of the chloroplast increased, which was noted under separate exposure to stress factors. The pyrenoid in both strains did not undergo significant changes in all cases.

Kinetic and thermodynamic evaluations of oil extraction from immobilized Chlorella vulgaris using different solvents

Investigating the effect of different solvents used for microalgae oil extraction on the determinations of kinetic values and thermodynamic parameters is a compelling novel approach for the current study. The primary concern of microalgae oil extraction is selecting a solvent system because it has a massive influence on the extraction yield, composition, and oil quality. Three different solvent systems were inspected to determine the suitable solvent for the oil extraction from immobilized Chlorella vulgaris: heptane, heptane:methanol, and heptane:ethanol. The research methodology begins with the immobilization of microalgae cells. The matured immobilized beads were then harvested, and dried biomass was recovered. The oil was extracted by solvent extraction method using the solvents mentioned. Finally, the extracted oil was trans-esterified and analyzed for fatty acid methyl ester profile. Based on the results calculated, heptane is determined to extract the most oil with 27.42 %, followed by H:M and H:E at the highest temperature and longest running time. When the polar solvent is mixed with a non-polar solvent, the mixture has the least kinetic value and immense activation energy. However, the activation energy for heptane is only 89.06 kJ/mol. Another essential objective of this research is to determine the effects of solvents on the thermodynamics parameters. The extraction process of microalgae oil using heptane alone is determined to have a ΔH value of 0.111 kJ/mol, followed by 0.201 and 0.102 kJ/mol for solvent system H:M and H:E respectively. All the values obtained are positive, thus making the extraction processes using various solvents to be endothermic. The positive values of ΔS and the negative values of ΔG indicated that the oil extraction using heptane is thermodynamically spontaneous and irreversible, thus making it economically feasible. The fatty acids methyl esters such as palmitic, stearic, oleic, linoleic, and linolenic were the prime constituents quantified in the oil extracted using heptane compared to other solvent systems. Therefore, with the larger and increasing kinetic values and lowest activation energy, it can be concluded that heptane is the most suitable solvent for the oil extraction of immobilized Chlorella vulgaris compared to other solvent systems employed in the current research work.

Application of Sequencing Batch Biofilm Reactor (SBBR) Using Microalgae Chorella sp. to Removal Nutrient in Grey Water

Grey water contains organic matter that is directly disposed to the environment without any treatment previously, will cause pollution and impacting life in the water. Treatment that can be done is using microorganisms. One of its kind is the microalgae Chlorella sp. which utilizes organic matter as a source of nutrients for its growth. In this study, the Kaldness 1 (K1) bio carrier was added as a medium for attaching microorganisms using the Sequencing Batch Biofilm Reactor (SBBR) process. The research objectives were (1) to know the maximum number of Chlorella sp. both attached and suspended in the Sequencing Batch Biofilm Reactor (SBBR), (2) to obtain the best cycle time and stabilization time in the removal of COD, Ammonia, and MLSS in grey water. The research was conducted by varying the stabilization time (1.5; 2 and 2.5 hours) in each cycle for four cycles with a constant variation of charging time 30 minutes, reaction 120 minutes, 45 minutes, separation 45 minutes, and carried out with four cycles, stirring speed at 60 rpm, the concentration of algae suspension in SBBR was 25% and the volume of Kaldness K1 medium was 20%. The results showed the number of microalgae cells Chlorella sp. was suspended and attached to 1.85 x 106 and 1.46 x 106 cells/ml. The best removal of COD, ammonia, and MLSS was found in the stabilization time variation of 1.5 hours in 4 cycles with a removal efficiency of 84% and 76%, respectively, and an increase in the concentration of suspended and attached MLSS by 4780 mg/l and 4720 mg/l. It can be concluded that the faster stabilization time, the more removal efficient will be.

Nutrient consumption of green microalgae, Chlorella sp. during the bioremediation of shrimp aquaculture wastewater

Aquaculture products are among the biggest contributor to food supplies to meet the global food demands of the growing population over these past few years. For aquaculture to continue developing, an effective wastewater treatment is required to lessen the environmental effects. This study examined the potential of Chlorella sp. to reduce nutrients in shrimp aquaculture wastewater and correlate with the growth kinetics of the algae during the bioremediation process. Six different Chlorella sp. inoculation dosages ranging from 0 to 60 % (v/v) were used in this study. Marine water wastewater (MW) and Freshwater wastewater (FW) where the two types of shrimp wastewater were employed. Results indicated that the 30 % (v/v) and 40 % (v/v) were the optimum dosage for MW and FW. During the treatment, microalgae cell density increased more than tenfold compared to the initial value. Moreover, batch culture resulted in the specific growth rate concentration of 0.18 k day−1 and 0.15 k day−1, respectively. Those dosage also resulting the highest removal efficiencies with removal of ammonia, nitrite and orthophosphate of 96.77 %, 82.07 %, 75.96 % and 90.10 %, 87.09 %, 95.60 %, respectively. The application of FTIR spectroscopy was employed in this study to analyze the functional group in the microalgae biomass. The results of the scanning electron microscopy (SEM) and Energy Dispersive Spectroscopy Analysis (EDS) also included to further illustrate how microalgae biomass was affected by the treatment in this study. Therefore, the research from this study could be used in design novel microalgae treatments that offer a thorough and environmentally beneficial method of treating shrimp aquaculture wastewater.

Microplastics leachate may play a more important role than microplastics in inhibiting microalga Chlorella vulgaris growth at cellular and molecular levels

The leaching of microplastics (MPs) additives and their negative effects on aquatic organisms remain to be systematically elucidated. In this study, the toxicological effects of MPs leachate (micro-sized polyethylene (mPE) and micro-sized polyvinyl chloride (mPVC) acceleratedly leached by UVA for 15, 90, and 180 days in seawater) on microalga Chlorella vulgaris in terms of cell growth inhibition, oxidative stress, and transcriptomes were investigated. The leachate components of MPs aged for 90 days were further identified to elucidate the corresponding toxicity mechanisms of MPs on microalgal cells. The results revealed that both leachates of mPE and mPVC inhibited cell growth and increased oxidative stress in C. vulgaris, accompanied by a growth inhibition rate to microalgal cells of 4.0%–36.2% and 7.1%–48.2%, respectively. At the same mass concentration, the toxicological effects on C. vulgaris followed the order of mPVC leachate > mPE > mPE leachate > mPVC, whereas MPs leaching time indicated no change in MPs leaching toxicity. Furthermore, the gene functions of “translation, ribosomal structure and biogenesis” were mostly affected by MPs leachate. Compared to mPE leachate and pure MPs, the stronger inhibitory effects of mPVC leachate on microalgal cells may be attributed to the fact that more substances were leached from the polymer of mPVC, including Zn, farnesol isomer a, 2,6-di-tert-butyl-4-methylphenol, and acetyl castor oil methyl ester. In summary, this study provides a better understanding of the ecotoxicological influences of MPs and MPs leachate, and offers a warning on the ecological risk caused by plastic additives.

Combining AFM, XPS and chemical hydrolysis to understand the complexity and dynamics of C. vulgaris cell wall composition and architecture

The microalgae cell wall represents its interface with its environment, and is a dynamic and complex structure that evolves depending on the growth phase or culture conditions. To apprehend this complexity, an experimental approach combining AFM, XPS, and chemical hydrolysis followed by HPAEC-PAD was developed to understand the cell wall of Chlorella vulgaris, a biotechnologically-relevant green microalgae species. Exponential and stationary growth stages were investigated, as well as saline stress condition inducing lipid production. Results showed that both the cell wall composition and architecture changes in stationary phase, with an increase of the lipidic fraction at the expense of the proteic fraction, changes in the polysaccharidic composition, and a significant increase of its rigidity. Under saline stress, cell wall architecture seems to be affected as importantly, its rigidity is reduced. Altogether, this study demonstrates the power of combining these three techniques to give new insights into C. vulgaris cell wall, in terms of composition and architecture, and of its dynamics in different conditions.

The effects of CdSe/ZnS quantum dots on the photosynthesis rate of the Chlorella Vulgaris beads

Photosynthesizing microalgae produce >50 % of oxygen in the atmosphere and are crucial for the survival of many living systems such as coral reefs. To address the declining of coral reefs, artificial reefs have been introduced to encapsulate the microalgae cells in a polymer matrix but the effects of nanoscale pollutants on these engineered systems have not been fully understood. In this work, quantum dots with a size smaller than 10 nm are being used to elucidate the photosynthesis performance of the sodium alginate beads encapsulated with Chlorella vulgaris (C. vulgaris). The fluorescent quantum dots can move into the alginate matrix and the fluorescence intensity in the algae beads is correlated with the quantum dot concentration. We further show that the photosynthesis of the algae beads is sensitive to the quantum dot concentration and are also time sensitive. In the first 48 min of quantum dot exposure, both carbon dioxide absorption and oxygen production are low, suggesting limited photosynthesis. After the initial incubation, the photosynthesis rate quickly increases even though more inhibition is still observed with higher concentration of the quantum dots. The measured electron transport rate shows a similar trend and is also sensitive to the quantum dot concentration.

Effects of Light on the Growth and β-carotene Accumulation in the Green Algae Dunaliella salina

The marine microalgae Dunaliella salina holds significant economic value due to its rich content of β-carotene, a natural pigment with high antioxidant capacity, immune-stimulating properties, and a crucial role in antioxidant reactions with vitamins C and E. This study evaluated the effects of light factors on the growth and biosynthesis of β-carotene in the Dunaliella salina strain isolated from coastal waters in Central Vietnam. The microalgae strain was cultured in the static f/2 medium at 25°C, under different light spectra (blue, red, and white) and light intensities (13.5, 27.0, and 40.5 µmol photon/m2/s), with a similar light/dark cycle of 16/8. The results indicated that the microalgae strain showed the highest growth rate and production yield of β-carotene under the red LED light, whereas the highest β-carotene accumulation in each microalgal cell was obtained under the blue LED light. Regarding light intensity, the best growth and β-carotene production yield was observed under the highest experimental light intensity of 40.5 µmol photon/m2/s, but the highest β-carotene content per algal cell was reached under the lowest intensity (13.5 µmol photon/m2/s). These findings provide important scientific data for further research to identify the optimal conditions for increasing biomass and stimulating the accumulation of valuable secondary compounds of the microalgae Dunaliella salina in Vietnam.

Label-free and noninvasive analysis of microorganism surface epistructures at the single-cell level

Cell surface properties of microorganisms provide abundant information for their physiological status and fate choice. However, current methods for analyzing cell surface properties require labeling or fixation, which can alter the cell activity. This study establishes a label-free, rapid, noninvasive, and quantitative analysis of cell surface properties, including the presence and the dimension of epistructure, down to the single-cell level and at the nanometer scale. Simultaneously, electrorotation provides dielectric properties of intracellular contents. With the combined information, the growth phase of microalgae cells can be identified. The measurement is based on electrorotation of single cells, and an electrorotation model accounting for the surface properties is developed to properly interpret experimental data. The epistructure length measured by electrorotation is validated by scanning electron microscopy. The measurement accuracy is satisfactory in particular in the case of microscale epistructures in the exponential phase and nanoscale epistructures in the stationary phase. However, the measurement accuracy for nanoscale epistructures on cells in the exponential phase is offset by the effect of a thick double layer. Lastly, a diversity in epistructure length distinguishes exponential phase from stationary phase.

Reviewing biohydrogen production from microalgal cells through fundamental mechanisms, enzymes and factors that engendering new challenges and prospects

Hydrogen is the most promising alternative energy recently that is renewable and clean due to its sustainability, great potential as energy source and efficiency during the conversion. In fact, microalgal biohydrogen has attracted significant attentions as a clean and sustainable fuel to achieve the carbon neutrality and sustainability in environment. In this review, the mechanisms of biological methods and electrochemical methods, and the enzymes involved in the biohydrogen generation from microalgae have been inclusively and rudimentary detailed. The review also summarizes the benefits and drawbacks of each technique, and compares the biohydrogen yields from each method based on the results reported by the recent studies. Furthermore, the key factors that influence the biohydrogen yields including pH, light intensity, cell density, temperature and substrate introduced have been addressed as well in this review. Moreover, the review also highlights the primary shortcomings encountered which are oxygen sensitivity of enzymes and low biohydrogen yield. Lastly, the resolutions to enhance the biohydrogen production together with the future prospects for sustainable biohydrogen production from microalgae are opined systematically.