Photosynthetic activity assessment in mixotrophically cultured Chlorella vulgaris biofilms at various developmental stages

Pilot-scale microalgae cultivation and wastewater treatment using high-rate ponds: a meta-analysis

Microalgae cultivation in wastewater has been widely researched under laboratory conditions as per its potential to couple treatment with biomass production. Currently, only a limited number of published articles consider outdoor and long-term microalgae-bacteria cultivations in real wastewater environmental systems. The scope of this work is to describe microalgal cultivation steps towards high-rate algal pond (HRAP) scalability and identify key parameters that play a major role for biomass productivity under outdoor conditions and long-term cultivations. Reviewed pilot-scale HRAP literature is analysed using multivariate analysis to highlight key productivity parameters within environmental and operational factors. Wastewater treatment analysis indicated that HRAP can effectively remove 90% of NH4+, 70% of COD, and 50% of PO43-. Mean reference values of 210 W m-2 for irradiation, 18 °C for temperature, pH of 8.2, and HRT of 7.7 are derived from pilot-scale cultivations. Microalgae biomass productivity at a large scale is governed by solar radiation and NH4+ concentration, which are more important than retention time variations within investigated studies. Hence, selecting the correct type of location and a minimum of 70 mg L-1 of NH4+ in wastewater will have the greatest effect in microalgae productivity. A high nutrient wastewater content increases final biomass concentrations but not necessarily biomass productivity. Pilot-scale growth rates (~ 0.54 day-1) are half those observed in lab experiments, indicating a scaling-up bottleneck. Microalgae cultivation in wastewater enables a circular bioeconomy framework by unlocking microalgal biomass for the delivery of an array of products.

Effect of light intensity on nitrogen transformation, enzymatic activity, antioxidant system and transcriptional response of Chlorella pyrenoidosa during treating mariculture wastewater

The nitrogen transformation, enzymatic activity, antioxidant ability and transcriptional response of Chlorella pyrenoidosa (C. pyrenoidosa) treating mariculture wastewater were compared under different light intensities. The microalgal growth, chlorophyll synthesis and nitrogen removal ability of C. pyrenoidosa increased with the light intensity from 3000 to 7000 Lux, whereas they slightly decreased under 9000 and 11,000 Lux. The nitrogen metabolism enzymatic activities displayed obvious differences under different light intensities and affected the nitrogen transformation process. The reactive oxygen species (ROS) production increased with the increase of operational time, whereas it had distinct differences under different light intensities. The changes of antioxidant enzymatic activities were positively correlated with the ROS production. The transcriptional response of C. pyrenoidosa was in accordance with the variation of the photosynthesis, nitrogen assimilation and antioxidant system under different light intensities. This study provides theoretical basis and technical support to select suitable light intensity for algae treating mariculture wastewater.

A Response Surface Methodology Study for Chlorella vulgaris Mixotrophic Culture Optimization

Glycerol is a carbon source that produces good biomass under mixotrophic conditions. Enhancing the composition of culture media in algae biomass production improves growth rates, biomass yield, nutrient utilization efficiency, and overall cost-effectiveness. Among the key nutrients in the medium, nitrogen plays a pivotal role. Urea can be effectively used as a nitrogen source and is considered a low-cost form of nitrogen compared to other sources. Urea metabolism releases some CO2 in photosynthesis, and magnesium plays a major role in urea uptake. Magnesium is another key nutrient that is key in photosynthesis and other metabolic reactions. To maximize glycerol consumption in the mixotrophic system and to obtain high biomass and lipid productions, the variations in MgSO4·7H2O and urea concentrations were evaluated in the growth medium of the microalgae. A response surface methodology (RSM) using a central composite design (CCD) was designed to maximize glycerol consumption at the initial cellular growth rates (up to four days). The magnesium and urea supply varied from 0.3 to 1.7 g L-1. Response surface methodology was utilized to analyze the results, and the highest glycerol consumption rate, 770.2 mg L-1 d-1, was observed when C. vulgaris was grown at 1.7 g L-1 urea, 1.0 g L-1 MgSO4·7H2O. Using the optimal urea and magnesium concentrations with acetate, glucose, and glycerol as carbon sources, the same lipid content (10% average) was achieved on day 4 of mixotrophic C. vulgaris culture. Overall, the results show that mixotrophic growth of C. vulgaris using urea with an optimum magnesium concentration yields large amounts of fatty acids and that the carbon source greatly influences the profile of the fatty acids.

Identification of Metabolites with Antibacterial Activities by Analyzing the FTIR Spectra of Microalgae

Biologically active substances from microalgae can exhibit antioxidant, immunostimulating, antibacterial, antiviral, antitumor, antihypertensive, regenerative, and neuroprotective effects. Lipid complexes of microalgae Chlorella vulgaris and Arthrospira platensis exhibit antibacterial activity and inhibit the growth of the Gram-positive strain Bacillus subtilis; the maximum zone of inhibition is 0.7 ± 0.03 cm at all concentrations. The carbohydrate-containing complex of C. vulgaris exhibits antibacterial activity, inhibits the growth of the Gram-positive strain B. subtilis, Bacillus pumilus; the maximum zone of inhibition is 3.5 ± 0.17 cm at all concentrations considered. The carbohydrate complex of A. platensis has antimicrobial activity against the Gram-negative strain of Escherichia coli at all concentrations, and the zone of inhibition is 2.0–3.0 cm. The presence of mythelenic, carbonyl groups, ester bonds between fatty acids and glycerol in lipid molecules, the stretching vibration of the phosphate group PO₂, neutral lipids, glyco- and phospholipids, and unsaturated fatty acids, such as γ-linolenic, was revealed using FTIR spectra. Spectral peaks characteristic of saccharides were found, and there were cellulose and starch absorption bands, pyranose rings, and phenolic compounds. Both algae in this study had phenolic and alcohol components, which had high antibacterial activity. Microalgae can be used as biologically active food additives and/or as an alternative to antibiotic feed in animal husbandry due to their antibacterial properties.

Effect of digestate loading rates on microalgae-based treatment under low LED light intensity

Low red-LED irradiances are an attractive alternative for enhancing microalgae photobioreactors treating digestate due to their potential contribution in decreasing area footprints with low energy consumptions. However, more information is required regarding the influence of digestate load on treatment performance and biomass valorisation when low-intensity red-LEDs are applied. Thus, this study assessed microalgae-based photobioreactors treating food waste digestate under different concentrations (5%, 25%, 50%, and 75%, v/v) at low red-LED irradiance (15 µmol·m^-2·s^-1). The removal efficiencies of soluble chemical oxygen demand (sCOD) at the end of the experiment ranged from 45% to 75% when treating influent loads between 5.3 and 79.1 g sCOD·m^-3·d^-1 (5% and 75%-digestate), respectively. Total ammonia nitrogen (TAN) was applied in loading rates between 3.2 and 48.5 g TAN·m^-3·d^-1 (5% and 75%, respectively) and removed with maximum efficiencies of 90%-100% in all trials. Nitrification-denitrification was proportionally more relevant when treating 5%-digestate, whereas volatilisation was the primary process in 25%, 50% and 75% concentrations. Microalgae presented adequate yields in all treatments, except in 75%-digestate, likely due to the blocking of light by the high solids concentrations. The assessment of the microalgae community and chlorophyll-a and carotenoids suggested that chlorophytes, mainly Dictyosphaerium pulchellum and Scenedesmus sp. grew autotrophically, whereas cyanobacteria Pseudanabaena sp. grew mixotrophically. Moreover, the sustainability of red LED lighting applications can be increased by anaerobic digestion or agricultural valorisation of the biomass, enabled by its high N and P contents. Low-intensity red-LEDs may have promissory applications in the treatment of high-strength wastewaters.

The architecture and metabolic traits of monospecific photosynthetic biofilms studied in a custom flow-through system

Microalgae biofilms have great ecological importance and high biotechnological potential. Nevertheless, an in-depth and combined structural (i.e., the architecture of the biofilm) and physiological characterization of microalgae biofilms is still missing. An approach able to provide the same time physiological and structural information during biofilm growth would be of paramount importance to understand these complex biological systems and to optimize their productivity. In this study, monospecific biofilms of a diatom and a green alga were grown under dynamic conditions in custom flow cells represented by UV/Vis spectroscopic cuvettes. Such flow cells were conceived to characterize the biofilms by several techniques mostly in situ and in a nondestructive way. Physiological traits were obtained by measuring variable chlorophyll a fluorescence by pulse amplitude modulated fluorometry and by scanning the biofilms in a spectrometer to obtain in vivo pigments spectral signatures. The architectural features were obtained by imaging the biofilms with a confocal laser scanning microscopy and an optical coherence tomography. Overall, this experimental setup allowed us to follow the growth of two biofilm-forming microalgae showing that cell physiology is more affected in complex biofilms likely as a consequence of alterations in local environmental conditions.

Detection and Characterization of Bacterial Biofilms and Biofilm-Based Sensors

Microbial biofilms have caused serious concerns in healthcare, medical, and food industries because of their intrinsic resistance against conventional antibiotics and cleaning procedures and their capability to firmly adhere on surfaces for persistent contamination. These global issues strongly motivate researchers to develop novel methodologies to investigate the kinetics underlying biofilm formation, to understand the response of the biofilm with different chemical and physical treatments, and to identify biofilm-specific drugs with high-throughput screenings. Meanwhile microbial biofilms can also be utilized positively as sensing elements in cell-based sensors due to their strong adhesion on surfaces. In this perspective, we provide an overview on the connections between sensing and microbial biofilms, focusing on tools used to investigate biofilm properties, kinetics, and their response to chemicals or physical agents, and biofilm-based sensors, a type of biosensor using the bacterial biofilm as a biorecognition element to capture the presence of the target of interest by measuring the metabolic activity of the immobilized microbial cells. Finally we discuss possible new research directions for the development of robust and rapid biofilm related sensors with high temporal and spatial resolutions, pertinent to a wide range of applications.

Mutual impacts and interactions of antibiotic resistance genes, microcystin synthetase genes, graphene oxide, and Microcystis aeruginosa in synthetic wastewater

The physiological impacts and interactions of antibiotic resistance gene (ARG) abundance, microcystin synthetase gene expression, graphene oxide (GO), and Microcystis aeruginosa in synthetic wastewater were investigated. The results demonstrated that the absolute abundance of sul1, sul2, tetW, and tetM in synthetic wastewater dramatically increased to 365.2%, 427.1%, 375.2%, and 231.7%, respectively, when the GO concentration was 0.01 mg/L. Even more interesting is that the sum gene copy numbers of mcyA-J also increased to 243.2%. The appearance of GO made the significant correlation exist between ARGs abundance and mcyA-J expression. Furthermore, M. aeruginosa displayed better photosynthetic performance and more MCs production at 0.01 mg/L GO. There were 65 pairs of positive correlations between the intracellular differential metabolites of M. aeruginosa and the abundance of sul1, sul2, tetM, and tetW with various GO concentrations. The GO will impact the metabolites and metabolic pathway in M. aeruginosa. The metabolic changes impacted the ARGs, microcystin synthetase genes, and physiological characters in algal cells. Furthermore, there were complex correlations among sul1, sul2, tetM, tetW, mcyA-J, MCs, photosynthetic performance parameters, and ROS. The different concentration of GO will aggravate the hazards of M. aeruginosa by promoting the expression of mcyA-J, producing more MCs; simultaneously, it may cause the spread of ARGs.

Production, Purification, and Study of the Amino Acid Composition of Microalgae Proteins

Microalgae are known to be rich in protein. In this study, we aim to investigate methods of producing and purifying proteins of 98 microalgae including Chlorella vulgaris, Arthrospira platensis, Nostoc sp., Dunaliella salina, and Pleurochrysis carterae (Baltic Sea). Therefore, we studied their amino acid composition and developed a two-stage protein concentrate purification method from the microalgae biomass. After an additional stage of purification, the mass fraction of protein substances with a molecular weight greater than 50 kDa in the protein concentrate isolated from the biomass of the microalga Dunaliella salina increased by 2.58 times as compared with the mass fraction before filtration. In the protein concentrate isolated from the biomass of the microalga Pleurochrysis cartera, the relative content of the fraction with a molecular weight greater than 50.0 kDa reached 82.4%, which was 2.43 times higher than the relative content of the same fractions in the protein concentrate isolated from this culture before the two-stage purification. The possibilities of large-scale industrial production of microalgae biomass and an expanded range of uses determine the need to search for highly productive protein strains of microalgae and to optimize the conditions for isolating amino acids from them.

Effects of stepwise changes in dissolved carbon dioxide concentrations on metabolic activity in Chlorella for spaceflight applications

This paper assesses the impacts to the growth rate, health, oxygen production, and carbon dioxide fixation and nitrogen assimilation of Chlorella vulgaris while sparging the culture with various influent concentrations of carbon dioxide. Selected concentrations reflect a cabin environment with one crew member (0.12% v/v) and four crew members (0.45% v/v). Stepwise, sustained changes in influent carbon dioxide concentration on day four of the eight-day experiments simulated a dynamic crew size, reflective of a planetary surface mission. Control experiments used constant influent concentrations across eight days. Significant changes in growth rate (0.12%-to-0.45%: 57% increase; 0.45%-to-0.12%: 59% reduction) suggest a positive correlation between metabolic activity of C. vulgaris and environmental carbon dioxide concentration. Statistical tests illustrate that algae are more sensitive to reductions in influent carbon dioxide. No specific correlation of the nitrogen assimilation rate to influent carbon dioxide, suggesting a nitrogen-limited or irradiance-limited system. Photosynthetic yield results (0.59-0.72) indicate that the culture was minimally stressed in all tested conditions. This paper compares these results to findings of published, steady-state experiments conducted under similar carbon dioxide environments. The findings presented here imply that a sufficient volume of C. vulgaris, with nutrient supplementation or biomass harvesting, could support the respiratory requirements of a long duration human mission with a dynamic cabin environment and these data can be used in future dynamic models.

Insight into nitrogen and phosphorus coupling effects on mixotrophic Chlorella vulgaris growth under stably controlled nutrient conditions

In water environment, nitrogen (N) and phosphorus (P) are biochemically dependent nutrients following the co-limitation concept for algae growth under mixotrophic mode. From a practical viewpoint, algae growth may not bring about significant change of the background nutrient concentration of an actual waterbody in contrast to a conventional batch system. In order to better understand the growth pattern of microalgae in aquatic environments, a series of experiments were conducted under stably controlled N-P levels for studying the N-P coupling effect on mixotrophic Chlorella vulgaris growth process, with attention paid to the physiological and biochemical characteristics. It was found that within the concentration range of N = 1-8 mg·L^-1 and P = 0.1-1.0 mg·L^-1, the variation of the N-P level slightly affected the specific growth rate, but significantly influenced nutrients uptake, biomass dry weight, chlorophyll contents of the grown C. vulgaris. The biochemical and elemental composition of the microalgae tended to be more sensitive to the N-P concentrations and ratios in the lower nutrient range (1-2 mg N·L^-1, 0.1-0.4 mg P·L^-1) in which the highest N and P conversion rates were gained as 90.18 ± 1.23% and 60.47 ± 1.59%, respectively. The P assimilation and conversion efficiencies were much affected by both N and P supplies, while the P supply showed little influence on N assimilation and conversion efficiencies. It was also noticed that the N level greatly affected the metabolic pathway involving nutrient assimilation, carbohydrate fixation and monosaccharide profile, resulting in conversion of the dominant fraction of protein at N ≤ 2 mg·L^-1 into other biochemical compositions including lipids at N ≥ 3 mg·L^-1. The fatty acid methyl esters (FAMEs) composition tended to differ with varied nutrient levels. These findings may deepen our understanding of algal growth in aquatic environment and provide perspective for eutrophication control.

Effects of pyrolysis temperature on soil-plant-microbe responses to Solidago canadensis L.-derived biochar in coastal saline-alkali soil

Because salinity of coastal soils is drastically increasing, the application of biochars to saline-alkali soil amendments has attracted considerable attention. Various Solidago-canadensis-L.-derived biochars prepared through pyrolysis from 400 to 600 °C were applied to coastal saline-alkali soil samples to optimise the biochar pyrolysis temperature and investigate its actual ecological responses. All biochars reduced the soil bulk density and exchangeable sodium stress and increased soil water-holding capacity, cation exchange capacity, and organic matter content. Principal-component-analysis results showed that pyrolysis temperature played an important role in the potential application of biochars to improve the coastal saline-alkali soil, mainly contributed to ameliorating exchangeable sodium stress and decreasing biochar-soluble toxic compounds. Furthermore, soil bulk density and organic matter, as well as carboxylic acids, phenolic acids and amines of biochar were major driving factors for bacterial community composition. Compared to low-temperature biochar (pyrolyzed below 550 °C), which showed higher toxicity for Brassica chinensis L. growth due to the higher content of carboxylic acids, phenols and amines, high-temperature biochar (pyrolyzed at or above 550 °C) possessed less amounts of these toxic functional groups, more beneficial soil bacteria and healthier for plant growth. Therefore, high-temperature biochar could be applied as an effective soil amendment to ameliorate the coastal saline-alkali soil with acceptable environmental risk.

Microalgae: A Promising Source of Valuable Bioproducts

Microalgae are a group of autotrophic microorganisms that live in marine, freshwater and soil ecosystems and produce organic substances in the process of photosynthesis. Due to their high metabolic flexibility, adaptation to various cultivation conditions as well as the possibility of rapid growth, the number of studies on their use as a source of biologically valuable products is growing rapidly. Currently, integrated technologies for the cultivation of microalgae aiming to isolate various biologically active substances from biomass to increase the profitability of algae production are being sought. To implement this kind of development, the high productivity of industrial cultivation systems must be accompanied by the ability to control the biosynthesis of biologically valuable compounds in conditions of intensive culture growth. The review considers the main factors (temperature, pH, component composition, etc.) that affect the biomass growth process and the biologically active substance synthesis in microalgae. The advantages and disadvantages of existing cultivation methods are outlined. An analysis of various methods for the isolation and overproduction of the main biologically active substances of microalgae (proteins, lipids, polysaccharides, pigments and vitamins) is presented and new technologies and approaches aimed at using microalgae as promising ingredients in value-added products are considered.

Photosynthetic toxicity of non-steroidal anti-inflammatory drugs (NSAIDs) on green algae Scenedesmus obliquus

The widespread use of pharmaceuticals and personal care products (PPCPs) has raised serious concerns regarding their potential ecotoxicological effects. We examined the photosynthetic toxicity of four non-steroidal anti-inflammatory drugs (NSAIDs), i.e. ibuprofen (rac-IBU and S-(+)-IBU), aspirin (ASA) and ketoprofen (KEP) on the green alga Scenedesmus obliquus. Our results showed that NSAIDs exerted inhibitory effects on algal growth; the IC_50-24h of S-(+)-IBU, rac-IBU, ASA, and KEP was 123.29, 107.91, 103.05, and 4.03 mg/L, respectively. KEP was the most toxic, ASA was slightly more toxic than rac-IBU, and S-(+)-IBU was the least toxic. NSAIDs adversely affected the cellular ultrastructure, as evident from plasmolysis, chloroplast deformation and disintegration. NSAID treatments decreased the chlorophyll and carotenoid content, and chlorophyll fluorescence parameters such as minimum fluorescence yield (F₀), maximum fluorescence yield (F_m), maximum photochemical quantum yield (F_v/F_m), PSII (photosystem II) effective quantum yield [Y(II)], photosynthetic electron transfer rate (ETR), and the photochemical quenching (qP), were also adversely affected. Algal photosynthetic and respiratory rates decreased following NSAID treatments, and the expression of genes involved in photosynthetic electron transport (psaA, psaB, psbB, psbD, and rbcL) was down-regulated. Furthermore, the functioning of the photosynthetic electron transport chain from PSI (photosystem I) to PSII, carbon assimilation, and photorespiration were affected. Our results suggest that NSAIDs can exert considerable toxic effects on the photosynthetic system of S. obliquus. These results provide a basis for evaluating the environmental safety of NSAIDs.

Remediation of cadmium-contaminated soil with biochar simultaneously improves biochar's recalcitrance

Biochar sequesters cadmium (Cd) by immobilisation, but the process is often less effective in field trials than in the laboratory. Therefore, the involvement of soil components should be considered for predicting field conditions that could potentially improve this process. Here, we used biochar derived from Spartina alterniflora as the amendment for Cd-contaminated soil. In simulation trials, a mixture of kaolin, a representative soil model component, and S. alterniflora-derived biochar immobilised Cd by forming silicon-aluminium-Cd-containing complexes. Interestingly, the biochar recalcitrance index value increased from 48% to 53%-56% because of the formation of physical barriers consisting of kaolinite minerals and Cd complexes. Pot trials were performed using Brassica chinensis for evaluating the effect of S. alterniflora-derived biochar on plant growth in Cd-contaminated soil. The bio-concentration factor values in B. chinensis were 24%-31% after soil remediation with biochar than in control plants. In summary, these results indicated that soil minerals facilitated Cd sequestration by biochar, which reduced Cd bioavailability and improved the recalcitrance of this soil amendment. Thus, mechanisms for effective Cd remediation should include biochar-soil interactions.

Effects of mixotrophic cultivation on antioxidation and lipid accumulation of Chlorella vulgaris in wastewater treatment

The effects of mixotrophic cultivation on antioxidation and lipid production of Chlorella vulgaris in wastewater treatment were analyzed. The biomass and lipid content of the mixotrophic C. vulgaris cultured in wastewater were higher compared with the autotrophic C. vulgaris cultured in BG-11. The mixotrophic C. vulgaris provided more fatty acids as the contents of total fatty acids rose. The unsaturated fatty acid/total fatty acid ratio under mixotrophic cultivation was up to 0.91, indicating the mixotrophic cultivation system was applicable for the generation of unsaturated fatty acids. Activities of antioxidant enzymes such as superoxide dismutase and glutathione peroxidase were improved after the addition of wastewater to algal cultures. Moreover, the activity and starch formation of ADP-glucose pyrophosphorylase decreased and the activity of acetyl-CoA carboxylase was enhanced, which contributed to the lipid production in the mixotrophic C. vulgaris in wastewater. This study suggests mixotrophic cultivation of microalgae in wastewater is an efficient way to improve lipid production.

The Architecture of Monospecific Microalgae Biofilms

Microalgae biofilms have been proposed as an alternative to suspended cultures in commercial and biotechnological fields. However, little is known about their architecture that may strongly impact biofilm behavior, bioprocess stability, and productivity. In order to unravel the architecture of microalgae biofilms, four species of commercial interest were cultivated in microplates and characterized using a combination of confocal laser scanning microscopy and FTIR spectroscopy. In all the species, the biofilm biovolume and thickness increased over time and reached a plateau after seven days; however, the final biomass reached was very different. The roughness decreased during maturation, reflecting cell division and voids filling. The extracellular polymeric substances content of the matrix remained constant in some species, and increased over time in some others. Vertical profiles showed that young biofilms presented a maximum cell density at 20 μm above the substratum co-localized with matrix components. In mature biofilms, the maximum density of cells moved at a greater distance from the substratum (30–40 μm), whereas the maximum coverage of matrix components remained in a deeper layer. Carbohydrates and lipids were the main macromolecules changing during biofilm maturation. Our results revealed that the architecture of microalgae biofilms is species-specific. However, time similarly affects the structural and biochemical parameters.