Effect of culture conditions on biomass yield of acclimatized microalgae in ozone pre-treated tannery effluent: A simultaneous exploration of bioremediation and lipid accumulation potential

Advancements and hurdles in symbiotic microalgal co-cultivation strategies for wastewater treatment

Microalgae offer significant potential in various industrial applications, such as biofuel production and wastewater treatment, but the economic barriers to their cultivation and harvesting have been a major obstacle. However, a promising strategy involving co-cultivating microalgae in wastewater treatment could overcome the limitations of monocultivation and open the possibility for increased integration of microalgae into various industrial processes. This symbiotic relationship between microalgae and other microbes can enhance nutrient removal efficiency, increase value-added bioproduct production, promote carbon capture, and decrease energy consumption. However, unresolved challenges, such as the competition between microalgae and other microbes within the wastewater treatment system, may result in imbalances and reduced efficiency. The complexity of managing multiple microbes in a co-cultivation system poses difficulties in achieving stability and consistency in bioproduct production. In response to these challenges, strategies such as optimizing nutrient ratios, manipulating environmental conditions, understanding the dynamics of microbial relationships, and employing genetic modification to enhance the metabolic capabilities of microalgae and improve their competitiveness are critical in transitioning to a more sustainable path. Hence, this review will provide an in-depth analysis of recent advancements in symbiotic microalgal co-cultivation for applications in wastewater treatment and CO2 utilization, as well as discuss approaches for improving microalgal strains through genetic modification. Furthermore, the review will explore the use of efficient bioreactors, advanced control systems, and advancements in biorefinery processes.

Enhancing the production of PHA in Scenedesmus sp. by the addition of green synthesized nitrogen, phosphorus, and nitrogen-phosphorus-doped carbon dots

Plastic consumption has increased globally, and environmental issues associated with it have only gotten more severe; as a result, the search for environmentally friendly alternatives has intensified. Polyhydroxyalkanoates (PHA), as biopolymers produced by microalgae, might be an excellent option; however, large-scale production is a relevant barrier that hinders their application. Recently, innovative materials such as carbon dots (CDs) have been explored to enhance PHA production sustainably. This study added green synthesized multi-doped CDs to Scenedesmus sp. microalgae cultures to improve PHA production. Prickly pear was selected as the carbon precursor for the hydrothermally synthesized CDs doped with nitrogen, phosphorous, and nitrogen-phosphorous elements. CDs were characterized by different techniques, such as FTIR, SEM, ζ potential, UV-Vis, and XRD. They exhibited a semi-crystalline structure with high concentrations of carboxylic groups on their surface and other elements, such as copper and phosphorus. A medium without nitrogen and phosphorous was used as a control to compare CDs-enriched mediums. Cultures regarding biomass growth, carbohydrates, lipids, proteins, and PHA content were analyzed. The obtained results demonstrated that CDs-enriched cultures produced higher content of biomass and PHA; CDs-enriched cultures presented an increase of 26.9% in PHA concentration and an increase of 32% in terms of cell growth compared to the standard cultures.

Effects of culture temperature and light regimes on biomass and lipid accumulation of Chlamydomonas reinhardtii under carbon-rich and nitrogen-limited conditions

This study investigated the impacts of various culture temperatures and light regimes on growth and biochemical constituents of Chlamydomonas reinhardtii under carbon-supply and nitrogen-limited conditions to improve oil production in algal cells. Results displayed that under a 30 ℃ and 150 μE/m2/s regime, there was a significant increase in biomass, total lipids, and lipid productivity. Specifically, these parameters reached 1.83 g/L, 36.25 %, and 130.73 mg/L/d, respectively. Remarkably, prolonging the photoperiod further enhanced the aforementioned three parameters, reaching peak levels of 1.92 g/L, 41.10 %, and 157.54 mg/L/d, respectively, recorded at a 24/0h photoperiod. Compared with cultures grown under normal conditions, these values displayed increments of 1.21-fold, 74.88 %, and 3.01-fold, respectively. Additionally, under optimal conditions, the soluble sugar content reached 79.72 mg/g, and the biodiesel properties were improved. These findings indicate that moderately increasing temperature, light intensity, and photoperiod could achieve the co-production of biomass, lipids, and sugars in C. reinhardtii.

Effects of low temperature and highlight stress on lipid accumulation and cell structure of Tropidoneis maxima

Tropidoneis maxima is a marine diatom with a rapid growth rate that produces high levels of lipids. To explore whether the lipid content could be further enhanced, cultures were first incubated under optimal conditions and then stressed under low temperature (10°C), a high light intensity level (80 μmol/m2 ·s), and the two factors together (interaction treatment). The results indicated that high light intensity and the temperature-light interaction exhibited greater impacts on lipid synthesis of T. maxima than low temperature. The two stress treatments increased lipid content by 17.16% and 16.6% compared to the control. In particular, higher biomass concentration was obtained with high light intensity (1.082 g L-1 ) and low temperature (1.026 g L-1 ). Moreover, high light intensity (9.06%) and interaction (10.3%) treatments yielded lower starch content compared to low temperature (14.27%) at the end of the stress culture. After 3 days of stress culture, the high light intensity treatment resulted in a 97.01% increase in cell wall thickness and an 18.46% decrease in cell diameter. The results suggest that high light intensity stress on T. maxima would open a new approach to cost-effective biolipid production.

Microalgae: A green eco-friendly agents for bioremediation of tannery wastewater with simultaneous production of value-added products

Tannery wastewater (TWW) has high BOD, COD, TS and variety of pollutants like chromium, formaldehydes, biocides, oils, chlorophenols, detergents and phthalates etc. Besides these pollutants, TWW also rich source of nutrients like nitrogen, phosphorus, carbon and sulphur etc. that can be utilized by microalgae during their growth. Direct disposal of TWW into the environment may lead severe environmental and health threats, therefore it needs to be treated adequately. Microalgae are considered as an efficient microorganisms (fast growing, adaptability and strain robustness, high surface to volume ratio, energy saving) for remediation of wastewaters with simultaneous biomass recovery and generation of value added products (VAPs) such as biofuels, biohydrogen, biopolymer, biofertilizer, pigments, bioethanol, bioactive compounds, nutraceutical etc. Most microalgae are photosynthetic and use CO₂ and light energy to synthesise carbohydrate and reduces the emission of greenhouse gasses. Microalgae are also reported to remove heavy metals and antibiotics from wastewaters by bioaccumulation, biodegradation and biosorption. Microalgal treatment can be an alternative of conventional processes with generation of VAPs. The use of biotechnology in wastewater remediation with simultaneous generation of VAPs is trending. The validation of economic viability and environmental sustainability, life cycle assessment studies and techno-economic analysis is undergoing. Thus, in this review, the characteristics of TWW and microalgae are summarized, which manifest microalgae as potential candidates for wastewater remediation with simultaneous production of VAPs. Further, the treatment mechanisms, various factors (physical, chemical, mechanical and biological etc.) affecting treatment efficiency as well as challenges associated with microalgal remediation are also discussed.

Efficient removal of thiamethoxam by freshwater microalgae Scenedesmus sp. TXH: Removal mechanism, metabolic degradation and application

Neonicotinoids, as the most widely used pesticides in the world, help improve the production of crops. Meanwhile, it also brings potential threats to surrounding environments and other organisms because of its wide use and even abuse. In this study, Scenedesmus sp. TXH isolated from a wastewater treatment plant was used to remove the neonicotinoid pesticide thiamethoxam (THIA). The removal efficiency, degradation pathway, metabolite fate of THIA and physicochemical effects on microalgae cells were studied. Meanwhile, the feasibility of using microalgal technology to remove THIA from municipal wastewater was also explored. The results showed that 5-40 mg/L of THIA slightly promoted the growth of microalgae, while 60 mg/L THIA severely inhibited microalgal growth. It was observed that malondialdehyde content and superoxide dismutase activity in 60 mg/L THIA group increased significantly (p < 0.05) in the early stage of the experiment, indicating that THIA caused oxidative damage to microalgae. Scenedesmus sp. TXH showed high-efficient degradation ability and high resistance to THIA, with 100% removal of THIA at 5, 20 and 40 mg/L groups and 97.5% removal of THIA at 60 mg/L group on day 12. THIA was mainly removed by biodegradation, accounting for 78.18%, 93.50%, 96.81% and 91.35% under 5, 20, 40 and 60 mg/L on day 12, respectively. Six degradation products were identified, and four potential degradation pathways were proposed. In practical wastewater, the removal efficiency of total dissolved nitrogen, total dissolved phosphorus, ammonia nitrogen and THIA reached 85.68%, 90.00%, 98.43% and 100%, respectively, indicating that Scenedesmus sp. TXH was well adapted to the wastewater and effectively removed THIA and conventional pollutants.

Purification efficiency of Pyropia-processing wastewater and microalgal biomass production by the combination of Chlorella sp. C2 cultivated at different culture temperatures and chitosan

To elucidate the impacts of culture temperature on nutrient removal efficiency of Pyropia-processing wastewater (PPW) and microalgal biomass production, Chlorella sp. C2 was employed and cultivated in raw PPW under different temperatures. Results showed that, after incubating for 7 days, higher biomass (0.50 g/L) and total lipids (21.84 %) were attained at 35 °C. The maximal chemical oxygen demand (COD), phycobiliprotein, total nitrogen and total phosphorus removal rates were observed at 30-35 °C and separately reached 62.41 %, 92.61 %, 92.19 % and 98.33 %. Interestingly, COD removal efficiencies of Chlorella cells, cultivated for 3, 5 and 7 days at 30-35 °C, 15-25 °C and 10 °C respectively, could reach >75 % with assistance from 60-80 mg/L chitosan. Meanwhile, the clarification efficiency of chitosan on algal cells reached >95 %. It suggests that Chlorella strain cultured at altered temperatures could efficiently remove PPW nutrients assisted by moderate chitosan, simultaneously achieving the rapid harvest of microalgae.

Optimization of the culture of Chlorella sorokiniana PA.91 by RSM: effect of temperature, light intensity, and MgAC-NPs

The unique physicochemical properties of magnesium amino clay nanoparticles (MgAC-NPs) tends to be beneficial in the application as a co-additive in treating microalgae. Also, MgAC-NPs can create oxidative stress in the environment, concurrently elective control bacteria in mixotrophic culture, and stimulate CO₂ biofixation. The condition of the cultivation of newly isolated strains, Chlorella sorokiniana PA.91, was optimized for the first time for MgAC-NPs at various temperatures and light intensities in the culture medium of municipal wastewater (MWW) by central composite design in the response surface methodology (RSM-CCD). This study examined synthesized MgAC-NP with their FE-SEM, EDX, XRD, and FT-IR characteristics. The synthesized MgAC-NPs were naturally stable, cubic shaped, and within the size range of 30-60 nm. The optimization results show that at culture conditions of 20 °C, 37 μmol m^-2 s^-1, and 0.05 g L^-1, microalga MgAC-NPs have the best growth productivity and biomass performance. Maximum dry biomass weight (55.41%), specific growth rate (30.26%), chlorophyll (81.26%), and carotenoids (35.71%) were achieved under the optimized condition. Experimental results displayed that C.S. PA.91 has a high capacity for lipid extraction (1.36 g L^-1) and significant lipid efficiency (45.1%). Also, in 0.2 and 0.05 g L^-1 of the MgAC-NPs, COD removal efficiency 91.1% and 81.34% from C.S. PA.91 showed, respectively. These results showed the potential of C.S. PA.91-MgAC-NPs for nutrient removal in wastewater treatment plants and their quality as sources of biodiesel.

Screening of microalgae species and evaluation of algal-lipid stimulation strategies for biodiesel production

Microalgae is considered an alternative source for biodiesel production producing renewable, sustainable and carbon-neutral energy. Microalgae property changes among species, which determines the efficiency of biodiesel production. Besides the lipid content evaluation, multi-principles (including high lipid productivity, high biomass yield, pollution resistance and desired fatty acid, etc.) for superior oil-producing species screening was proposed in this review and three microalgae species (Chlorella vulgaris, Scenedesmus obliquus and Mychonastes afer) with high bio-lipid producing prospect were screened out based on big data digging and analysis. The multilateral strategies for algal-lipid stimulating were also compared, among which, nutrient restriction, temperature control, heterotrophy and chemicals addition showed high potential in enhancing lipid accumulation; while electromagnetic field showed little effect. Interestingly, it was found that the lipid accumulation was more sensitive to nitrogen (N)-limitation other than phosphorus (P). Nutrient restriction, salinity stress etc. enhanced lipid accumulation by creating a stressed environment. Hence, optimum conditions (e.g. N:15-35 mg/L and P:4-16 mg/L) should be set to balance the lipid accumulation and biomass growth, and further guarantee the algal-lipid productivity. Otherwise, two-step cultivation could be applied during all the stressed stimulation. Different from lab study, effectiveness, operability and economy should be all considered for stimulation strategy selection. Nutrient restriction, temperature control and heterotrophy were highly feasible after the multidimensional evaluation.

Dark stress for improved lipid quantity and quality in bioprospected acid-tolerant green microalgae

The cost of microalgae cultivation is one of the largest limitations to achieving sustainable, large-scale microalgae production of commercially desirable lipids. Utilizing CO2 as a 'free' carbon source from waste industrial flue gas emissions can offer wide-ranging cost savings. However, these gas streams typically create acidic environments, in which most microalgae cannot survive due to the concentration of CO2 and the presence of other acidic gasses such as NO2 and SO2. To address this situation, we investigated growth of a mixed acid-tolerant green microalgal culture (91% dominated by a single Coccomyxa sp. taxon) bioprospected at pH 2.8 from an acid mine drainage impacted water body. The culture was grown at pH 2.5 and fed with a simulated flue gas containing 6% CO2 and 94% N2. On reaching the end of the exponential growth phase, the culture was exposed to either continued light-dark cycle conditions or continual dark conditions. After three days in the dark, the biomass consisted of 28% of lipids, which was 42% higher than at the end of the exponential phase and 55% higher than the maximum lipid content achieved under light/dark conditions. The stress caused by being continually in the dark also favoured the production of omega-3 and omega-6 polyunsaturated fatty acids (PUFAs; 19.47% and 21.04%, respectively, after 7 days) compared to 7-days of light-dark treatment (1.94% and 9.53%, respectively) and showed an increase in nitrogen content (C:N ratio of 6.4) compared to light-dark treatment (C:N ratio of 11.9). The results of the research indicate that use of acid tolerant microalgae overcomes issues using flue gasses that will create an acidic environment and that applying dark stress is a low-cost stressor stimulates production of desirable dietary lipids.

Evaluation of the effects of input variables on the growth of two microalgae classes during wastewater treatment

Wastewater treatment carried out by microalgae is usually affected by the type of algal strain and the combination of cultivation parameters provided during the process. Every microalga strain has a different tolerance level towards cultivation parameters, including temperature, pH, light intensity, CO2 content, initial inoculum level, pretreatment method, reactor type and nutrient concentration in wastewater. Therefore, it is vital to supply the right combination of cultivation parameters to increase the wastewater treatment efficiency and biomass productivity of different microalgae classes. In the current investigation, the decision tree was used to analyse the dataset of class Trebouxiophyceae and Chlorophyceae. Various combinations of cultivation parameters were determined to enhance their performance in wastewater treatment. Nine combinations of cultivation parameters leading to high biomass production and eleven combinations each for high nitrogen removal efficiency and high phosphorus removal efficiency for class Trebouxiophyceae were detected by decision tree models. Similarly, eleven combinations for high biomass production, nine for high nitrogen removal efficiency, and eight for high phosphorus removal efficiency were detected for class Chlorophyceae. The results obtained through decision tree analysis can provide the optimum conditions of cultivation parameters, saving time in designing new experiments for treating wastewater at a large scale.

Low temperature combined with high inoculum density improves alpha-linolenic acid production and biochemical characteristics of Chlamydomonas reinhardtii

Chlamydomonas reinhardtii grows fast and is rich in polyunsaturated fatty acids. To explore whether the alpha-linolenic acid (ALA) content can be further enhanced, the cultures were incubated under different culture temperatures, light intensities and inoculum densities. Results showed that temperature exhibited more great impact on ALA synthesis of C. reinhardtii than light intensity and inoculum size. The changes of light intensity and inoculum size displayed non-significant effects on ALA content. The optimal ALA proportion in cells was obtained under the condition of 10 °C, 50 μE/m²/s and 5% inoculum density, which reached ∼ 39%.The augmented initial inoculum density could markedly improve the biomass of C. reinhardtii under 10 °C. The maximum ALA productivity (16.42 mg/L/d) was gained under 10 °C coupled with 25% inoculum size, where higher intracellular sugar and protein yield were observed. These results suggest C. reinhardtii would be an alternative feedstock for the industrial production of ALA.

Reduction of the environmental impacts of the hydropower plant by microalgae cultivation and biodiesel production

Hydropower plants supply their energy needs for electricity generation from rivers or water canals, so these power plants cannot be used for sustainable electricity generation, and the best time to use these power plants is during peak power consumption. These power plants are less polluting in terms of environment than other power plants, but they also have negative environmental effects, such as freshwater eutrophication and water salinity or microalgae. This study focused its attention on microalgae extraction as an environmentally friendly method to reduce water salinity and how they can be used for biodiesel production as an auxiliary fuel to enhance the energy production by hydropower plants. The information of the sample hydroelectric power plant (Gotvand Dam) that was required for the processing and simulation process is stated. The step-by-step simulation is reviewed and the results and optimizations are described. The highest separation of microalgae for 1 min electrolysis with distance of 1 cm between the two electrodes is 90%, which reduces the salt content of the water in which the microalgae is grown by 13%. The maximum separation of salt from water is 19.5%, which is reduced to 9.5% in the centrifuge method. Water salinity reduction to microalgae extraction ratio is 14.45%. The optimal combination of diesel and biodiesel is 80%-20%. As can be seen from the results it is recommended to use microalgae for reducing negative environmental impacts in addition to increasing the power generation capacities of hydropower plants. Also more specific studies on terms of the culture of the microalgae and its individual cultivation methods for hydropower plants beneficial programs should be taken into account and be used by policy makers in the future.

Evaluation of the Light/Dark Cycle and Concentration of Tannery Wastewater in the Production of Biomass and Metabolites of Industrial Interest from Microalgae and Cyanobacteria

The tanning industry transforms animal skins into leather and produces liquid effluents with a high organic and inorganic pollutant load. This work evaluated the effect of the tannery wastewater (TWW) concentration and the light/dark cycle on the production of biomass, carbohydrates, proteins, lipids, and pigments (carotenoids and phycobiliproteins) on two microalgae (Chlorella sp. and Scenedesmus sp.) and one cyanobacterium (Hapalosiphon sp.). A non-factorial central experimental design with a response surface was implemented using the STATISTICA 7.0 software. High removal percentages for nitrates (97%), phosphates (73.3%), and chemical oxygen demand (93.2%) were achieved with the three strains. The results also highlight that the use of a constant light regime (24:0) and the concentration of real TWW affect the biomass production, since the highest concentration of biomass recorded was 1.31 g L−1 of Hapalosiphon sp. with 100% undiluted wastewater.

Advanced Oxidation Processes and Biotechnological Alternatives for the Treatment of Tannery Wastewater

The tannery industry is one of the economic sectors that contributes to the development of different countries. Globally, Europe and Asia are the main producers of this industry, although Latin America and Africa have been growing considerably in recent years. With this growth, the negative environmental impacts towards different ecosystem resources as a result of the discharges of recalcitrated pollutants, have led to different investigations to generate alternative solutions. Worldwide, different technologies have been studied to address this problem, biological and physicochemical processes have been widely studied, presenting drawbacks with some recalcitrant compounds. This review provides a context on the different existing technologies for the treatment of tannery wastewater, analyzing the physicochemical composition of this liquid waste, the impact it generates on human health and ecosystems and the advances in the different existing technologies, focusing on advanced oxidation processes and the use of microalgae. The coupling of advanced oxidation processes with biological processes, mainly microalgae, is seen as a viable biotechnological strategy, not only for the removal of pollutants, but also to obtain value-added products with potential use in the biorefining of the biomass.