Recent advances in sustainable hydrogen production from microalgae: Mechanisms, challenges, and future perspectives.

The depletion of fossil fuel reserves has resulted from their application in the industrial and energy sectors. As a result, substantial efforts have been dedicated to fostering the shift from fossil fuels to renewable energy sources via technological advancements in industrial processes. Microalgae can be used to produce biofuels such as biodiesel, hydrogen, and bioethanol. Microalgae are particularly suitable for hydrogen production due to their rapid growth rate, ability to thrive in diverse habitats, ability to resolve conflicts between fuel and food production, and capacity to capture and utilize atmospheric carbon dioxide. Therefore, microalgae-based biohydrogen production has attracted significant attention as a clean and sustainable fuel to achieve carbon neutrality and sustainability in nature. To this end, the review paper emphasizes recent information related to microalgae-based biohydrogen production, mechanisms of sustainable hydrogen production, factors affecting biohydrogen production by microalgae, bioreactor design and hydrogen production, advanced strategies to improve efficiency of biohydrogen production by microalgae, along with bottlenecks and perspectives to overcome the challenges. This review aims to collate advances and new knowledge emerged in recent years for microalgae-based biohydrogen production and promote the adoption of biohydrogen as an alternative to conventional hydrocarbon biofuels, thereby expediting the carbon neutrality target that is most advantageous to the environment.

LASSO Regression with Multiple Imputations for the Selection of Key Variables Affecting the Fatty Acid Profile of Nannochloropsis oculata.

The marine microalga Nannochloropsis oculata has garnered significant interest as a potential source of lipids, both for biofuel and nutrition, containing significant amounts of C16:0, C16:1, and C20:5, n-3 (EPA) fatty acids (FA). Growth parameters such as temperature, pH, light intensity, and nutrient availability play a crucial role in the fatty acid profile of microalgae, with N. oculata being no exception. This study aims to identify key variables for the FA profile of N. oculata grown autotrophically. To that end, the most relevant literature data were gathered and combined with our previous work as well as with novel experimental data, with 121 observations in total. The examined variables were the percentages of C14:0, C16:0, C16:1, C18:1, C18:2, and C20:5, n-3 in total FAs, their respective ratios to C16:0, and the respective content of biomass in those fatty acids in terms of ash free dry weight. Many potential predictor variables were collected, while dummy variables were introduced to account for bias in the measured variables originating from different authors as well as for other parameters. The method of multiple imputations was chosen to handle missing data, with limits based on the literature and model-based estimation, such as using the software PHREEQC and residual modelling for the estimation of pH. To eliminate unimportant predictor variables, LASSO (Least Absolute Shrinkage and Selection Operator) regression analysis with a novel definition of optimal lambda was employed. LASSO regression identified the most relevant predictors while minimizing the risk of overfitting the model. Subsequently, stepwise linear regression with interaction terms was used to further study the effects of the selected predictors. After two rounds of regression, sparse refined models were acquired, and their coefficients were evaluated based on significance. Our analysis confirms well-known effects, such as that of temperature, and it uncovers novel unreported effects of aeration, calcium, magnesium, and manganese. Of special interest is the negative effect of aeration on polyunsaturated fatty acids (PUFAs), which is possibly related to the enzymatic kinetics of fatty acid desaturation under increased oxygen concentration. These findings contribute to the optimization of the fatty acid profile of N. oculata for different purposes, such as production of, high in PUFAs, food or feed, or production of, high in saturated and monounsaturated FA methyl esters (FAME), biofuels.

Comparative life cycle assessment of anaerobic digestion, lagoon evaporation, and direct land application of olive mill wastewater.

Olive mill wastewater is a prominent waste stream in the Mediterranean countries, with its uncontrolled disposal in water recipients causing significant environmental issues. Anaerobic digestion has been extensively studied for the treatment of various agricultural waste streams. The scope of the present study was the environmental evaluation of the anaerobic digestion of three-phase olive mill wastewater for energy production in an anaerobic bioreactor. Regarding the environmental assessment of the process, the results indicate a lead in the proposed process compared with the baseline scenarios. Moreover, several environmental issues in terrestrial acidification and water eutrophication midpoint categories were exhibited by the digestate utilization. The implementation of the anaerobic digestion method averts an overall environmental damage of 5 mPt per 1000 kg of waste treated. For this reason, the implementation of the proposed method could be a sustainable alternative for wastewater treatment in olive oil production regions, aiming to circular economy.

A novel rice hull - microalgal biorefinery for the production of natural phenolic compounds comprising of rice hull acid pretreatment and a two-stage Botryococcus braunii cultivation process.

Recently, the rising demand of the industry for natural phenolic antioxidant compounds has turned to the study of microalgae as potential sources. Yet, more economic substrates for microalgal cultivation are sought to lower production costs. To this end, the present work deals with the utilization of rice hull hydrolysate (RHH) as substrate for microalgae Botryococcus braunii through a novel two-stage cultivation system. Initially, RHH was optimized to maximize the contained nutrients while minimizing its inhibitors content. The optimum point was reached under 121 °C, 60 min, 2% (v/v) H2SO4, 30% (w/v) loading. Next, B. braunii was successfully grown first heterotrophically in RHH (25%, v/v), obtaining high biomass production (6.67 g L-1) and then autotrophically to enhance phenolics accumulation. At the end, a high phenolic content of 7.44 ± 0.60 mg Gallic Acid Equivalents g-1 DW was achieved from the produced biomass, thus highlighting the potential of this novel biotechnological method.

Toxicity of Heavy Metals and Recent Advances in Their Removal: A Review.

Natural and anthropogenic sources of metals in the ecosystem are perpetually increasing; consequently, heavy metal (HM) accumulation has become a major environmental concern. Human exposure to HMs has increased dramatically due to the industrial activities of the 20th century. Mercury, arsenic lead, chrome, and cadmium have been the most prevalent HMs that have caused human toxicity. Poisonings can be acute or chronic following exposure via water, air, or food. The bioaccumulation of these HMs results in a variety of toxic effects on various tissues and organs. Comparing the mechanisms of action reveals that these metals induce toxicity via similar pathways, including the production of reactive oxygen species, the inactivation of enzymes, and oxidative stress. The conventional techniques employed for the elimination of HMs are deemed inadequate when the HM concentration is less than 100 mg/L. In addition, these methods exhibit certain limitations, including the production of secondary pollutants, a high demand for energy and chemicals, and reduced cost-effectiveness. As a result, the employment of microbial bioremediation for the purpose of HM detoxification has emerged as a viable solution, given that microorganisms, including fungi and bacteria, exhibit superior biosorption and bio-accumulation capabilities. This review deals with HM uptake and toxicity mechanisms associated with HMs, and will increase our knowledge on their toxic effects on the body organs, leading to better management of metal poisoning. This review aims to enhance comprehension and offer sources for the judicious selection of microbial remediation technology for the detoxification of HMs. Microbial-based solutions that are sustainable could potentially offer crucial and cost-effective methods for reducing the toxicity of HMs.

Effect of Nitrogen, Salinity, and Light Intensity on the Biomass Composition of Nephroselmis sp.: Optimization of Lipids Accumulation (Including EPA).

Microalgal biomass is characterized by high protein, carbohydrates, and lipids concentrations. However, their qualitative and quantitative compositions depend not only on the cultivated species but also on the cultivation conditions. Focusing on the microalgae's ability to accumulate significant fatty acids (FAs) amounts, they can be valorized either as dietary supplements or for biofuel production, depending on the accumulated biomolecules. In this study, a local isolate (Nephroselmis sp.) was precultured under autotrophic conditions, while the Box-Behnken experimental design followed using the parameters of nitrogen (0-250 mg/L), salinity (30-70 ppt) and illuminance (40-260 µmol m-2 s-1) to evaluate the accumulated biomolecules, with an emphasis on the amount of FAs and its profile. Regardless of the cultivation conditions, the FAs of C14:0, C16:0, and C18:0 were found in all samples (up to 8% w/w in total), while the unsaturated C16:1 and C18:1 were also characterized by their high accumulations. Additionally, the polyunsaturated FAs, including the valuable C20:5n3 (EPA), had accumulated when the nitrogen concentration was sufficient, and the salinity levels remained low (30 ppt). Specifically, EPA approached 30% of the total FAs. Therefore, Nephroselmis sp. could be considered as an alternative EPA source compared to the already-known species used in food supplementation.

Clean-Up of Heavy Metals from Contaminated Soil by Phytoremediation: A Multidisciplinary and Eco-Friendly Approach.

Pollution from heavy metals is one of the significant environmental concerns facing the world today. Human activities, such as mining, farming, and manufacturing plant operations, can allow them access to the environment. Heavy metals polluting soil can harm crops, change the food chain, and endanger human health. Thus, the overarching goal for humans and the environment should be the avoidance of soil contamination by heavy metals. Heavy metals persistently present in the soil can be absorbed by plant tissues, enter the biosphere, and accumulate in the trophic levels of the food chain. The removal of heavy metals from contaminated soil can be accomplished using various physical, synthetic, and natural remediation techniques (both in situ and ex situ). The most controllable (affordable and eco-friendly) method among these is phytoremediation. The removal of heavy metal defilements can be accomplished using phytoremediation techniques, including phytoextraction, phytovolatilization, phytostabilization, and phytofiltration. The bioavailability of heavy metals in soil and the biomass of plants are the two main factors affecting how effectively phytoremediation works. The focus in phytoremediation and phytomining is on new metal hyperaccumulators with high efficiency. Subsequently, this study comprehensively examines different frameworks and biotechnological techniques available for eliminating heavy metals according to environmental guidelines, underscoring the difficulties and limitations of phytoremediation and its potential application in the clean-up of other harmful pollutants. Additionally, we share in-depth experience of safe removing the plants used in phytoremediation-a factor frequently overlooked when choosing plants to remove heavy metals in contaminated conditions.

Bioplastic production in terms of life cycle assessment: A state-of-the-art review.

The current transition to sustainability and the circular economy can be viewed as a socio-technical response to environmental impacts and the need to enhance the overall performance of the linear production and consumption paradigm. The concept of biowaste refineries as a feasible alternative to petroleum refineries has gained popularity. Biowaste has become an important raw material source for developing bioproducts and biofuels. Therefore, effective environmental biowaste management systems for the production of bioproducts and biofuels are crucial and can be employed as pillars of a circular economy. Bioplastics, typically plastics manufactured from bio-based polymers, stand to contribute to more sustainable commercial plastic life cycles as part of a circular economy in which virgin polymers are made from renewable or recycled raw materials. Various frameworks and strategies are utilized to model and illustrate additional patterns in fossil fuel and bioplastic feedstock prices for various governments' long-term policies. This review paper highlights the harmful impacts of fossil-based plastic on the environment and human health, as well as the mass need for eco-friendly alternatives such as biodegradable bioplastics. Utilizing new types of bioplastics derived from renewable resources (e.g., biowastes, agricultural wastes, or microalgae) and choosing the appropriate end-of-life option (e.g., anaerobic digestion) may be the right direction to ensure the sustainability of bioplastic production. Clear regulation and financial incentives are still required to scale from niche polymers to large-scale bioplastic market applications with a truly sustainable impact.

Microalgae-based wastewater treatment: Mechanisms, challenges, recent advances, and future prospects.

The rapid expansion of both the global economy and the human population has led to a shortage of water resources suitable for direct human consumption. As a result, water remediation will inexorably become the primary focus on a global scale. Microalgae can be grown in various types of wastewaters (WW). They have a high potential to remove contaminants from the effluents of industries and urban areas. This review focuses on recent advances on WW remediation through microalgae cultivation. Attention has already been paid to microalgae-based wastewater treatment (WWT) due to its low energy requirements, the strong ability of microalgae to thrive under diverse environmental conditions, and the potential to transform WW nutrients into high-value compounds. It turned out that microalgae-based WWT is an economical and sustainable solution. Moreover, different types of toxins are removed by microalgae through biosorption, bioaccumulation, and biodegradation processes. Examples are toxins from agricultural runoffs and textile and pharmaceutical industrial effluents. Microalgae have the potential to mitigate carbon dioxide and make use of the micronutrients that are present in the effluents. This review paper highlights the application of microalgae in WW remediation and the remediation of diverse types of pollutants commonly present in WW through different mechanisms, simultaneous resource recovery, and efficient microalgae-based co-culturing systems along with bottlenecks and prospects.

Cobalt oxide nanoparticles as a new strategy for enhancing methane production from anaerobic digestion of noxious aquatic weeds.

This study investigated the effect of cobalt oxide nanoparticles (Co3O4-NPs) supplementation on anaerobic microbial population changes and anaerobic digestion (AD) performance and production. Co3O4-NPs (3 mg/L) showed the maximum enhancement of biogas yield over the cow dung (CD) as control and the co-digestion process of CD with water hyacinth (WH) by 58.9 and 27.2 %, respectively. Furthermore, methane (CH4) yield was enhanced by 89.96 and 43.4 % over CD and co-digestion processes, respectively. Additionally, the microbiological assessment analysis using VIT® gene probe technology showed that Co3O4-NPs enhance the viability of total bacterial cells by 9 %. The techno-economic analysis reflects the revenue of this strategy on the highest net energy content of biogas, which was achieved with 3 mg/L Co3O4-NPs and was 428.05 kWh with a net profit of 67.66 USD/m3 of the substrate. Therefore, nanoparticle supplementation to the AD process can be considered a promising approach to enhance biogas and CH4.

Production of Antioxidants and High Value Biomass from Nannochloropsis oculata: Effects of pH, Temperature and Light Period in Batch Photobioreactors

Nannochloropsis oculata is a marine microalgal species with a great potential as food or feed due to its high pigment, protein and eicosapentaenoic acid contents. However, for such an application to be realized on a large scale, a biorefinery approach is necessary due to the high cost of microalgal biomass production. For example, techno economic analyses have suggested the co-production of food or feed with antioxidants, which can be extracted and supplied separately to the market. The aim of this study was to investigate the effect of cultivation conditions on the antioxidant capacity of Nannochlosopsis oculata extracts, derived with ultrasound-assisted extraction at room temperature, as well as the proximate composition and fatty acid profile of the biomass. A fractional factorial approach was applied to examine the effects of temperature (20-35 °C), pH (6.5-9.5) and light period (24:0, 12:12). At the end of each run, biomass was collected, washed with 0.5M ammonium bicarbonate and freeze-dried. Antioxidant capacity as gallic acid equivalents as well as pigment content were measured in the ethanolic extracts. Optimal conditions were different for productivity and biomass composition. Interesting results regarding the effect of light period (LP) and pH require further investigation, whereas the effect of moisture on the extraction process was confounded with biomass composition. Finally, further data is provided regarding the relation between chlorophyll content and apparent phenolic content using the Folin-Ciocalteu assay, in agreement with our previous work.

Biowastes for biodegradable bioplastics production and end-of-life scenarios in circular bioeconomy and biorefinery concept.

Due to global urbanization, industrialization, and economic development, biowastes generation represents negative consequences on the environment and human health. The use of generated biowastes as a feedstock for biodegradable bioplastic production has opened a new avenue for environmental sustainability from the circular (bio)economy standpoint. Biodegradable bioplastic production can contribute to the sustainability pillars (environmental, economic, and social). Furthermore, bioenergy, biomass, and biopolymers production after recycling of biodegradable bioplastic can help to maintain the energy-environment balance. Several types of biodegradable bioplastic, such as starch-based, polyhydroxyalkanoates, polylactic acid, and polybutylene adipate terephthalate, can achieve this aim. In this review, an overview of the main biowastes valorization routes and the main biodegradable bioplastic types of production, application, and biodegradability are discussed to achieve the transition to the circular economy. Additionally, end-of-life scenarios (up-cycle and down-cycle) are reviewed to attain the maximum environmental, social, and economic benefit from biodegradable bioplastic products under biorefinery concept.

High-rate in-vessel continuous composting of olive mill byproducts.

The increasing production of agro-industrial organic residues in modern society is extremely concerning. One of the most polluting procedures in the agricultural industry is the production of olive oil. This process creates a large amount of waste with high organic load and phytotoxic components. In this study, composting of two-phase olive pomace (OP), olive leaves (OL) and dewatered anaerobic sludge (DAS) from an olive mill wastewater anaerobic digestion process was conducted in a pilot-scale in-vessel high-rate continuous composter. Five different feed scenarios were studied with different OP/OL ratio in the feed material, while the effect of the addition of pine tree bark pieces (PB) and DAS was examined. The OP:OL 95:5 % w/w ratio exhibited the best results in terms of product quality, while OL proved capable of acting as a bulking agent for the better aeration of the material. The final product in the optimum feed ratio was free of Salmonella spp., was stable in terms of static respiratory index (lower than 0.5 g O2 kg-1 VS h-1) but contained elevated E. coli levels (3.5 × 104 CFU g-1 with a limit of 1 × 103 CFU g-1), which was the only EU proposed compost quality criteria not met. The addition of a more easily degradable material in the feed mixture is expected to lead to elevated composting temperature and amend the presence of pathogens.

Total Phenolic Content, Biomass Composition, and Antioxidant Activity of Selected Marine Microalgal Species with Potential as Aquaculture Feed.

There has been growing interest in microalgal biomolecules for health and cosmetics, as well as in the use of microalgae as aquaculture feed due to the need to replace fishmeal and fish oil with sustainable yet equally nutritious alternatives. Aim of this study is to evaluate the potential of five marine microalgal species, namely Chlorella minutissima, Dunaliella salina, Isochrysis galbana, Nannochloropsis oculata and Tisochrysis lutea, for the co-production of antioxidants and aquaculture feed. Batch cultivation was performed under saturating light intensity and continuous aeration. Freeze-dried biomass was extracted sequentially with water and methanol and evaluated for phenolic content and antioxidant activity, as well as proximate composition and fatty acid profile. Methanolic extracts of C. minutissima presented the highest phenolic content, measured with the Folin-Ciocalteu assay, and antioxidant activity. However, HPLC and LC-MS showed the presence of non-pigment compounds only in T. lutea. Total phenolic content and antioxidant activity were correlated to chlorophyll content. N. oculata and T. lutea were rich in eicosapentaenoic acid and docosahexaenoic acid, respectively, as well as in protein. In conclusion, N. oculata and T. lutea are suitable candidates for further optimization, while the data presented suggest that pigment effects on the Folin-Ciocalteu method require reconsideration.

Proposed protocol for rate-limiting step determination during anaerobic digestion of complex substrates.

Anaerobic digestion is a complex process, involving various microorganism groups and, consequently, several reactions. An easy-to-use protocol for the rate-limiting step determination of the process is proposed. The hydrogen production, acetate production, and acetate consumption rates can be calculated, according to a structured algorithm. During the rate limiting step determination, several compounds (biopolymer and monomer representatives, as well as sodium acetate) were used, combined or not with the substrate, to draw the corresponding conclusions. Three substrates were tested, characterized by specific organic compound groups (carbohydrates, proteins, and fats). All three substrates followed the acetate-consuming pathway for the organic matter conversion to methane. In this study, the rate-limiting step for the pathway of acetate consumption was acetate production. Determining the rate-limiting step through the proposed protocol can point to the appropriate actions needed to boost methane production, like substrate pretreatment, using an acidogenic reactor, or checking for the presence of inhibitors.

Comparative Assessment of Nitrogen Concentration Effect on Microalgal Growth and Biochemical Characteristics of Two Chlorella Strains Cultivated in Digestate.

Microalgae have been recently recognized as a promising alternative for the effective treatment of anaerobic digestion effluents. However, to date, a widely applied microalgae-based process is still absent, due to several constraints mainly attributed to high ammonia concentrations and turbidity, both hindering microalgal growth. Within this scope, the purpose of the present study was to investigate the performance of two Chlorella strains, SAG 211-11b and a local Algerian isolate, under different nitrogen levels, upon ammonia stripping. The experiments were performed on cylindrical photobioreactors under controlled pH (7.8 ± 0.2) and temperature (25 ± 2 °C). Cultures were monitored for biomass production and substrate consumption. After sampling at the beginning of the stationary phase of growth (12th day) and after the maturation of the cells (24th day), an analysis of the produced biomass was conducted, in terms of its biochemical components. The local isolate grew better than C. vulgaris 211-11b, resulting in 1.43 mg L-1 biomass compared to 1.02 mg L-1 under 25 mg NH4-N L-1, while organic carbon and nutrient consumption varied between the two strains and different conditions. Concerning biomass quality, a high initial NH4-N concentration led to high protein content, while low nitrogen levels favored fatty acid (FA) accumulation, though the production of pigments was inhibited. In particular, the protein content of the final biomass was determined close to 45% of the dry weight in all experimental scenarios with adequate nitrogen, while proteins decreased, and the fatty acids approached 20% in the case of the local isolate grown on the substrate with the lowest initial ammonium nitrogen (25 mg NH4-N L-1). The novelty of the present work lies in the comparison of a microalga with industrial applications against a local isolate of the same species, which may prove to be even more robust and profitable.

Ultrasound-Assisted Extraction of Nannochloropsis oculata with Ethanol and Betaine: 1,2-Propanediol Eutectic Solvent for Antioxidant Pigment-Rich Extracts Retaining Nutritious the Residual Biomass.

The aim of this study was the development of an efficient "green" extraction method of Nannochloropsis oculata to produce antioxidant extracts and nutritious residual biomass. Twenty-one extraction methods were evaluated by measuring the reactivity with the Folin-Ciocalteu reagent: ultrasonication or maceration at different temperatures with different organic solvents, extraction at different pH values, enzyme-assisted extraction, encapsulation with ß-cyclodextrin, and the use of natural deep eutectic solvents. Ultrasound-assisted extraction with ethanol or betaine: 1,2-propanediol in a molar ratio of 2:5 (BP) had optimal extractive capacity. Both extracts were evaluated with antioxidant assays and the ethanol extract exhibited significantly higher (at least twofold) values. The determination of carotenoids by LC-MS and HPLC-DAD revealed the dominance of violaxanthin and antheraxanthin and their fourfold higher concentrations in the ethanol extract. The 1H-NMR characterization of the ethanol extract confirmed the results of the colorimetric and chromatographic assays. The microalgal biomass was characterized before and after the extraction in terms of humidity, ash, carbohydrates, proteins, chlorophyll-a, carotenoids, and lipids; the identity and content of the latter were determined with gas chromatography. BP caused a smaller depletion of the lipids from the biomass compared to ethanol, but proteins, carbohydrates, and ash were at a higher content in the biomass obtained after ethanol extraction, whereas the biomass was dry and easy to handle. Although further optimization may take place for the scale-up of those procedures, our study paves the way for a green strategy for the valorization of microalgae in cosmetics without generating waste, since the remaining biomass can be used for aquafeed.

Biopolymers production from microalgae and cyanobacteria cultivated in wastewater: Recent advances.

Plastic materials are used to manufacture a broad variety of items with a short useful lifespan, resulting in significant amounts of waste material generation. This form of waste is often observed floating at sea, and different microplastics have been discovered in fish stomachs and women's placentas. Bioplastics are a more biodegradable substitute for fossil-based polymers. Microalgae are capable of producing poly (hydroxy alkanoate) esters (PHAs), aliphatic polyesters that are biodegradable. The most prevalent and well-characterized biopolymer is the poly (3-hydroxy butyrate) ester (PHB), which belongs to the short-chain PHAs. Under aerobic conditions, PHB compounds degrade fully to carbon dioxide and water. They are ecologically neutral, having thermal and mechanical qualities comparable to those of petrochemical polymers. Numerous microalgae species have been reported in the literature to be capable of making bioplastics under certain conditions (N-P restriction, light exposure, etc.), which may be exploited as a source of energy and carbon. To further ameliorate the environmental impact of microalgae culture for bioplastics production, a limited number of published studies have examined the accumulation of bioplastics, from microalgae grown in wastewater, at a concentration of 5.5-65% of dry biomass weight.

Biochemical Μethane potential of most promising agricultural residues in Northern and Southern Greece.

Greece produces significant amounts of residual biomass due to its intense agricultural and agro-industrial sector. The anaerobic digestion process has been frequently considered as the best environmental and economic solution for energy recovery from different biodegradable waste such as agricultural waste, livestock manure, agro-industrial waste, as well as for their co-digestion. The aim of this study was the assessment of biochemical methane potential (BMP) of biomass feedstocks representative of Northern and Southern Greece, which are available during the fall/winter and spring/summer seasons, through the implementation of BMP assays. The raw residues evaluated in the current work included: (a) crop residues (corn silage and unsuitable for human consumption watermelon), (b) agro-industrial residues (malt, tomato processing residues, orange peels and olive pomace) and (c) livestock (cattle) manure. Tests of both single substrates and various mixtures were conducted for the evaluation of their methane yields. The results of the mono-substrates are in accordance with other studies in the literature, with watermelon presenting the highest methane potential (421.0 ± 3.4 ml CH4/g VSadded). After the evaluation of the mixtures and mono-substrates results, the most promising mixtures seemed to be the following: a) for Northern Greece, 10% corn silage-80% cattle manure-10% malt, b) for Southern Greece spring/summer season, 10% corn silage-14% cattle manure-66% watermelon-10% tomato processing residues, and c) for Southern Greece fall/winter season, 10% corn silage-57% cattle manure-23% orange peels-10% olive pomace.

Utilization of the microalga Scenedesmus quadricauda for hexavalent chromium bioremediation and biodiesel production.

The purpose of this study was to evaluate the bioremediation potential of the microalga Scenedesmus quadricauda in removing hexavalent chromium (Cr (VI)) from synthetic wastewater, under autotrophic and heterotrophic conditions and different inoculum concentrations. In both cultivation modes, the highest inoculum density of 0.8 g L-1 led to the highest Cr (VI) removal efficiency. In addition, Cr (VI) stress was more severe in 10 ppm compared to 5 ppm, while heavy metal effects were alleviated under heterotrophic conditions. Concurrently, Cr (VI) stress affected biomass quality, resulting in an increase in lipid and carbohydrate production and decreased proteins. Furthermore, under higher Cr (VI) concentration more saturated and monounsaturated fatty acids were produced, while monounsaturated fatty acids content was also greater under heterotrophic conditions. In total, the findings of this study highlight the advantages of heterotrophic cultivation of microalgae for concomitant industrial wastewater treatment and biofuel production.