A critical review on anaerobic digestion of microalgae and macroalgae and co-digestion of biomass for enhanced methane generation

Prospects and challenges of thermal hydrolysis pretreatment of microalgae for enhancing bioenergy and resource recovery in anaerobic bioprocesses

Microalgae have emerged as a promising feedstock for bioenergy production through anaerobic digestion and fermentation, gaining significant attention due to their rapid growth rate, ability to adapt to diverse environments, and rich biochemical composition. However, the recalcitrant nature of the microalgal cell wall necessitates pretreatment to enhance the accessibility of intracellular components and improve overall bioenergy yields from anaerobic digestion/fermentation. Among the various pretreatment methods, the thermal hydrolysis process has proven to be a promising strategy for enhancing the efficiency of bioenergy recovery from microalgal biomass. The benefits of thermal hydrolysis pretreatment of microalgae include improved organic matter solubilization, enhanced digestibility, and increased product yields in subsequent anaerobic digestion/fermentation processes for biomethane, biohydrogen, and volatile fatty acids production. However, thermal pretreatment poses challenges, such as forming future research by-products like furfural and ammonia, which can adversely affect microbial activities and reduce process efficiency. Thus, addressing its associated challenges is critical for maximizing its effectiveness in bioenergy and resource recovery. This review provides a comprehensive analysis of these challenges and offers recommendations for future research, emphasizing the need for optimized pretreatment strategies for advancing the sustainable and efficient use of microalgae in bioenergy production.

Exploring macroalgae biorefinery: Extraction of bioactive compounds and production of volatile fatty acids

Macroalgae have gained significant attention in recent research owing to their potential as novel food source and their noteworthy nutritional properties. However, a substantial amount of these macroalgae accumulates along the coast without being utilized, highlighting the need for proper treatment and disposal methods to mitigate secondary pollution effects. Previous studies on macroalgae have primarily focused on extracting bioactive compounds or anaerobic digestion processes to produce methane or volatile fatty acids (VFA), with observed improvements following different pre-treatments. In this study, three biorefinery options for macroalgae have been compared. Additionally, the extraction of bioactive compounds followed by VFA production is proposed as a promising new valorization strategy. Milled macroalgae exhibited a low methane production yield (138 ± 17 NmL CH4·g volatile solid-1), corresponding to 31 ± 4 % biodegradability, while the acidification percentage was higher (45 ± 1%). Among the three solvents applied (water, ethanol and acetone), ethanol (80%) at 25 °C was the most effective in recovering bioactive compounds, such as chlorophylls, sugars, and phenolic compounds with antioxidant activity. The extraction of chlorophylls and phenolic compounds was not influenced by particle size reduction. However, a more efficient extraction of sugars was observed with lower particle size. Moreover, ethanol treatment demonstrated the good efficiency in VFA production, reaching up to 3.6 ± 0.2 g VFA-(chemical oxygen demand, COD)·L-1, with a VFA spectrum (in COD basis) consisting of 51% acetic acid, 29% propionic acid, 5% i-butyric acid, 7% butyric acid, and 7% i-valeric acid. These findings highlight the potential of ethanol for efficient compound recovery and VFA production from macroalgae.

Integration of Digestate-Derived Biochar into the Anaerobic Digestion Process through Circular Economic and Environmental Approaches-A Review

The growing energy consumption and the need for a circular economy have driven considerable interest in the anaerobic digestion (AD) of organic waste, offering potential solutions through biogas and digestate production. AD processes not only have the capability to reduce greenhouse gas emissions but also contribute to the production of renewable methane. This comprehensive review aims to consolidate prior research on AD involving different feedstocks. The principles of AD are explored and discussed, including both chemical and biological pathways and the microorganisms involved at each stage. Additionally, key variables influencing system performance, such as temperature, pH, and C/N ratio are also discussed. Various pretreatment strategies applied to enhance biogas generation from organic waste in AD are also reviewed. Furthermore, this review examines the conversion of generated digestate into biochar through pyrolysis and its utilization to improve AD performance. The addition of biochar has demonstrated its efficacy in enhancing metabolic processes, microorganisms (activity and community), and buffering capacity, facilitating Direct Interspecies Electron Transfer (DIET), and boosting CH4 production. Biochar also exhibits the ability to capture undesirable components, including CO2, H2S, NH3, and siloxanes. The integration of digestate-derived biochar into the circular economy framework emerges as a vital role in closing the material flow loop. Additionally, the review discusses the environmental benefits derived from coupling AD with pyrolysis processes, drawing on life cycle assessment investigations. Techno-economic assessment (TEA) studies of the integrated processes are also discussed, with an acknowledgment of the need for further TEA to validate the viability of integrating the biochar industry. Furthermore, this survey examines the techno-economic and environmental impacts of biochar production itself and its potential application in AD for biogas generation, aiming to establish a more cost-effective and sustainable integrated system.

Enhancing environmental performance in biogas production from wastewater-grown microalgae: A life cycle assessment perspective

The production of biogas from microalgae has gained attention due to their rapid growth, CO2 sequestration, and minimal land use. This study uses life cycle assessment to assess the environmental impacts of biogas production from wastewater-grown microalgae through anaerobic digestion within an optimized microalgae-based system. Using SimaPro® 9 software, 3 scenarios were modeled considering the ReCiPe v1.13 midpoint and endpoint methods for environmental impact assessment in different categories. In the baseline scenario (S1), a hypothetical system for biogas production was considered, consisting of a high rate algal pond (HRAP), a settling, an anaerobic digester, and a biogas upgrading unit. The second scenario (S2) included strategies to enhance biogas yield, namely co-digestion and thermal pre-treatment. The third scenario (S3), besides considering the strategies of S2, proposed the biogas upgrading in the HRAP and the digestate recovery as a biofertilizer. After normalization, human carcinogenic toxicity was the most positively affected category due to water use in the cultivation step, accounted as avoided product. However, this category was also the most negatively affected by the impacts of the digester heating energy. Anaerobic digestion was the most impactful step, constituting on average 60.37% of total impacts. Scenario S3 performed better environmentally, primarily due to the integration of biogas upgrading within the cultivation reactor and digestate use as a biofertilizer. Sensitivity analysis highlighted methane yield's importance, showing potential for an 11.28% reduction in ionizing radiation impacts with a 10% increase. Comparing S3 biogas with natural gas, the resource scarcity impact was reduced sixfold, but the human health impact was 23 times higher in S3.

Enhanced biomethane production from Scenedesmus sp. using polymer harvesting and expired COVID-19 disinfectant for pretreatment

This study evaluates the repurposing of expired isopropanol (IPA) COVID-19 disinfectant (64% w/w) to pretreat algal biomass for enhancing methane (CH4) yield. The impact of harvesting methods (centrifugation and polymer flocculation) and microwave pretreatment on CH4 production from Scenedesmus sp. microalgal biomass were also investigated. Results show minimal impact of harvesting methods on the CH4 yield, with wet centrifuged and polymer-harvested biomass exhibiting comparable and low CH4 production at 66 and 74 L/kgvolatile solid, respectively. However, microalgae drying significantly increased CH4 yield compared to wet biomass, attributed to cell shrinkage and enhanced digestibility. Consequently, microwave and IPA pretreatment significantly enhanced CH4 production when applied to dried microalgae, yielding a 135% and 212% increase, respectively, compared to non-pretreated wet biomass. These findings underscore the advantage of using dried Scenedesmus sp. over wet biomass and highlight the synergistic effect of combining oven drying with IPA treatment to boost CH4 production whilst reducing COVID-19 waste.

Anaerobic co-digestion of microalgal biomass, sugarcane vinasse, and residual glycerol from biodiesel using simplex-centroid mixture design: methane potential, synergic effect, and microbial diversity

Microalgal biomass (MB) is a promising feedstock for bioenergy production. Nonetheless, the cell recalcitrance and the low C/N ratio limit the methane yield during anaerobic digestion. As an alternative to overcome these challenges, MB co-digestion with different feedstocks has been proposed. Thus, this study evaluated the anaerobic co-digestion (AcoD) of MB cultivated in wastewater with sugarcane vinasse (VIN) and residual glycerol from biodiesel production (GLY). Batch tests were conducted using augmented simplex-centroid mixture design to investigate the impact of AcoD on methane production (SMP), synergistic effects, and the influence on microbial community. When compared to MB digestion, 150 NmL CH4.g-1VS, binary and ternary AcoD achieved SMP increases from 120 to 337%. The combination of 16.7:16.7:66.7 (MB:VIN:GLY) showed the highest SMP for a ternary mixture (631 NmL CH4.g-1VS). Optimal synergies ranged from 1.3 to 1.4 and were primarily found for the MB:GLY AcoD. Acetoclastic Methanosaeta genus was predominant, regardless the combination between substrates. Despite the largest SMP obtained from the MB:GLY AcoD, other ternary mixtures were also highly synergetic and therefore had strong potential as a strategic renewable energy source.

Incorporating saline microalgae biomass in anaerobic digester treating sewage sludge: Impact on performance and microbial populations

The aim of this study was to acclimate anaerobic prokaryotes to saline microalgae biomass. Semi-continuous experiments were conducted using two 1.5 L mesophilic reactors for 10 weeks, (hydraulic retention time of 21 days). The first reactor was solely fed with sewage sludge (control), while the second received a mixture of sewage sludge and microalgal biomass (80/20 %w/w) cultivated at 70 g·L-1 salinity. The in-reactor salinity reached after the acclimation phase was 14 g·L-1. Biomethane production was comparable between the control and acclimated reactors (205 ± 29 NmLMethane·gVolatileSolids-1). Salinity tolerance assessment of methanogenic archaea revealed that salinity causing 50% inhibition of methane production increased from 10 to 27 g·L-1 after acclimation. Microbial diversity analyses revealed notable changes in methanogenic archaea populations during co-digestion of saline microalgae biomass, particularly methylotrophic (+27%) and acetotrophic (-26%) methanogens. This study has highlighted the possibility of treating efficiently saline microalgae in co-digestion with sewage sludge in future industrial biogas plants.

Fermentation of micro- and macroalgae as a way to produce value-added products

Fermentation of both microalgae and macroalgae is one of the most efficient methods of obtaining valuable value-added products due to the minimal environmental pollution and the availability of economic benefits, as algae do not require arable land and drift algae and algal bloom biomass are considered waste and must be recycled and their fermentation waste utilized. The compounds found in algae can be effectively used in the fuel, food, cosmetic, and pharmaceutical industries, depending on the type of fermentation used. Products such as methane and hydrogen can be produced by anaerobic digestion and dark fermentation of algae, and lactic acid and its polymers can be produced by lactic acid fermentation of algae. Article aims to provide an overview of the different types potential of micro- and macroalgae fermentation, the advantages and disadvantages of each type considered, and the economic feasibility of algal fermentation for the production of various value-added products.

Effect of harvesting time in the methane production on the anaerobic digestion of microalgae

Microalgae are being proposed as excellent substrates for different biorefinery processes. Anaerobic digestion process of microalgae is one of these interesting processes but has some limitations in deleting cell walls. For this reason, many studies proposed different types of pre-treatments, entailing energy, operation, and investment costs. This work aims to optimize the anaerobic digestion of the microalgae Chlorella sorokiniana and Chlorella sorokiniana (strain S12/S13/S16) without any pre-treatment by selecting the optimal harvesting time. The greatest influence is seen at 5:00 PM in methane production for both microalgae. For Chlorella sorokiniana, it is the most optimal moment for anaerobic digestion, whereas Chlorella sorokiniana (strain S12/S13/S16) is the least optimal. In the other harvesting times, both microalgae present a similar methane production, i.e. 173 ± 12 mL CH4/g of total volatile solids. The highest methane production rate values were obtained during peak sunlight, 1:00 PM and 8:00 AM, respectively, and lower overnight.

Performance of sewage sludge treatment for the removal of antibiotic resistance genes: Status and prospects

Wastewater treatment plants (WWTPs) receive wastewater containing antibiotic resistant bacteria (ARB) and antibiotic resistant genes (ARGs), which are predominant contributors to environmental pollution in water and soil. Of these sources, sludge is a more significant contributor than effluent. Knowing how sludge treatment affects the fate of ARGs is vital for managing the risk of these genes in both human and natural environments. This review therefore discusses the sources and transmission of ARGs in the environment and highlights the risks of ARGs in sludge. The effects of co-existing constituents (heavy metals, microplastics, etc.) on sludge and ARGs during treatment are collated to highlight the difficulty of treating sludge with complex constituents in ARGs. The effects of various sludge treatment methods on the abundances of ARGs in sludge and in soil from land application of treated sludge are discussed, pointing out that the choice of sludge treatment method should take into account various potential factors, such as soil and soil biology in subsequent land application. This review offers significant insights and explores the abundances of ARGs throughout the process of sludge treatment and disposal. Unintentional addition of antibiotic residues, heavy metals, microplastics and organic matter in sludge could significantly increase the abundance and reduce the removal efficiency of ARGs during treatment, which undoubtedly adds a barrier to the removal of ARGs from sludge treatment. The complexity of the sludge composition and the diversities of ARGs have led to the fact that no effective sludge treatment method has so far been able to completely eliminate the ecological risk of ARGs. In order to reduce risks resulting by transmission of ARGs, technical and management measures need to be implemented.

Algae-based approaches for Holistic wastewater management: A low-cost paradigm

Aquatic algal communities demonstrated their appeal for diverse industrial applications due to their vast availability, ease of harvest, lower production costs, and ability to biosynthesize valuable molecules. Algal biomass is promising because it can multiply in water and on land. Integrated algal systems have a significant advantage in wastewater treatment due to their ability to use phosphorus and nitrogen, simultaneously accumulating heavy metals and toxic substances. Several species of microalgae have adapted to thrive in these harsh environmental circumstances. The potential of algal communities contributes to achieving the United Nations' sustainable development goals in improving aquaculture, combating climate change, reducing carbon dioxide (CO2) emissions, and providing biomass as a biofuel feedstock. Algal-based biomass processing technology facilitates the development of a circular bio-economy that is both commercially and ecologically viable. An integrated bio-refinery process featuring zero waste discharge could be a sustainable solution. In the current review, we will highlight wastewater management by algal species. In addition, designing and optimizing algal bioreactors for wastewater treatment have also been incorporated.

A comparative cradle-to-gate life cycle assessment of three cotton stalk waste sustainable applications

This paper presents a novel approach to utilizing agricultural waste. It compares three different applications for cotton stalks: fabrication of wood composites, bioethanol production, and biogas cradle-to-gate Life cycle assessment production processes. Cotton cultivation generates a lot of debris, mostly cotton stalks, which are incinerated or landfilled, Sustainable resource management is critical for maintaining the ecosystem, and economic stability, and promoting social fairness since it ensures the long-term availability of resources while minimizing environmental damage. The investigation uses the Ecological Footprint, Impact 2002 +, Global Warming Damage Potential, Greenhouse Gas Protocol, Recipe Midpoint, Ecosystem Damage Potential, and CML IA Baseline-open LCA-enabled environmental sustainability assessments. The analysis showed that bioethanol has a lower carbon footprint and climate change impact than both wood composite and biogas production processes, as a result, this could cause a preference for bioethanol production as an environmentally friendly strategy for cotton stalks utilization. While human toxicity was higher in the biogas production process, it emits less fossil CO2 than biogenic CO2. The total climate change of wood composite, bioethanol, and biogas production processes was 0.01761, 0.011300, and 0.01083 points, respectively. This research helps accomplish wider ecological and economic aims by giving insights into sustainable waste management practices.

Microalgae as tools for bio-circular-green economy: Zero-waste approaches for sustainable production and biorefineries of microalgal biomass

Microalgae are promising organisms that are rapidly gaining much attention due to their numerous advantages and applications, especially in biorefineries for various bioenergy and biochemicals. This review focuses on the microalgae contributions to Bio-Circular-Green (BCG) economy, in which zero-waste approaches for sustainable production and biorefineries of microalgal biomass are introduced and their possible integration is discussed. Firstly, overviews of wastewater upcycling and greenhouse gas capture by microalgae are given. Then, a variety of valuable products from microalgal biomass, e.g., pigments, vitamins, proteins/peptides, carbohydrates, lipids, polyunsaturated fatty acids, and exopolysaccharides, are summarized to emphasize their biorefinery potential. Techno-economic and environmental analyses have been used to evaluate sustainability of microalgal biomass production systems. Finally, key issues, future perspectives, and challenges for zero-waste microalgal biorefineries, e.g., cost-effective techniques and innovative integrations with other viable processes, are discussed. These strategies not only make microalgae-based industries commercially feasible and sustainable but also reduce environmental impacts.

Impact of natural degradation of the invasive alga Rugulopteryx okamurae on anaerobic digestion: Heavy metal pollution and kinetic performance

This study shows, for the first time, how the natural biodegradation of the Phaeophyceae Rugulopteryx okamurae (R.o.) affects its methane yield, by biochemical methane potential assays, and the methane production kinetics. Additionally, a mechanical (zeolite-assisted milling) and a thermal (120 °C, 45 min) pretreatments were assessed. The highest methane yield was obtained from the mechanically pretreated fresh ashore biomass (219 (15) NLCH4 kgVS-1), which presents the use of zeolite during milling as an economical alternative for heavy metal toxicity reduction. Moreover, no significant differences were observed between the other tests (with the exception of the lowest value obtained for the mechanically pretreated fresh R.o.). Low methane yields were linked to the heavy metal content. However, an increase of 28.5 % and 20.0 % in the k value was found for the untreated fresh R.o. biomass and fresh ashore biomass, respectively, when subjected to thermal pretreatment. Finally, an enhancement of 80.5 % in the maximum methane production rate was obtained for the fresh ashore biomass milled with zeolite compared to the untreated fresh ashore biomass.

Algal sorbents and prospects for their application in the sustainable recovery of rare earth elements from E-waste

Efficient and sustainable secondary sourcing of Rare-Earth Elements (REE) is essential to counter supply bottlenecks and the impacts associated with primary mining. Recycled electronic waste (E-waste) is considered a promising REE source and hydrometallurgical methods followed by chemical separation techniques (usually solvent extraction) have been successfully applied to these wastes with high REE yields. However, the generation of acidic and organic waste streams is considered unsustainable and has led to the search for "greener" approaches. Sorption-based technologies using biomass such as bacteria, fungi and algae have been developed to sustainably recover REE from e-waste. Algae sorbents in particular have experienced growing research interest in recent years. Despite its high potential, sorption efficiency is strongly influenced by sorbent-specific parameters such as biomass type and state (fresh/dried, pre-treatment, functionalization) as well as solution parameters such as pH, REE concentration, and matrix complexity (ionic strength and competing ions). This review highlights differences in experimental conditions among published algal-based REE sorption studies and their impact on sorption efficiency. Since research into algal sorbents for REE recovery from real wastes is still in its infancy, aspects such as the economic viability of a realistic application are still unexplored. However, it has been proposed to integrate REE recovery into an algal biorefinery concept to increase the economics of the process (by providing a range of additional products), but also in the prospect of achieving carbon neutrality (as large-scale algae cultivation can act as a CO₂ sink).

Integration of microalgae cultivation and anaerobic co-digestion with dairy wastewater to enhance bioenergy and biochemicals production

A sequential anaerobic digestion and phycoremediation process was employed to recover nutrients and remove pollutants from dairy wastewater (DW), while simultaneously producing biomethane and biochemicals. Anaerobic digestion of 100% DW achieved a methane content and production rate of 53.7% and 0.17 L/L/d, respectively. This was accompanied by the removal of 65.5% chemical oxygen demand (COD), 86% total solid (TS), and 92.8% volatile fatty acids (VFAs). The anaerobic digestate was then used to grow Chlorella sorokiniana SU-1. Using 25% diluted digestate as the medium, SU-1 could reach 4.64 g/L biomass concentration, with total nitrogen (TN), total phosphorus (TP) and COD removal efficiencies of 77.6%, 87.1% and 70.4%, respectively. The obtained microalgal biomass (contained 38.5% carbohydrates, 24.9% proteins, 8.8% lipids) was used to co-digest with DW, resulting in good methane production performance. Co-digestion with 25% (w/v) algal biomass obtained a higher CH₄ content (65.2%) and production rate (0.16 L/L/d) than other ratios.

Improvement of Asterarcys quadricellulare biomass solubilization and subsequent biogas production via pretreatment approaches: structural changes and kinetic modeling evaluation

This study investigated the effect of enzymatic and hydrothermal pretreatment approaches on the solubilization of organic matter, structure, and biogas yield from microalgal biomass. The soluble chemical oxygen demand (sCOD) concentration increased by 1.21-3.30- and 5.54-6.60-fold compared to control by enzymatic and hydrothermal pretreatments respectively. The hydrothermal pretreatment affected the structural changes in the microalgal biomass markedly; nonetheless, increased enzymatic concentration also had a definite effect on it as determined by qualitative approaches like scanning electron microscopy and Fourier transform infrared spectroscopy. Also, the hydrothermal pretreatment (100 °C for 30 min) resulted in the highest biogas production potential (P) of 765.37 mLg^-1 VS at a maximum biogas production rate (R_m) of 22.66 mLg^-1 day^-1 with a very short lag phase (λ) of 0.07 days. The biogas production of pretreated microalgal biomass particularly at higher enzyme dose (20%, 24 h) and higher hydrothermal pretreatment temperature (120 °C, 30 min) showed a significant but weak correlation (R = 0.53) with sCOD, thus demonstrating that the less organic matter was used up for the biogas production. The modified Gompertz model explained the anaerobic digestion of microalgal biomass more accurately and had a better fit to the experimental data comparatively because of the low root mean square error (3.259-16.728), residual sum of squares (78.887-177.025), and Akaike's Information Criterion (38.605-62.853).

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.

A Recent Review of Primary Hydrogen Carriers, Hydrogen Production Methods, and Applications

Hydrogen is a promising energy carrier, especially for transportation, owing to its unique physical and chemical properties. Moreover, the combustion of hydrogen gas generates only pure water; thus, its wide utilization can positively affect human society to achieve global net zero CO2 emissions by 2050. This review summarizes the characteristics of the primary hydrogen carriers, such as water, methane, methanol, ammonia, and formic acid, and their corresponding hydrogen production methods. Additionally, state-of-the-art studies and hydrogen energy applications in recent years are also included in this review. In addition, in the conclusion section, we summarize the advantages and disadvantages of hydrogen carriers and hydrogen production techniques and suggest the challenging tasks for future research.

In-situ sulfite treatment enhanced the production of short-chain fatty acids from waste activated sludge in the side-stream anaerobic fermentation

Sulfite-based technology could enhance methane production from anaerobic sludge digestion. However, its potential for in-situ direct sludge treatment without anaerobic sludge addition in the side-stream remains unclear. This study investigated the feasibility of using in-situ sulfite treating sludge for short-chain fatty acids (SCFAs) production via anaerobic fermentation of waste activated sludge (WAS) as a side-stream treatment. In-situ sulfite direct sludge treatment enhanced SCFAs and acetic acid production by 2.03 and 4.89 times at 500 mg S/L compared to the control. With in-situ sulfite treatment, WAS hydrolysis and acidification were enhanced while methanogenesis was spontaneously hindered. The in-situ sulfite treatment inactivated pathogens and improved the sludge dewatering properties. The relative abundances of SCFAs-production microbial were stimulated, facilitating the sludge bioconversion. The produced SCFAs from in-situ sulfite side-stream treatment could be applied as an "internal carbon source" to enhance biological nutrient removal to improve economic and environmental value from sludge treatment.

Turning an invasive alien species into a valuable biomass: Anaerobic digestion of Rugulopteryx okamurae after thermal and new developed low-cost mechanical pretreatments

The invasive alien seaweed Rugulopteryx okamurae (R.o.) has spread quickly through the Mediterranean Sea causing an unprecedented ecological impact. A solution integrated into a circular economy model is needed in order to curb the negative effects of its presence. Anaerobic digestion (AD) is proposed as a feasible process able to transform biomass into renewable energy. Nevertheless, in order to improve the methane yield and surpass the drawbacks associated with AD processes, this research proposes a thermal pretreatment and a new developed method where the macroalgae is mechanically pretreated with zeolite. Chemical and microstructure characterization of the algal biomass after pretreatments involved scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). The highest methane yields of 240 (28) and 250 (20) NLCH4 kg-1 VSadded were obtained with the new mechanical pretreatment and the thermal pretreatment at 120 °C for 45 min without zeolite, achieving a 35 % improvement against the non-pretreated algae. A direct relationship between the crystallinity index of the samples and methane production was observed. The experimental data of methane production versus time were found to be in accordance with both first-order kinetic and Transference Function mathematical models.

Synergistic Effect of Surfactant on Disperser Energy and Liquefaction Potential of Macroalgae (Ulva intestinalis) for Biofuel Production

The objective of this study was to evaluate the effect of surfactant on disperser homogenization pretreatment for macroalgae (Ulva intestinalis) to enhance biogas production. The macroalgae are subjected to surfactant coupled disperser pretreatment, which enhanced the liquefaction and improved the biomethane production. The outcome of this study revealed that 10,000 rpm at 20 min with a specific energy input of 1748.352 kJ/ kg total solids (TS) are the optimum conditions for surfactant disperser pretreatment (SDP), which resulted in the liquefaction rate of 20.08% with soluble organics release of 1215 mg/L and showed a better result than disperser pretreatment (DP) with a liquefaction rate of 14%. Biomethane production through the SDP method was found to be 0.2 g chemical oxygen demand (COD)/g COD, which was higher than DP (0.11 g COD/g COD). SDP was identified to be a synergetic pretreatment method with an energy ratio and net profit of about 0.91 and 104.04 United States dollars (USD)/ton, respectively.

Research on Biogas Yield from Macroalgae with Inoculants at Different Organic Loading Rates in a Three-Stage Bioreactor

Macroalgae can be a viable alternative to replace fossil fuels that have a negative impact on the environment. By mixing macroalgae with other substrates, higher quality biogas can be obtained. Such biogas is considered one of the most promising solutions for reducing climate change. In the work, new studies were conducted, during which biogas yield was investigated in a three-stage bioreactor (TSB) during the anaerobic digestion of Cladophora glomerata macroalgae with inoculants from cattle manure and sewage sludge at different organic loading rates (OLR). By choosing the optimal OLR in this way, the goal was to increase the energy potential of biomass. The research was performed at OLRs of 2.87, 4.06, and 8.13 Kg VS/m³ d. After conducting research, the highest biogas yield was determined when OLR was 2.87 Kg VS/m³ d. With this OLR, the average biogas yield was 439.0 ± 4.0 L/Kg VS_added, and the methane yield was 306.5 ± 9.2 L CH₄/Kg VS_added. After increasing the OLR to 4.06 and 8.13 Kg VS/m³ d, the yield of biogas and methane decreased by 1.55 times. The higher yield was due to better decomposition of elements C, N, H, and S during the fermentation process when OLR was 2.87 Kg VS/m³ d. At different OLRs, the methane concentration remained high and varied from 68% to 80%. The highest biomass energy potential with a value of 3.05 kWh/Kg VS_added was determined when the OLR was 2.87 Kg VS/m³ d. This biomass energy potential was determined by the high yield of biogas and methane in TSB.

A Concise Review on the Potential Applications of Rugulopteryx okamurae Macroalgae

The brown macroalgae of the species Rugulopteryx okamurae has reached European waters and the Strait of Gibraltar as an invasive species. The proliferation and colonization of the species in subtidal and intertidal zones of these regions imposes significant threats to local ecosystems and additionally represents a significant socioeconomic burden related to the large amounts of biomass accumulated as waste. As a way to minimize the effects caused by the accumulation of algae biomass, investigations have been made to employ this biomass as a raw material in value-added products or technologies. The present review explores the potential uses of R. okamurae, focusing on its impact for biogas production, composting, bioplastic and pharmaceutical purposes, with potential anti-inflammatory, antibacterial and α-glucosity inhibitory activities being highlighted. Overall, this species appears to present many attributes, with remarkable potential for uses in several fields of research and in various industries.

Seaweed for climate mitigation, wastewater treatment, bioenergy, bioplastic, biochar, food, pharmaceuticals, and cosmetics: a review

The development and recycling of biomass production can partly solve issues of energy, climate change, population growth, food and feed shortages, and environmental pollution. For instance, the use of seaweeds as feedstocks can reduce our reliance on fossil fuel resources, ensure the synthesis of cost-effective and eco-friendly products and biofuels, and develop sustainable biorefinery processes. Nonetheless, seaweeds use in several biorefineries is still in the infancy stage compared to terrestrial plants-based lignocellulosic biomass. Therefore, here we review seaweed biorefineries with focus on seaweed production, economical benefits, and seaweed use as feedstock for anaerobic digestion, biochar, bioplastics, crop health, food, livestock feed, pharmaceuticals and cosmetics. Globally, seaweeds could sequester between 61 and 268 megatonnes of carbon per year, with an average of 173 megatonnes. Nearly 90% of carbon is sequestered by exporting biomass to deep water, while the remaining 10% is buried in coastal sediments. 500 gigatonnes of seaweeds could replace nearly 40% of the current soy protein production. Seaweeds contain valuable bioactive molecules that could be applied as antimicrobial, antioxidant, antiviral, antifungal, anticancer, contraceptive, anti-inflammatory, anti-coagulants, and in other cosmetics and skincare products.

Algal biomass to biohydrogen: Pretreatment, influencing factors, and conversion strategies

Hydrogen has gained attention as an alternative source of energy because of its non-polluting nature as on combustion it produces only water. Biological methods are eco-friendly and have benefits in waste management and hydrogen production simultaneously. The use of algal biomass as feedstock in dark fermentation is advantageous because of its low lignin content, high growth rate, and carbon-fixation ability. The major bottlenecks in biohydrogen production are its low productivity and high production costs. To overcome these issues, many advances in the area of biomass pretreatment to increase sugar release, understanding of algal biomass composition, and development of fermentation strategies for the complete recovery of nutrients are ongoing. Recently, mixed substrate fermentation, multistep fermentation, and the use of nanocatalysts to improve hydrogen production have increased. This review article evaluates the current progress in algal biomass pretreatment, key factors, and possible solutions for increasing hydrogen production.

Variations in the substrate composition and microbial community structure in the anaerobic fermentation process using the green algae Enteromorpha prolifera

Enteromorpha prolifera is a nutrient-rich green alga and abound in the Yellow Sea and the Bohai Sea of China. In this study, E. prolifera was anaerobically digested for biogas production. The variations of chemical compositions and microbial community structure as well as the physical structure of E. prolifera in anaerobic digestion process were investigated. This is the first report of multiple ways to deeply analysis the process of E. prolifera anaerobic digestion. Results from the present work showed that the biogas obtained from E. prolifera anaerobic digestion could achieve 409.7 mL•g− 1 TS with an average methane concentration of 53.2%, and the VFAs content in substrate played a vital role for driving the biogas production of flora. Moreover, S1 of Thermotogaceae and Cenarchaeum, the dominant bacteria and archaea in digestion flora, respectively, played important roles in degrading E. prolifera, acidizing slurry, and providing methanogenic substrate for methanogens.

Graphical Abstract

Fermentation Wastes from Chrypthecodinium cohnii Lipid Production for Energy Recovery by Anaerobic Digestion

Wastes generated during the cultivation of marine microalga Crypthecodinium cohnii and after the lipid extraction process, were energetically valorized into biogas production through anaerobic digestion (AD). The tested wastes were extracted microalgae (Ae) with hexane (AeH) using supercritical extraction methods (AeS) and the supernatant obtained after culture medium centrifugation (M). The digestion of the algae biomass in the admixture with the supernatant medium (AeH+M+I and AeS+M+I) provided a higher methane content and a higher methane yield (582 and 440 L CH4/kg VS) than the substrates Ae and M, individually digested (155 and 96 L CH4/kg VS, respectively). Flow cytometry monitoring processes during AD indicated that the yield of the accumulated biogas was influenced by the operating conditions. The mixture of AeH+M+I was the only assay with a proportion of cells with less damaged membranes after AD, providing the highest methane yield and productivity (582 L CH4/kg VS and 31 L CH4/kg VS.d, respectively) and the highest energetic potential of 5.8 KWh/kg VS of all the substrates. From the results, AD integration to lipid production by C. cohnii to recover energy from the generated wastes enhanced the sustainability of the entire process and promoted the practice of zero waste.

Overview of recent progress in exogenous hydrogen supply biogas upgrading and future perspective

With the rapid development of renewable and sustainable energy, biogas upgrading for producing high-quality biomethane as an alternative to natural gas has attracted worldwide attention. This paper comprehensively reviews the current state of biogas upgrading technologies. The advances in physicochemical, photosynthetic autotrophic, and chemical autotrophic biogas upgrading technologies are briefly described with particular attention to the key challenges. New chemical autotrophic biogas upgrading strategies, such as direct and indirect exogenous hydrogen supply, for overcoming barriers to biogas upgrading and realizing highly efficient bioconversion of carbon dioxide are summarized. For each approach to exogenous hydrogen supply for biogas upgrading, the key findings and technical limitations are summarized and critically analyzed. Finally, future developments are also discussed to provide a reference for the development of biogas upgrading technology that can address the global energy crisis and climate change issues related to the application of biogas.

Antibiotics in anaerobic digestion: Investigative studies on digester performance and microbial diversity

The ever-increasing consumption of antibiotics in both humans and animals has increased their load in municipal and pharmaceutical industry waste and may cause serious damage to the environment. Impact of antibiotics on the performance of commercially used anaerobic digesters in terms of bioenergy output, antibiotics' removal and COD removal have been compared critically with a few studies indicating >90% removal of antibiotics. AnMBR performed the best in terms of antibiotic removal, COD removal and methane yield. Most of the antibiotics investigated have adverse effects on microbiome associated with different stages and methane generation pathways of AD which has been assessed using high throughput technologies like metatranscriptomics, metaproteomics and flow cytometry. Perspectives have been given for understanding the fate and elimination of antibiotics from AD. The challenge of optimization and process improvement needs to be addressed to increase efficiency of the anaerobic digesters.

Microalgae cultivation strategies using cost-effective nutrient sources: Recent updates and progress towards biofuel production

Scientists are grabbing huge attention as well as consciousness on non-renewable energy sources for the global energy crises because of gradual increase in oil price, fast depletion or low availability of resources, and the release of more toxic-gases (CO₂, SO_x, N_xO) during exhaustion, etc. Due to such hitches, the key need is to find alternative biofuels or feedstocks to replace fossil fuel energy demands worldwide. Currently, microalgae have become intrigued feedstock candidates (3rd generation source of biofuel) to replace nearly 50-60 % of fossil fuels due to high production of biomass and oil, mitigating CO₂ and wastewater remediation. The present work demonstrated the current developments and future perspectives on large-scale algal cultivation strategies for the biorefinery economy. In addition, various advanced cultivation techniques adopted for enhanced biomass production and cost-effective methods for bioenergy production were detailly discussed.

Microalgae and Cyanobacteria Biomass Pretreatment Methods: A Comparative Analysis of Chemical and Thermochemical Pretreatment Methods Aimed at Methane Production

Anaerobic digestion of microalgae and cyanobacteria was first proposed as a destination for algal biomass accumulated on stabilization ponds since it could not be disposed of directly in the environment. Now, the versatility of algal biomass makes them a suitable candidate to produce biofuels and other biomolecules in biorefineries. Anaerobic digestion of biomass is advantageous because it does not require the extraction of specific cellular constituents or drying of the biomass. Nevertheless, challenges remain regarding biomass concentration and their resistant cell walls, which are factors that could hamper methane yield. Many pretreatment methods, including chemical and thermochemical, have been proposed to break down the complex polymers present on the cell wall into smaller molecules. Unfortunately, the relationship between biomass solubilization and methane yield is not well defined. This article intends to review the anaerobic digestion of algal biomass and the role of chemical and thermochemical pretreatments in enhancing methane production. Several pretreatment conditions selected from the scientific literature were compared to verify which conditions actually improve methane yield. The severity of the selected pretreatments was also assessed using the combined severity factor. Results suggest that thermochemical pretreatment in less severe conditions is the most efficient, leading to a greater increase in methane yield. Only enzymatic pretreatments and some thermal pretreatments result in a positive energy balance. The large-scale implementation of pretreatment methods requires technological innovations to reduce energy consumption and its integration with other processes in wastewater treatment plants.

Characterization of microbial communities in anaerobic acidification reactors fed with casein and/or lactose

Abstract

Protein-rich agro-industrial waste streams are high in organic load and represent a major environmental problem. Anaerobic digestion is an established technology to treat these streams; however, retardation of protein degradation is frequently observed when carbohydrates are present. This study investigated the mechanism of the retardation by manipulating the carbon source fed to a complex anaerobic microbiota and linking the reactor performance to the variation of the microbial community. Two anaerobic acidification reactors were first acclimated either to casein (CAS reactor) or lactose (LAC reactor), and then fed with mixtures of casein and lactose. Results showed that when lactose was present, the microbial community acclimated to casein shifted from mainly Chloroflexi to Proteobacteria and Firmicutes, the degree of deamination in the CAS reactor decreased from 77 to 15%, and the VFA production decreased from 75 to 34% of the effluent COD. A decrease of 75% in protease activity and 90% in deamination activity of the microbiota was also observed. The microorganisms that can ferment both proteins and carbohydrates were predominant in the microbial community, and from a thermodynamical point of view, they consumed carbohydrates prior to proteins. The frequently observed negative effect of carbohydrates on protein degradation can be mainly attributed to the substrate preference of these populations.

Keypoints

• The presence of lactose shifted the microbial community and retarded anaerobic protein degradation.

• Facultative genera were dominant in the presence and absence of lactose.

• Substrate-preference caused retardation of anaerobic protein degradation.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-022-12132-5.

Improvement in Methane Production from Pelagic Sargassum Using Combined Pretreatments

The constant golden tides of Sargassum spp., identified to be a mixture of Sargassum natans and Sargassum fluitans, observed recently in the Mexican Caribbean have affected the marine ecosystem and the local economy and have created the need for solutions for their management and use. The Sargassum arrivals have thus been considered as third-generation feedstock for biofuel. Their potential for energetic conversion to biomethane was investigated, with hydrolysis as the limiting step due to its complex composition; therefore, in the present study, different physical, chemical, and enzymatic pretreatments and a combination of them have been evaluated, with the additional use of granular activated carbon, to determine the best yield and methane quality. The combined pretreatments of 2.5% hydrogen peroxide, followed by an enzymatic pretreatment (enzymatic extract from Trametes hirsuta isolated from decomposing wood in the Yucatán Peninsula-Mexico), was the best option, reaching a biodegradability of 95% and maximum methane yield of 387 ± 3.09 L CH₄/kg volatile solid. The use of a conductive material, such as granular activated carbon, did not generate significant changes in performance and methane concentration.

Field test of water-net based wastewater treatment for nutrient removal and bioethanol production

In this study, an open pond constructed in Myanmar, a region with tropical climate and favorable environmental conditions for algae growth, was considered to conduct field experiments on sewage inflow river water. The nutrient removal efficiency and productivity of Hydrodictyon reticulatum (H. reticulatum) were analyzed, and the maximum fermentation limit concentration for bioethanol production was determined. Three ponds were operated in batch mode to investigate the effect of light intensity. Photoinhibition was caused due to excessive light intensity in summer season in the region with tropical climate resulting in reduced facility efficiency in the absence of shade. For light blocking, a transparent film was found to be more effective than a translucent film. In the transparent film shading facility, the nitrogen and phosphorus removal efficiencies were maintained above 76% and 81%, respectively, and the productivity of H. reticulatum was 2.27 g m^-2 d^-1. For a raceway open pond facility shaded with transparent film, the performance was evaluated based on hydraulic retention time (HRT), and the productivity of algae was found to increase with increasing supply of nitrogen and phosphorous. Maximum biomass production of 3.21 g m^-2 d^-1 was observed with an HRT of 3 d, suggesting the possibility of long-term operation. As a result of evaluating the ethanol production based on the initial concentration of H. reticulatum, the yield of bioethanol at the initial reducing sugar content of 120 g L^-1 was 89.4%, but bioethanol production was only 8.9 g L^-1.

Dynamic Foam Characteristics during Cultivation of Arthrospira platensis

This study is aimed at understanding the serious foaming problems during microalgal cultivation in industrial raceway ponds by studying the dynamic foam properties in Arthrospira platensis cultivation. A. platensis was cultivated in a 4 L bowl bioreactor for 4 days, during which the foam height above the algal solution increased from 0 to 30 mm with a bubble diameter of 1.8 mm, and biomass yield reached 1.5 g/L. The algal solution surface tension decreased from 55 to 45 mN/m, which favored the adsorption of microalgae on the bubble to generate more stable foams. This resulted in increased foam stability (FS) from 1 to 10 s, foam capacity (FC) from 0.3 to 1.2, foam expansion (FE) from 15 to 43, and foam maximum density (FMD) from 0.02 to 0.07. These results show a decrease in CO₂ flow rate and operation temperature when using the Foamscan instrument, which minimized the foaming phenomenon in algal solutions to a significantly lower and acceptable level.

Sustainable microalgal biomass valorization to bioenergy: Key challenges and future perspectives

The global trend is shifting toward circular economy systems. It is a sustainable environmental approach that sustains economic growth from the use of resources while minimizing environmental impacts. The multiple industrial use of microalgal biomass has received great attention due to its high content of essential nutrients and elements. Nevertheless, low biomass productivity, unbalanced carbon to nitrogen (C/N) ratio, resistant cellular constituents, and the high cost of microalgal harvesting represent the major obstacles for valorization of algal biomass. In recent years, microalgae biomass has been a candidate as a potential feedstock for different bioenergy generation processes with simultaneous treating wastewater and CO₂ capture. An overview of the appealing features and needed advancements is urgently essential for microalgae-derived bioenergy generation. The present review provides a timely outlook and evaluation of biomethane production from microalgal biomass and related challenges. Moreover, the biogas recovery potential from microalgal biomass through different pretreatments and synergistic anaerobic co-digestion (AcoD) with other biowastes are evaluated. In addition, the removal of micropollutants and heavy metals by microalgal cells via adsorption and bioaccumulation in their biomass is discussed. Herein, a comprehensive review is presented about a successive high-throughput for anaerobic digestion (AD) of the microalgal biomass in order to achieve for sustainable energy source. Lastly, the valorization of the digestate from AD of microalgae for agricultural reuse is highlighted.

Phytochemical and Potential Properties of Seaweeds and Their Recent Applications: A Review

Since ancient times, seaweeds have been employed as source of highly bioactive secondary metabolites that could act as key medicinal components. Furthermore, research into the biological activity of certain seaweed compounds has progressed significantly, with an emphasis on their composition and application for human and animal nutrition. Seaweeds have many uses: they are consumed as fodder, and have been used in medicines, cosmetics, energy, fertilizers, and industrial agar and alginate biosynthesis. The beneficial effects of seaweed are mostly due to the presence of minerals, vitamins, phenols, polysaccharides, and sterols, as well as several other bioactive compounds. These compounds seem to have antioxidant, anti-inflammatory, anti-cancer, antimicrobial, and anti-diabetic activities. Recent advances and limitations for seaweed bioactive as a nutraceutical in terms of bioavailability are explored in order to better comprehend their therapeutic development. To further understand the mechanism of action of seaweed chemicals, more research is needed as is an investigation into their potential usage in pharmaceutical companies and other applications, with the ultimate objective of developing sustainable and healthier products. The objective of this review is to collect information about the role of seaweeds on nutritional, pharmacological, industrial, and biochemical applications, as well as their impact on human health.

Microalgal Biomass as Feedstock for Bacterial Production of PHA: Advances and Future Prospects

The search for biodegradable plastics has become the focus in combating the global plastic pollution crisis. Polyhydroxyalkanoates (PHAs) are renewable substitutes to petroleum-based plastics with the ability to completely mineralize in soil, compost, and marine environments. The preferred choice of PHA synthesis is from bacteria or archaea. However, microbial production of PHAs faces a major drawback due to high production costs attributed to the high price of organic substrates as compared to synthetic plastics. As such, microalgal biomass presents a low-cost solution as feedstock for PHA synthesis. Photoautotrophic microalgae are ubiquitous in our ecosystem and thrive from utilizing easily accessible light, carbon dioxide and inorganic nutrients. Biomass production from microalgae offers advantages that include high yields, effective carbon dioxide capture, efficient treatment of effluents and the usage of infertile land. Nevertheless, the success of large-scale PHA synthesis using microalgal biomass faces constraints that encompass the entire flow of the microalgal biomass production, i.e., from molecular aspects of the microalgae to cultivation conditions to harvesting and drying microalgal biomass along with the conversion of the biomass into PHA. This review discusses approaches such as optimization of growth conditions, improvement of the microalgal biomass manufacturing technologies as well as the genetic engineering of both microalgae and PHA-producing bacteria with the purpose of refining PHA production from microalgal biomass.

Recovery of Nutrients from Residual Streams Using Ion-Exchange Membranes: Current State, Bottlenecks, Fundamentals and Innovations

The review describes the place of membrane methods in solving the problem of the recovery and re-use of biogenic elements (nutrients), primarily trivalent nitrogen N^III and pentavalent phosphorus P^V, to provide the sustainable development of mankind. Methods for the recovery of NH₄⁺ − NH₃ and phosphates from natural sources and waste products of humans and animals, as well as industrial streams, are classified. Particular attention is paid to the possibilities of using membrane processes for the transition to a circular economy in the field of nutrients. The possibilities of different methods, already developed or under development, are evaluated, primarily those that use ion-exchange membranes. Electromembrane methods take a special place including capacitive deionization and electrodialysis applied for recovery, separation, concentration, and reagent-free pH shift of solutions. This review is distinguished by the fact that it summarizes not only the successes, but also the “bottlenecks” of ion-exchange membrane-based processes. Modern views on the mechanisms of NH₄⁺ − NH₃ and phosphate transport in ion-exchange membranes in the presence and in the absence of an electric field are discussed. The innovations to enhance the performance of electromembrane separation processes for phosphate and ammonium recovery are considered.

Breakthrough in hydrolysis of waste biomass by physico-chemical pretreatment processes for efficient anaerobic digestion

Anaerobic digestion (AD) is the most comprehended process to stabilise the waste biomass efficiently and to obtain bioenergy. The AD starts with the hydrolysis process, where the major liability is the action of inhibitors during the hydrolysis process. The biomass pretreatment preceding anaerobic digestion is obligatory to improve feedstock biodegradability for enhanced biogas generation. It can be prevailed by the application of various pretreatment processes. This review explains the major inhibiting compounds and their formation during hydrolysis that affect the efficiency of anaerobic digestion and the benefits of the physico-chemical pretreatment (PCP) method for enhancing hydrolysis in the digestion of waste biomass. The synergistic effect of PCP on macromolecular release, liquefaction and biodegradability were presented. The feasibility of the pretreatment process was evaluated in terms of energy and cost assessment for pilot scale implementation. The outcome of this review reveals that the physico-chemical process is one of the best pretreatment methods to enhance anaerobic digestion by optimising various parameters and increasing the solubilization by about 90%. The thermochemical pretreatment at lower temperature (<100) increases the net energy yield. The solubilization of waste biomass in terms of macromolecular release and liquefaction cannot describe the pretreatment potential. The effectiveness of pretreatment was evaluated by the substrate pre-treatment followed by anaerobic digestibility of pretreated substrate.

Integrated marine microalgae biorefineries for improved bioactive compounds: A review

Marine microalgae offer a promising feedstock for biofuels and other valuable compounds for biorefining and carry immense potential to contribute to a clean energy and environment future. However, it is currently not economically feasible to use marine algae to produce biofuels, and the potential bioactive chemicals account for only a small market share. The production of algal biomass with multiple valuable chemicals is closely related to the algal species, cultivation conditions, culture systems, and production modes. Thus, higher requirements for screening of dominant algal strains, developing integrated technologies with the optimum culture conditions, efficient cultivation systems, and production modes to exploit algal biomass for biorefinery applications, are all needed. This review summarizes the screening of dominant microalgae, discusses the environmental conditions that may affect the growth, as well as the culture systems and production modes, and further emphasizes the valorization options of the algal biomass, which should help to offer a sustainable approach to run a profitable marine algae production system.

Algae: Study of Edible and Biologically Active Fractions, Their Properties and Applications

The beneficial properties of algae make them perfect functional ingredients for food products. Algae have a high energy value and are a source of biologically active substances, proteins, fats, carbohydrates, vitamins, and macro- and microelements. They are also rich in polyunsaturated fatty acids, proteins, mycosporine-like amino acids, polysaccharides, polyphenols, carotenoids, sterols, steroids, lectins, halogenated compounds, polyketides, alkaloids, and carrageenans. Different extraction parameters are used depending on the purpose and the substances to be isolated. In this study, the following parameters were used: hydromodule 1:10 and an extraction duration of 1-2 h at the extraction temperature of 25-40 °C. A 30-50% solution of ethanol in water was used as an extractant. Algae extracts can be considered as potential natural sources of biologically active compounds with antimicrobial activity and antiviral properties. The content of crude protein, crude fat, and carbohydrates in U. Prolifera, C. racemosa var. peltata (Chlorophyta), S. oligocystum and S. fusiforme (SF-1) was studied. It was found that C. muelleri (Bacillariophyta), I. galbana (Haptophyta), and T. weissflogii (Bacillariophyta) contain about 1.9 times more omega-3 than omega-6 fatty acids. N. gaditana (Ochrophyta), D. salina (Chlorophyta), P. tricornutum (Bacillaryophyta) and I. galbana (Haptophyta) extracts showed inhibitory activity of varying intensities against E. coli or P. aeruginosa. In addition, algae and algae-derived compounds have been proposed to offer attractive possibilities in the food industry, especially in the meat sector, to evolve functional foods with myriad functionalities. Algae can increase the biological activity of food products, while the further study of the structure of compounds found in algae can broaden their future application possibilities.