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Extensive anthropogenic activity has led to the accumulation of organic and inorganic contaminants in diverse ecosystems, which presents significant challenges for the environment and its inhabitants. Utilizing microalgae as a bioremediation tool can present a potential solution to these challenges. Microalgae have gained significant attention as a promising biotechnological solution for detoxifying environmental pollutants. This is due to their advantages, such as rapid growth rate, cost-effectiveness, high oil-rich biomass production, and ease of implementation. Moreover, microalgae-based remediation is more environmentally sustainable for not generating additional waste sludge, capturing atmospheric CO2, and being efficient for nutrient recycling and sustainable algal biomass production for biofuels and high-value-added products generation. Hence, microalgae can achieve sustainability's three main pillars (environmental, economic, and social). Microalgal biomass can mediate contaminated wastewater effectively through accumulation, adsorption, and metabolism. These mechanisms enable the microalgae to reduce the concentration of heavy metals and organic contaminants to levels that are considered non-toxic. However, several factors, such as microalgal strain, cultivation technique, and the type of pollutants, limit the understanding of the microalgal removal mechanism and efficiency. Furthermore, adopting novel technological advancements (e.g., nanotechnology) may serve as a viable approach to address the challenge of refractory pollutants and bioremediation process sustainability. Therefore, this review discusses the mechanism and the ability of different microalgal species to mitigate persistent refractory pollutants, such as industrial effluents, dyes, pesticides, and pharmaceuticals. Also, this review paper provided insight into the production of nanomaterials, nanoparticles, and nanoparticle-based biosensors from microalgae and the immobilization of microalgae on nanomaterials to enhance bioremediation process efficiency. This review may open a new avenue for future advancing research regarding a sustainable biodegradation process of refractory pollutants. [ABSTRACT FROM AUTHOR]

Simple Summary: The accumulation of recalcitrant pollutants (e.g., lignin biomass and synthetic textile dyes) in diverse ecosystems has exacerbated the problem of environmental pollution. The complex polymer structure of lignin brings huge challenges to the development of high-efficiency transformation technology, making it difficult to realize its industrial utilization. On the other hand, the use of dyes in the textile industry poses significant challenges worldwide. Dyes are very similar to lignin biomass in chemical structure. Similarly, under natural conditions, it is difficult for microorganisms in the environment to achieve rapid biodegradation and complete detoxification. Clearly, the integration of bioremediation and biorefinery technology towards such recalcitrant organic wastes is considered a novel concept for mitigating the toxicity of such pollutants using termite gut symbionts. Modern biorefinery and bioremediation applications can integrate the termite gut system, a unique bioresource that comprises distinct bacterial species valued for lignocellulosic material processing and synthetic dye degradation. Therefore, this review paper provides a new strategy for efficient management of recalcitrant pollutants by exploring the potential application of termite gut bacteria in terms of science and industry. Lignocellulosic biomass (LCB) in the form of agricultural, forestry, and agro-industrial wastes is globally generated in large volumes every year. The chemical components of LCB render them a substrate valuable for biofuel production. It is hard to dissolve LCB resources for biofuel production because the lignin, cellulose, and hemicellulose parts stick together rigidly. This makes the structure complex, hierarchical, and resistant. Owing to these restrictions, the junk production of LCB waste has recently become a significant worldwide environmental problem resulting from inefficient disposal techniques and increased persistence. In addition, burning LCB waste, such as paddy straws, is a widespread practice that causes considerable air pollution and endangers the environment and human existence. Besides environmental pollution from LCB waste, increasing industrialization has resulted in the production of billions of tons of dyeing wastewater from several industries, including textiles, pharmaceuticals, tanneries, and food processing units. The massive use of synthetic dyes in various industries can be detrimental to the environment due to the recalcitrant aromatic structure of synthetic dyes, similar to the polymeric phenol lignin in LCB structure, and their persistent color. Synthetic dyes have been described as possessing carcinogenic and toxic properties that could be harmful to public health. Environmental pollution emanating from LCB wastes and dyeing wastewater is of great concern and should be carefully handled to mitigate its catastrophic effects. An effective strategy to curtail these problems is to learn from analogous systems in nature, such as termites, where woody lignocellulose is digested by wood-feeding termites and humus-recalcitrant aromatic compounds are decomposed by soil-feeding termites. The termite gut system acts as a unique bioresource consisting of distinct bacterial species valued for the processing of lignocellulosic materials and the degradation of synthetic dyes, which can be integrated into modern biorefineries for processing LCB waste and bioremediation applications for the treatment of dyeing wastewaters to help resolve environmental issues arising from LCB waste and dyeing wastewaters. This review paper provides a new strategy for efficient management of recalcitrant pollutants by exploring the potential application of termite gut bacteria in biorefinery and bioremediation processing. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The presence of high salinity levels in textile wastewater poses a significant obstacle to the process of decolorizing azo dyes. The present study involved the construction of a yeast consortium HYC, which is halotolerant and was recently isolated from wood-feeding termites. The consortium HYC was mainly comprised of Sterigmatomyces halophilus SSA-1575 and Meyerozyma guilliermondii SSA-1547. The developed consortium demonstrated a decolourization efficiency of 96.1% when exposed to a concentration of 50 mg/l of Reactive Black 5 (RB5). The HYC consortium significantly decolorized RB5 up to concentrations of 400 mg/l and in the presence of NaCl up to 50 g/l. The effects of physicochemical factors and the degradation pathway were systematically investigated. The optimal pH, salinity, temperature, and initial dye concentration were 7.0, 3%, 35 °C and 50 mg/l, respectively. The co-carbon source was found to be essential, and the addition of glucose resulted in a 93% decolorization of 50 mg/l RB5. The enzymatic activity of various oxido-reductases was assessed, revealing that NADH-DCIP reductase and azo reductase exhibited greater activity in comparison to other enzymes. UV-Visible (UV–vis) spectrophotometry, Fourier-transform infrared spectroscopy (FTIR), high-performance liquid chromatography (HPLC), and gas chromatography–mass spectrometry (GC-MS) were utilized to identify the metabolites generated during the degradation of RB5. Subsequently, a metabolic pathway was proposed. The confirmation of degradation was established through alterations in the functional groups and modifications in molecular weight. The findings indicate that this halotolerant yeast consortium exhibits promising potential of degrading dye compounds. The results of this study offer significant theoretical basis and crucial perspectives for the implementation of halotolerant yeast consortia in the bioremediation of textile and hypersaline wastewater. This approach is particularly noteworthy as it does not produce aromatic amines. • HYC is a promising halotolerant yeast consortium for azo dye degradation. • HYC decolorized Reactive Black 5 (RB5) up to 400 mg/l and 50 g/l NaCl. • GC-MS, FTIR, and enzyme analysis are used to study decolorization mechanism. • Environmental and nutritional factors affecting RB5 decolorization are discussed. • HYC provides a useful strategy to remediate dyeing contaminated saline wastewater. [ABSTRACT FROM AUTHOR]

• A new dye-degrading oleaginous yeast consortium MG-Y-SH, was constructed. • FAME of MG-Y-SH could represent a promising alternative to commonly used vegetable oils. • Significant enhancement of lipase activity in MG-Y-SH as catalyst for FAME production. • COD reduction of the dye mixtures by MG-Y-SH suggests its high potential for dye treatment. • MG-Y-SH can efficiently decolorize and detoxify RR120 due to its unique enzyme system. A new lipid-accumulating and cold-adapted oleaginous yeast consortium MG-Y-SH which stands for molecularly identified species Meyerozyma guilliermondii , Yarrowia sp. and Sterigmatomyces halophilus was successfully constructed in this study. Its total saturated fatty acid content (34.64 ± 0.95%) was higher than that of jatropha oil (21.52%). The oil obtained from oleaginous yeast consortium MG-Y-SH is advantageous for biodiesel production, since it contains low amount (21.48 ± 1.1%) of polyunsaturated fatty acids (C18:2 and C18:3). Lipase, which is a biocatalyst for the production of biodiesel by oleaginous yeasts, reached its highest specific activity of 8.35 ± 0.14 U/mg (extracellular) and 7.901 ± 0.12 U/mg (intracellular) after 36 h of incubation compared to the individual strains. Seven dye mixtures, six dyes in each group, were constructed and the maximum decolorization efficiency ranged between 55.81% (mixture III) and 80.56% (mixture VI) within 24 h of treatment with MG-Y-SH at 18 °C and under static conditions. The maximum decolorization efficiency by MG-Y-SH reached 100% at 100 mg/L Reactive Red 120 (RR120) within 3 h. Based on our investigation and analysis on those metabolites drawn from the mass spectrum as well as various induced enzymes, a possible dye biodegradation pathway linked to fatty acid synthesis was proposed. The results of phytotoxicity indicate a capability of MG-Y-SH in converting the toxic azo dye RR120 into some non-toxic metabolites, suggesting MG-Y-SH as a promising multipurpose oleaginous yeast consortium suitable for biodiesel production in the future, while degrading recalcitrant dyes and lignin valorization in cold environments. [ABSTRACT FROM AUTHOR]

Protecting the environment from harmful pollutants has become increasingly difficult in recent decades. The presence of heavy metal (HM) pollution poses a serious environmental hazard that requires intricate attention on a worldwide scale. Even at low concentrations, HMs have the potential to induce deleterious health effects in both humans and other living organisms. Therefore, various strategies have been proposed to address this issue, with extremophiles being a promising solution. Bacteria that exhibit resistance to metals are preferred for applications involving metal removal due to their capacity for rapid multiplication and growth. Extremophiles are a special group of microorganisms that are capable of surviving under extreme conditions such as extreme temperatures, pH levels, and high salt concentrations where other organisms cannot. Due to their unique enzymes and adaptive capabilities, extremophiles are well suited as catalysts for environmental biotechnology applications, including the bioremediation of HMs through various strategies. The mechanisms of resistance to HMs by extremophilic bacteria encompass: (i) metal exclusion by permeability barrier; (ii) extracellular metal sequestration by protein/chelator binding; (iii) intracellular sequestration of the metal by protein/chelator binding; (iv) enzymatic detoxification of a metal to a less toxic form; (v) active transport of HMs; (vi) passive tolerance; (vii) reduced metal sensitivity of cellular targets to metal ions; and (viii) morphological change of cells. This review provides comprehensive information on extremophilic bacteria and their potential roles for bioremediation, particularly in environments contaminated with HMs, which pose a threat due to their stability and persistence. Genetic engineering of extremophilic bacteria in stressed environments could help in the bioremediation of contaminated sites. Due to their unique characteristics, these organisms and their enzymes are expected to bridge the gap between biological and chemical industrial processes. However, the structure and biochemical properties of extremophilic bacteria, along with any possible long-term effects of their applications, need to be investigated further. • Extremophiles provide a promising approach for the removal of heavy metals (HMs). • Extremophilic bacteria have unique enzymatic activities and adaptive capabilities. • Species of bacteria for the tolerance of HMs are reviewed. • Resistance mechanisms of tolerance of HMs by extremophilic bacteria are discussed. [ABSTRACT FROM AUTHOR]