New insights on improved growth and biogas production potential of Chlorella pyrenoidosa through intermittent iron oxide nanoparticle supplementation

Effect of pretreated and anaerobically digested microalgae on the chemical and biochemical properties of soil and wheat grown on fluvisol

In this study, the effects of the potential application of digestate as an agricultural fertiliser obtained from anaerobically digested microalgae treated by three pretreatment methods, namely alkaline hydrogen peroxide (AHP), high temperature and pressure (HTP), and hydrodynamic cavitation (HC) on some properties of soil, and wheat growth and yield were investigated. For this purpose, pretreated and anaerobically digested microalgae digestates alone or together with diammonium phosphate (DAP) as a chemical fertiliser were applied to soil for wheat growth. The highest dosage of AHP pretreated digestate combined with a half dose of DAP applied to soil was rich in nutrients as 0.25%N and 7.19 mg kg-1 compared to all groups. The properties of the soils were enhanced by applying the highest dosage (0.06 g kg-1) of microalgae digestate combined with a half dose of DAP. 0.02 g kg-1 dosage of HC pretreated digestate combined with a half dose of DAP also greatly improved nitrogen use efficiency indices by up to 104%. The soils' enzyme activities increased in wheat growth experiments by applying either raw or pretreated microalgae digestates. The soils' β-glycosidase, alkaline phosphatase, and urease enzyme activities increased to 1.38 mg pNP g-1 soil, 4.91 mg pNP g-1 soil, and 2.27 mg NH4-N 100 g-1 soil respectively by the application of highest dosage of HC pretreated digestate. The digestates did not have a toxic effect on wheat growth, it was determined that applied pretreatment processes did not cause significant changes in wheat plant height or wet and dry weight.

Interaction of Fe2O3 nanoparticles with marine microalga Chlorella sorokiniana: Analysis of growth, morphological changes and biochemical composition

The wide utilization of iron-based nanoparticles (NPs) based on their preferential properties has led to the discharge and accumulation of these materials into the aquatic environment. In this regard, a comparative study of different concentrations of α-Fe2O3 NPs and their micro form was conducted using microalga Chlorella sorokiniana up to the stationary growth phase. This study revealed that high concentrations of NPs (100 and 200 mg L-1) imposed a stressful condition on algal cells documented by a reduction in microalga growth, including cell number and specific growth rate. The physical contact between the algal cells and NPs resulted in a shading effect as well as morphological changes validated by scanning electron microscope results. The biochemical composition of C. sorokiniana exposed to high levels of Fe2O3 NPs was also evaluated. The increase in total carbohydrate content of algal cells along with a significant reduction in unsaturated fatty acids was found. Moreover, Fe2O3 NPs exposure induced oxidative stress evidenced by an increase in lipid peroxidation. To cope with oxidative stress, superoxide dismutase activity and antioxidant potential of microalga as defensive mechanisms increased in the culture with high concentrations of NPs. Besides, due to the interactions, microalga tended to form a protective layer from further cell-NP interactions through the secretion of extracellular polymeric substances. Nonetheless, the nano form of Fe2O3 was more toxic than its micro form due to its small size. Overall, this trial may provide additional insight into the toxicological mechanism and safety assessments of Fe2O3 NPs in the aquatic environment.

Influence of Fe2O3 Nanoparticles on the Anaerobic Digestion of Macroalgae Sargassum spp

The anaerobic digestion (AD) of biomass is a green technology with known environmental benefits for biogas generation. The biogas yield from existing substrates and the biodegradability of biomasses can be improved by conventional or novel enhancement techniques, such as the addition of iron-based nanoparticles (NPs). In this study, the effect of different concentrations of Fe2O3-based NPs on the AD of brown macroalga Sargassum spp. has been investigated by 30 days trials. The effect of NPs was evaluated at different concentrations. The control sample yielded a value of 80.25 ± 3.21 NmLCH4/gVS. When 5 mg/g substrate and 10 mg/g substrate of Fe2O3 NPs were added to the control sample, the yield increased by 24.07% and 26.97%, respectively. Instead, when 50 mg/g substrate of Fe2O3 NPs was added to the control sample, a negative effect was observed, and the biomethane yield decreased by 38.97%. Therefore, low concentrations of Fe2O3 NPs favor the AD process, whereas high concentrations have an inhibitory effect. Direct interspecies electron transfer (DIET) via Fe2O3 NPs and their insolubility play an important role in facilitating the methanogenesis process during AD.

Anaerobic digestion as a tool to manage eutrophication and associated greenhouse gas emission

Eutrophicated inland water bodies are noticed to be one of the contributing factors to greenhouse gas (GHGs) emissions. Direct discharge of untreated or partially treated water is a major concern. Microalgae-based technology and management are regarded as one of the potential nature-based approaches to combat eutrophication. In turn, the microalgae facilitate the recovery of GHGs contributing compounds in the form of organic biomass. The recovered algal biomass can be harnessed for the production of biofuels and other bio-products, like biofertilizer, using anaerobic digestion. By virtue, circular bio-economy can be achieved alongside mitigating GHGs emissions. Before implementing, it is vital to thoroughly explore the links between the process and potential alternatives for wastewater treatment, waste valorization, biofuel production, and land usage. Thus, the present review discusses the impact of eutrophication on ecology and environment, current technologies for mitigating eutrophication and GHGs, and energy recovery through the anaerobic digestion of algal biomass. Further, the processes at the intercept of wastewater treatment and biogas production were reviewed to leverage the potential of anaerobic digestion for making a circular bioeconomy framework.

Effects of synthesized AlFe2O4 and MgFe2O4 nanoparticles on biogas production from anaerobically digested sugar refinery wastewater

The application of magnetic nanoparticles (MNPs) toward sustainable water economy and bioenergy production has become a subject of great interest. Anaerobic digestion (AD) has been widely exploited in wastewater treatment settings, whereby utilization of MNP additives by microorganism response for degradation of organics into biogas is seen to be eco-friendly and economically viable. The present study investigated the impact of two synthesized MNPs such as aluminum ferrite (AlFe₂O₄) and magnesium ferrite (MgFe₂O₄) on biogas production via biochemical methane potential (BMP) tests. A BMP set-up of three (3) bioreactors was tested with a working volume of 0.8 L which comprises 0.5 L wastewater (WW) and 0.3 L activated sludge (AS) with 1.5 g of the MNPs and a control system without MNPs. The degradation of chemical oxygen demand (COD) was increased with set-ups that contained MNPs as compared to the control system, MgFe₂O₄ (93.96%) > AlFe₂O₄ (85.95%) > control (68.83%). In terms of biogas production, the methane yield was also recorded as MgFe₂O₄ (85.7%) > AlFe₂O₄ (84.3%) > control (65.7%). The physical and chemical stability of MNPs makes them more advantageous for application in biogas production. In the prospects of biogas enhancement and biodegradability, integrating MNPs in an anaerobic digestion system will result in a more efficient anaerobic process performance for biogas production.

Magnetite-Bacillus subtilis synergy on the metabolic selection of products in electrofermentation system

The study examines the role of magnetite (1-150 mg/L) at the interface of Bacillus subtilis-electrode under poised-condition (-0.2 V) for product-formation and catalytic-conduct with the relative-gene-expression encoding lactate dehydrogenase (lctE), pyruvate dehydrogenase (pdhA), acetate kinase (ackA), pyruvate carboxylase (pycA), and NADH dehydrogenase (ndh). The magnetite load of 25 mg/L showed positive influence on acidogenesis resulting in H₂ production of 264.7 mol/mL and fatty acids synthesis of 3.6 g/L. Additionally, this condition showed higher succinic acid productivity (2.8 g/L) which correlates with the upregulated pycA gene and fumarate to succinate redox peak. With 10 mg/L loading, production of higher acetic acid (3.1 g/L) along with H₂ (181.6 mol/mL) was depicted wherein upregulation of pdhA, ackA and ndh genes was observed. In absence of magnetite, lctE gene was upregulated which resulted higher lactate production. The findings suggest that the mutual-interactions between magnetite-active sites of specific enzymes enhances the biocatalytic activity triggering product-formation.

Use of reverse osmosis reject from drinking water plant for microalgal biomass production

The present study evaluates the use of reverse osmosis (RO) reject, termed as ROR, for microalgal biomass production. The supplementation of ROR from two different sources, namely domestic RO unit (ROR1) and commercial-scale RO plant (ROR2), showed a synergistic effect on the growth and biochemical composition of Chlorella pyrenoidosa. Among the tested ROR1 doses, the highest biomass production (1.27±0.06 g L^-1) was observed with 25% ROR1 supplemented growth media. In contrast, the lipid content (28.85±3.13% of TS) in C. pyrenoidosa at 50% ROR1 dose was almost twice that in BG11 (positive control). Interestingly, the microalgae showed relatively higher biomass production (1.37±0.07 g L^-1) and higher lipid content (33.23±3.92% of TS) when 50% ROR2 was used in growth media. At the same time, the estimated carbohydrate and protein contents were 28.41±0.73 and 29.75±0.31% of TS, respectively. Furthermore, the lipid productivity (28.98±2.79 mg L^-1 d^-1) was relatively higher than the nutrient media (12.35±1.34 mg L^-1 d^-1). The present findings revealed that the RO reject from drinking water purifiers can efficiently be utilized for lipid-rich microalgal biomass production. Hence, the dependency on freshwater resources for mass scale microalgae cultivation through recycling of RO reject can be reduced.

Overview on agricultural potentials of biogas slurry (BGS): applications, challenges, and solutions

The residual slurry obtained from the anaerobic digestion (AD) of biogas feed substrates such as livestock dung is known as BGS. BGS is a rich source of nutrients and bioactive compounds having an important role in establishing diverse microbial communities, accelerating nutrient use efficiency, and promoting overall soil and plant health management. However, challenges such as lower C/N transformation rates, ammonia volatilization, high pH, and bulkiness limit their extensive applications. Here we review the strategies of BGS valorization through microbial and organomineral amendments. Such cohesive approaches can serve dual purposes viz. green organic inputs for sustainable agriculture practices and value addition of biomass waste. The literature survey has been conducted to identify the knowledge gaps and critically analyze the latest technological interventions to upgrade the BGS for potential applications in agriculture fields. The major points are as follows: (1) Bio/nanotechnology-inspired approaches could serve as a constructive platform for integrating BGS with other organic materials to exploit microbial diversity dynamics through multi-substrate interactions. (2) Advancements in next-generation sequencing (NGS) pave an ideal pathway to study the complex microflora and translate the potential information into bioprospecting of BGS to ameliorate existing bio-fertilizer formulations. (3) Nanoparticles (NPs) have the potential to establish a link between syntrophic bacteria and methanogens through direct interspecies electron transfer and thereby contribute towards improved efficiency of AD. (4) Developments in techniques of nutrient recovery from the BGS facilities' negative GHGs emissions and energy-efficient models for nitrogen removal. (5) Possibilities of formulating low-cost substrates for mass-multiplication of beneficial microbes, bioprospecting of such microbes to produce bioactive compounds of anti-phytopathogenic activities, and developing BGS-inspired biofertilizer formulations integrating NPs, microbial inoculants, and deoiled seed cakes have been examined.

Fate and impact of maghemite (γ-Fe2O3) and magnetite (Fe3O4) nanoparticles in barley (Hordeum vulgare L.)

The increasing demand for food in the world has made sustainable agriculture practices even more important. Nanotechnology applications in many areas have also been used in sustainable agriculture in recent years for the purposes to improve plant yield, pest control, etc. However, ecotoxicology and environmental safety of nanoparticles must be evaluated before large-scale applications. This study comparatively explores the efficacy and fate of different iron oxide NPs (γ-Fe₂O₃-maghemite and Fe₃O₄-magnetite) on barley (Hordeum vulgare L.). Various NP doses (50, 100, and 200 mg/L) were applied to the seeds in hydroponic medium for 3 weeks. Results revealed that γ-Fe₂O₃ and Fe₃O₄ NPs significantly improved the germination rate (~37% for γ-Fe₂O₃; ~63% for Fe₃O₄), plant biomass, and pigmentation (P < 0.005). Compared to the control, the iron content of tissues gradually raised by the increasing NPs doses revealing their translocation, which is confirmed by VSM analysis as well. The findings suggest that γ-Fe₂O₃ and Fe₃O₄ NPs have great potential to improve barley growth. They can be recommended for breeding programs as nanofertilizers. However, special care should be paid before the application due to their unknown effects on other living beings.

Harnessing Solar Energy using Phototrophic Microorganisms: A Sustainable Pathway to Bioenergy, Biomaterials, and Environmental Solutions

Phototrophic microorganisms (microbial phototrophs) use light as an energy source to carry out various metabolic processes producing biomaterials and bioenergy and supporting their own growth. Among them, microalgae and cyanobacteria have been utilized extensively for bioenergy, biomaterials, and environmental applications. Their superior photosynthetic efficiency, lipid content, and shorter cultivation time compared to terrestrial biomass make them more suitable for efficient production of bioenergy and biomaterials. Other phototrophic microorganisms, especially anoxygenic phototrophs, demonstrated the ability to survive and flourish while producing renewable energy and high-value products under harsh environmental conditions. This review presents a comprehensive overview of microbial phototrophs on their (i) production of bioenergy and biomaterials, (ii) emerging and innovative applications for environmental conservation, mitigation, and remediation, and (iii) physical, genetic, and metabolic pathways to improve light harvesting and biomass/biofuel/biomaterial production. Both physical (e.g., incremental irradiation) and genetic approaches (e.g., truncated antenna) are implemented to increase the light-harvesting efficiency. Increases in biomass yield and metabolic products are possible through the manipulation of metabolic pathways and selection of a proper strain under optimal cultivation conditions and downstream processing, including harvesting, extraction, and purification. Finally, the current barriers in harnessing solar energy using phototrophic microorganisms are presented, and future research perspectives are discussed, such as integrating phototrophic microorganisms with emerging technologies.

Microalgal Biodiesel Production: Realizing the Sustainability Index

Search for new and renewable sources of energy has made research reach the tiny little tots, microalgae for the production of biodiesel. But despite years of research on the topic, a definitive statement, declaring microalgae as an economically, environmentally, and socially sustainable resource is yet to be seen or heard of. With technological and scientific glitches being blamed for this delay in the progress of the production system, an assessment of the sustainability indices achieved so far by the microalgal biodiesel is important to be done so as to direct future research efforts in a more coordinated manner to achieve the sustainability mark. This article provides a review of the current economic, environmental, and social status of microalgal biodiesel and the strategies adopted to achieve them, with suggestions to address the challenges faced by the microalgal biodiesel production system.