Production of artificial humic acid from rice straw for fertilizer production and soil improvement

Transcriptomic insights into methanol utilization in Pichia pastoris lacking AOX genes under co-feeding conditions

The methylotrophic yeast Pichia pastoris (P. pastoris) exhibits remarkable capability for methanol-driven protein biosynthesis, positioning it as an attractive platform for carbon-neutral biomanufacturing utilizing methanol as a renewable feedstock. However, challenges arising from methanol metabolism, particularly the accumulation of toxic formaldehyde intermediates, significantly hinder efficient methanol biotransformation. To address this limitation, we implemented a metabolic engineering strategy involving dual knockout of alcohol oxidase genes (aox1 and aox2) combined with glycerol co-substrate supplementation. Using enhanced green fluorescent protein (EGFP) as a model heterologous product, we demonstrated that the ΔAOX1/2 strain achieved superior protein productivity in glycerol-methanol co-feeding cultures. Under optimized conditions (0.5% methanol + 0.4% glycerol), the engineered strain attained a biomass density of 38.5 (OD600) and EGFP fluorescence intensity of 494,723 units, representing improvements of 32.8% and 53.6%, respectively, compared to the wild-type (WT) strain cultivated with 1% methanol alone. Transcriptome profiling revealed that the observed enhancement in protein synthesis originated from optimized methanol utilization through coordinated upregulation of both assimilatory and dissimilatory metabolic modules. This study demonstrates that alcohol oxidase suppression coupled with glycerol co-metabolism constitutes an effective strategy to alleviate methanol-derived metabolic stress while enhancing heterologous protein yields in P. pastoris.

Integrated biorefinery process for dual production of fermentable sugars and functional humic acids from lignocellulose

In traditional lignocellulosic biorefineries, lignocellulose undergoes pretreatment followed by enzymatic hydrolysis to produce fermentable sugars, often leaving behind lignin rich residual biomass underutilized. In this study, we present a novel and integrated approach for the comprehensive utilization of lignocellulosic biomass as fermentable sugars and functional humic acids. First, corn stover was subjected to a mild densification pretreatment with Ca(OH)₂ (DLCA(ch)), facilitating efficient enzymatic hydrolysis and yielding 172.09 g/L of sugars at 30 % solid loading. Further, the enzymatic hydrolysis residues were subsequently converted into artificial humic acid (AHA) through a mild artificial humification process at 50 °C, catalyzed by potassium hydroxide (KOH) alone or in combination with urea. This process achieved a high AHA yield of 44.17 %, with a final concentration of 78 g/L. The AHA synthesized with KOH and urea (KU-AHA) predominantly consisted of medium molecular weight fractions (30-100 kDa) and exhibited enhanced nitrogen incorporation due to the involvement of urea. Additionally, KU-AHA demonstrated comparable effectiveness to natural humic acid in promoting plant growth, leading to an 13.40 % increase in plant height, an 11.82 % increase in root length. This integrated approach not only maximizes the utilization of lignocellulosic biomass but also provides a sustainable and efficient pathway for residue valorization.

A review on the recovery of humic substances from anaerobic digestate of sludge as a potential fertilizer: Quantification, efficiency and interaction with pollutants

This study reviewed the recovery of humic substances (HS) from anaerobic digestate of sludge as a potential fertilizer, focusing on the quantification of HS, the efficiency of HS recovery, and its interaction with pollutants. The potential pitfalls of current misunderstanding for HS quantification in sludge were pointed out. HS present in sludge showed potential to be used as a fertilizer, which solubilized insoluble phosphates for enhanced soil fertility. Pretreatment prior to anaerobic sludge digestion and composting of anaerobic digestate of sludge (ADS) improved the amount of HS in anaerobic digestate. But the amount of HS extracted from the retentate of ADS was much lower than the level required for the common HS fertilizer. Therefore, an additional concentration was required to process the retentate of ADS into HS liquid fertilizer. The quinone moieties in HS accepted electrons from anaerobic microbial respiration and their role in the degradation of organic pollutants were summarized. By binding with HS, the speciation of metals in sludge was changed from water-soluble and exchangeable to organic- and sulfide-bound fractions. Future research should focus on the novel application of machine learning for quantifying HS within sludge, offering a practical approach to interpret complex fluorescence data and enhance understanding of HS characteristics and distribution. Further studies should explore the application of hydrothermal humification to enhance HS content in ADS, offering an energy-efficient method for rapid fertilization in sustainable agriculture. This study offers a window into HS recovery from ADS.

A review of advancements in humic acid removal: Insights into adsorption techniques and hybrid solutions

Humic acid (HA) is a prominent contaminant in wastewater, and its elimination is crucial to ensure purified drinking water. A variety of sources of HA in wastewater exist, ranging from agricultural runoff, industrial discharges, and natural decomposition. Adsorption is a technique that has been heavily investigated in this direction. The process complexities, technological advancements, and sustainable approaches are discussed in this review. A range of adsorbents can be employed for HA removal, including modified membranes, carbon nanotubes (CNTs), clay nanoparticles, and acid-modified natural materials. This work compares the effectiveness of the preceding adsorbents along with their advantages and limitations. This review also discusses the optimization of various process parameters, such as pH, ionic strength, and temperature, with an emphasis on response surface methodology for process optimization. Furthermore, the challenges and limitations associated with each removal technique are discussed, along with the potential areas for improvement and future directions in the field of wastewater treatment.

Evaluation of hydrochar-derived modifier and water-soluble fertilizer on saline soil improvement and pasture growth

Soil salinization poses a serious threat to crop growth. The selection of appropriate soil modifiers and water-soluble fertilizers for saline soils represents a crucial method for enhancing crop yields. The modifiers and medium-element water-soluble fertilizers were prepared using hydrochar derived from rice straw. Two distinct experiments were designed to study the effect of modifiers and water-soluble fertilizers on saline soils. The first experiment, designated as the "Soil Cultivation Experiment" , sought to investigate the impact of various modifiers on soil quality. The second experiment, designated as the "Method of Field Micro-Area Experiment", aimed to assess the influence of water-soluble fertilizers on saline soils. The results showed that the application of modifiers and water-soluble fertilizers significantly enhanced comprehensive soil physical and chemical properties, crop growth, soil enzyme activity, and other key indicators in saline and alkaline soils. The optimal dosage of the modifier was 20 g/kg, which reduced the pH value from 8.62 to 8.21 and the decreased alkalinity by 8.26%. Furthermore, their application effectively boosted nutrient levels, including organic matter, and increased soil enzyme activity. The biomass of alfalfa showed enhancements of 63.01% and 20.87% and the biomass of leymus chinensis increased by 29.39% and 9.02% for the two batches, respectively. Notably, the application of water-soluble fertilizer yielded achieved superior results. This study also provided a theoretical basis for their future application in soda saline-alkali soil.

Loofah sponge crosslinked polyethyleneimine loaded with biochar biofilm reactor for ecological remediation of oligotrophic water: Mechanism, performance, and functional characterization

The removal of complex pollutants from oligotrophic water is an important challenge for researchers. In this study, the HCl-modified loofah sponge crosslinked polyethyleneimine loaded with biochar (LS/PEI@biochar) biofilm reactor was adapted to achieve efficient removal of complex pollutants in oligotrophic water. On the 35 d, the average removal efficiency of chemical oxygen demand (COD), ammonia nitrogen (NH4+-N), calcium (Ca2+), and phosphate (PO43--P) in water was 51, 95, 81, and 77 %, respectively. Additionally, it effectively used a low molecular weight carbon source. Scanning electron microscopy (SEM) results showed that the LS/PEI@biochar biocarrier had superior biofilm suspension performance. Meanwhile, analysis of the biocrystals confirmed Ca2+ and PO43- removal through the generation of CaCO3 (calcite and vaterite) and Ca5(PO4)3OH. This study demonstrated that the system has great efficiency and application prospect in treating oligotrophic water on the laboratory scale, and will be further validated for practical application on large-scale oligotrophic water.

Integrating bioprocess and metagenomics studies to enhance humic acid production from rice straw

Rice straw burning annually (millions of tons) leads to greenhouse gas emissions, and an alternative solution is producing humic acid with high added-value. This study aimed to examine the influence of a microbial consortium and other additives (chicken manure, urea, olive mill waste, zeolite, and biochar) on the composting process of rice straw and the subsequent production of humic acid. Results showed that among the fungal species, Thermoascus aurantiacus exhibited the most prominent impact in expediting maturation and improving compost quality, and Bacillus subtilis was the most abundant bacterial species based on metagenomics analysis. The highest temperature, C/N ratio reduction, and amount of humic acid production (Respectively in lab 61 °C, 54.67%, 298 g kg-1 and in pilot level 65 °C, 72.11%, 310 g kg-1) were related to treatments containing these microorganisms and other additives except urea. Consequently, T. aurantiacus and B. subtilis can be employed on an industrial scale as compost additives to further elevate quality. Functional analysis showed that the bacterial enzymes in the treatments had the highest metabolic activities, including carbohydrate and amino acid metabolism compared to the control. The maximum enzymatic activities were in the thermophilic phase in treatments which were significantly higher than that in the control. The research emphasizes the importance of identifying and incorporating enzymatically active strains that are suitable for temperature conditions, alongside the native strains in decomposing materials. This strategy significantly improves the composting process and yields high-quality humic acid during the thermophilic phase.

Kitchen compost-derived humic acid application promotes ryegrass growth and enhances the accumulation of Cd: An analysis of the soil microenvironment and rhizosphere functional microbes

Phytoremediation is an environmentally friendly and safe approach for remediating environments contaminated with heavy metals. Humic acid (HA) has high biological activity and can effectively complex with heavy metals. However, whether HA affects available Cd storage and the Cd accumulation ability of plants by altering the soil microenvironment and the distribution of special functional microorganisms remains unclear. Here, we investigated the effects of applying kitchen compost-derived HA on the growth and Cd enrichment capacity of ryegrass (Lolium perenne L.). Additionally, the key role of HA in regulating the structure of rhizosphere soil bacterial communities was identified. HA promoted the growth of perennial ryegrass and biomass accumulation and enhanced the Cd enrichment capacity of ryegrass. The positive effect of HA on the soil microenvironment and rhizosphere bacterial community was the main factor promoting the growth of ryegrass, and this was confirmed by the significant positive correlation between the ryegrass growth index and the content of SOM, AP, AK, and AN, as well as the abundance of rhizosphere growth-promoting bacteria such as Pseudomonas, Steroidobacter, Phenylobacterium, and Caulobacter. HA passivated Cd and inhibited the translocation capacity of ryegrass. The auxiliary effect of resistant bacteria on plants drove the absorption of Cd by ryegrass. In addition, HA enhanced the remediation of Cd-contaminated soil by ryegrass under different Cd levels, which indicated that kitchen compost-derived HA could be widely used for the phytoremediation of Cd-contaminated soil. Generally, our findings will aid the development of improved approaches for the use of kitchen compost-derived HA for the remediation of Cd-contaminated soil.