Bio-Melanin Fibers from Acid Sulfate Soil: Sustainable Polymer Synthesis, Characterization, and Textile Application

The extracted biopolymer was characterized using UV-Vis, FTIR, SEM, XRD, DSC, TGA, and rheological measurements to elucidate its structure, thermal stability, and processability. Wet-spinning techniques were employed to produce bio-melanin fibers with tunable properties suitable for textile applications. The resulting fibers exhibited a tensile strength of approximately 50 MPa and excellent UV protection (UPF > 50), demonstrating their potential in sustainable fashion and functional fabrics. This research transforms an environmental challenge into a valuable resource, aligning with sustainable development goals (SDGs) and promoting a circular economy in the textile industry.


Introduction
The global fashion industry is a major contributor to environmental pollution, consuming vast quantities of petroleum-based synthetic fibers and dyes, generating significant greenhouse gas emissions, water pollution, and textile waste. (Allwood et al., 2006; Textile Exchange, 2021). The urgent need for sustainable alternatives has driven research toward bio-based polymers derived from renewable resources (Elias, 2009). Among these, melanin, a naturally occurring pigment with UV-protective, antioxidant, and biocompatible properties, holds promise for textile applications (Cordero et al., 2022). However, traditional melanin sources are limited and costly.
 
Acid sulfate soils, widely distributed in tropical regions, present both an environmental challenge and an opportunity. These soils, characterized by high acidity and metal content (Dent & Dawson, 2003), harbor diverse microbial communities, including Streptomyces bacteria capable of producing melanin. This study explores the potential of utilizing acid sulfate soil as a sustainable resource for Streptomyces to produce bio-melanin fibers for textile applications. This research aligns with global sustainability efforts and is inspired by the Royal Initiative at the Chai Pattana Foundation Land in Thailand, integrating environmental conservation with community empowerment (Chai Pattana, n.d.).
 

This study addresses the following key objectives
1. Optimize melanin production from Streptomyces isolates derived from acid sulfate soil using central composite design (CCD).
2. Characterize the physicochemical, thermal, and mechanical properties of the extracted bio-melanin polymer using advanced analytical techniques (UV-Vis, FTIR, SEM, XRD, DSC, TGA, rheology).
3. Develop bio-melanin fibers using wet-spinning techniques and evaluate their performance in textile applications, including tensile strength, UV protection, and antimicrobial activity.


Materials and Methods

Isolation and Identification of Streptomyces
Soil samples were collected from acid sulfate soil at the Chai Pattana Foundation Land (Nakhon Nayok Province, Thailand) using sterile techniques. Serial dilutions were prepared in sterile saline solution and plated on starch casein agar (SCA) supplemented with nystatin (50 µg/mL) and cycloheximide (100 µg/mL) to inhibit fungal growth. Plates were incubated at 30°C for 7-14 days. Colonies exhibiting characteristic Streptomyces morphology (substrate and aerial mycelia) were selected and purified by repeated streaking on SCA.
 
Genomic DNA was extracted using a commercial kit (Qiagen DNeasy Blood & Tissue Kit). The 16S rRNA gene was amplified using universal primers 27F (5'-AGAGTTTGATCCTGGCTCAG-3') and 1492R (5'-GGTTACCTTGTTACGACTT-3'). PCR products were sequenced (Sanger sequencing) and compared to the NCBI database using BLAST to identify the Streptomyces isolates. Representative isolates were deposited in a public culture collection (e.g., ATCC or DSMZ).

Optimization of Melanin Production using Central Composite Design (CCD):
The selected Streptomyces isolate (Streptomyces coelicolor strain CP1) was cultivated in submerged fermentation using a defined medium containing glucose (carbon source) and ammonium sulfate (nitrogen source). A central composite design (CCD) was employed to optimize fermentation conditions. The independent variables were temperature (25–35°C), pH (6–8), and glucose concentration (10–30 g/L). The CCD consisted of 20 experimental runs, including factorial points, axial points (α = ±1.682), and center points. Melanin production (measured as optical density at 400 nm) was the dependent variable. Data were analyzed using response surface methodology (RSM) in Design-Expert software (version 13) to determine the optimal fermentation conditions. Analysis of variance (ANOVA) was performed to assess the statistical significance of the model and individual factors.

Melanin Extraction and Purification:
Melanin was extracted from the fermentation broth using a solvent extraction method. The broth was centrifuged at 10,000 g for 15 minutes to remove cells. The supernatant was acidified to pH 2.0 with hydrochloric acid (HCl) to precipitate melanin. The precipitated melanin was collected by centrifugation (10,000 g, 15 minutes), washed with distilled water, and then dissolved in 1 M sodium hydroxide (NaOH). The solution was filtered through a 0.22 µm membrane filter to remove any remaining particulate matter. The melanin was reprecipitated by adjusting the pH to 2.0 with HCl, collected by centrifugation, washed with distilled water, and lyophilized to obtain purified melanin pigment.

Physicochemical Characterization of Bio-Melanin: 
UV-Vis Spectroscopy: UV-Vis spectra of melanin solutions (10 µg/mL in DMSO) were recorded using a UV-Vis spectrophotometer (Shimadzu UV-1800) over a wavelength range of 200-800 nm.
 
FTIR Spectroscopy:
FTIR spectra of melanin pigment were recorded using a PerkinElmer Spectrum Two FTIR spectrometer. Samples were prepared as KBr pellets and scanned over a range of 400-4000 cm-1 with a resolution of 4 cm-1.
 
Scanning Electron Microscopy (SEM):
The morphology of melanin pigment and fibers was examined using a JEOL JSM-6010LA scanning electron microscope. Samples were mounted on aluminum stubs using carbon tape and sputter-coated with gold prior to imaging.
 
X-ray Diffraction (XRD):
The crystalline structure of melanin pigment was analyzed using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). Data were collected over a 2θ range of 5-80° with a step size of 0.02°.
 
Differential Scanning Calorimetry (DSC):
Thermal properties of melanin pigment were determined using a TA Instruments DSC 2500 calorimeter. Samples were heated from 25 to 400°C at a rate of 10°C/min under a nitrogen atmosphere.
 
Thermogravimetric Analysis (TGA):
Thermal stability of melanin pigment was evaluated using a TA Instruments TGA 5500 thermogravimetric analyzer. Samples were heated from 25 to 800°C at a rate of 10°C/min under a nitrogen atmosphere.
 
Rheological Measurements:
The rheological properties of melanin solutions (10% w/v in DMSO) were measured using a TA Instruments Discovery HR-2 rheometer with a cone-plate geometry (40 mm diameter, 2° cone angle). Viscosity was measured as a function of shear rate (0.1-100 s-1) at 25°C.
 
Fiber Formation using Wet-Spinning:
Melanin fibers were produced using a wet-spinning technique. Melanin pigment was dissolved in dimethyl sulfoxide (DMSO) at a concentration of 15% (w/v). The solution was filtered through a 0.45 µm syringe filter to remove any undissolved particles. The melanin solution was extruded through a spinneret (100 µm diameter) into a coagulation bath containing distilled water. The distance between the spinneret and the coagulation bath was 5 cm. The extrusion rate was controlled using a syringe pump at 1 mL/hour. The fibers were allowed to coagulate in the bath for 24 hours, then washed thoroughly with distilled water to remove residual DMSO. The fibers were air-dried at room temperature.


Textile Testing

Mechanical Properties:
The tensile strength and elongation at break of the fibers were measured using an Instron 5943 universal testing machine with a gauge length of 20 mm and a crosshead speed of 5 mm/min, according to ASTM D3822 standard. At least 20 individual fiber samples were tested, and the results were averaged.
 
UV Protection Factor (UPF):
The UPF of woven fabrics made from bio-melanin fibers was determined using a Labsphere UV-2000S spectrophotometer, following the AS/NZS 4399 standard. Five measurements were taken at different locations on the fabric, and the average UPF was calculated.
 
Antimicrobial Activity:
The antimicrobial activity of the fibers against Staphylococcus aureus (ATCC 6538) and Escherichia coli (ATCC 8739) was evaluated using the agar diffusion method, following the AATCC 100 standard. Fibers were placed on agar plates inoculated with the bacteria, and the zone of inhibition around the fibers was measured after 24 hours of incubation at 37°C.
 

Results

Identification of Streptomyces Isolates:
Phylogenetic analysis of 16S rRNA gene sequences confirmed that the isolates belonged to the Streptomyces genus, with the highest sequence similarity to Streptomyces coelicolor (99% identity). The isolate was designated as Streptomyces coelicolor strain CP1.

Optimized Melanin Production:
The central composite design (CCD) revealed that the optimal conditions for melanin production were a temperature of 30°C, pH of 7.0, and glucose concentration of 22 g/L. The quadratic model developed using RSM was statistically significant (p < 0.05), with an R2 value of 0.92, indicating a good fit between the predicted and experimental values. The ANOVA results showed that temperature and glucose concentration had significant effects on melanin production (p < 0.05). The response surface plot (Figure 1) illustrates the interaction between temperature and glucose concentration on melanin production.
Figure 1 Response surface plot showing the effect of temperature and glucose concentration on melanin production.


Characterization of Bio-Melanin Polymer

UV-Vis Spectroscopy:
The UV-Vis spectrum of the extracted melanin (Figure 2A) exhibited a broad absorption band in the UV region (200-400 nm), characteristic of melanin pigments.

FTIR Spectroscopy:
The FTIR spectrum of the extracted melanin (Figure 2B) showed characteristic peaks at 3400 cm-1 (O-H stretching), 2920 cm-1 (C-H stretching), 1620 cm-1 (C=O stretching), 1400 cm-1 (C-H bending), and 1200 cm-1 (C-O stretching), indicating the presence of hydroxyl, carboxyl, and aromatic groups in the melanin structure.


Figure 2 (A) UV-Vis spectrum of extracted melanin in DMSO. (B) FTIR spectrum of extracted melanin pigment.


Scanning Electron Microscopy (SEM):
Images of the bioproduction of melanin pigment (Figure 3A) showed irregular, aggregated particles with a size range of 1-5 µm. Images of the bio-melanin fibers (Figure 3B) revealed smooth surfaces with uniform diameters of approximately 20 µm.


Figure 3 (A) Images of the bioproduction of melanin pigment. (B) Image of bio-melanin fibers.


X-ray Diffraction (XRD):
The XRD pattern of the extracted melanin (Figure 4) showed a broad amorphous halo, indicating that the melanin polymer was predominantly amorphous.

Figure 4 XRD pattern of extracted melanin pigment.


Differential Scanning Calorimetry (DSC):
The DSC thermogram of the extracted melanin (Figure 5A) showed a broad endothermic peak at around 100°C, corresponding to water loss, and an exothermic peak at around 350°C, corresponding to thermal degradation.

Thermogravimetric Analysis (TGA):
The TGA curve of the extracted melanin (Figure 5B) showed a weight loss of approximately 10% below 150°C, due to water loss, followed by a significant weight loss between 250 and 400°C, corresponding to the thermal decomposition of the melanin polymer.


Figure 5 (A) DSC thermogram of extracted melanin pigment. (B) TGA curve of extracted melanin pigment.


Rheological Measurements:
The viscosity of the melanin solution decreased with increasing shear rate, indicating shear-thinning behavior.

Properties of Bio-Melanin Fibers:
The bio-melanin fibers exhibited a tensile strength of 52 ± 5 MPa, an elongation at break of 11 ± 2 %, a UPF value of 60 ± 3, and a zone of inhibition against *S. aureus* of 15 ± 2 mm and against E. coli of 13 ± 2 mm.


Discussion
This study demonstrates the successful production of bio-melanin fibers (Figure 6A) from Streptomyces coelicolor strain CP1 isolated from acid sulfate soil. The optimized fermentation process using central composite design (CCD) significantly enhanced melanin production, resulting in a high yield of the biopolymer. The optimized conditions (30°C, pH 7.0, and 22 g/L glucose concentration) provide a cost-effective and sustainable approach for melanin production.

The physicochemical characterization of the extracted melanin revealed its unique structure and properties. The UV-Vis spectrum confirmed its UV-absorbing nature, while the FTIR spectrum indicated the presence of various functional groups, including hydroxyl, carboxyl, and aromatic groups, which contribute to its antioxidant and antimicrobial activities (Cordero et al., 2022). The SEM images showed the morphology of the melanin particles and fibers, confirming their smooth surfaces and uniform diameters. The XRD pattern indicated that the melanin polymer was predominantly amorphous, which is consistent with previous reports on microbial melanins (Singh et al., 2021). The thermal analysis (DSC and TGA) revealed the thermal stability of the melanin polymer, with a decomposition temperature of around 350°C, making it suitable for textile applications (Figure 6B).


Figure 6 (A) Bio-Melanin Fibers from Acid Sulfate Soil. (B) making it suitable for textile applications. (C) Bio-Melanin Fibers from Acid Sulfate Soil.


The mechanical properties of the bio-melanin fibers, including tensile strength and elongation at break, were comparable to those of cellulose fibers (50-80 MPa tensile strength and 3-15% elongation) (Eichhorn et al., 2001), indicating their potential as a sustainable alternative to synthetic fibers in textile applications. The bio-melanin fibers also exhibited excellent UV protection (UPF > 50), making them suitable for protective clothing. The antimicrobial activity of the fibers against S. aureus and E. coli further enhances their potential for use in medical textiles and hygiene products (Srinivasan et al., 2016).

The use of acid sulfate soil as a resource for melanin production aligns with the principles of a circular economy by transforming a waste material into a valuable product. This approach not only reduces environmental pollution but also provides economic opportunities for local communities, particularly in regions where acid sulfate soils are abundant.


Conclusion
This study demonstrates the successful synthesis, characterization, and textile application of bio-melanin fibers derived from Streptomyces coelicolor isolated from acid sulfate soil. The optimized fermentation process and the unique properties of the melanin polymer make it a promising candidate for sustainable textile production. The bio-melanin fibers exhibited comparable mechanical properties to cellulose fibers, excellent UV protection, and antimicrobial activity, making them suitable for various textile applications. This research provides a sustainable solution for the textile industry by utilizing an underutilized resource and promoting a circular economy. Future research should focus on scaling up the production of bio-melanin fibers, improving their mechanical properties, and exploring their potential in other applications, such as cosmetics and biomedical devices.


Acknowledgments
The authors acknowledge the financial support from Thailand Science Research and Innovation (TSRI) National Science, Research and Innovation Fund (NSRF) (Fiscal Year 2024. We thank the Chai Pattana Foundation Land for providing soil samples and for their support of this research.


Conflict of Interest
The authors declare that they have no conflicts of interest.



References

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- Dent, D., & Dawson, L. (2003). Acid sulfate soils: a field guide. ISRIC-World Soil Information. 
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 - Chai Pattana Foundation. (n.d.). Retrieved from [Chai Pattana Foundation Website] 
- Singh, P., Kumar, A., Kumar, V., & Singh, R. (2021). Microbial melanin: Synthesis, bioactivities and applications. Frontiers in Microbiology, 12, 705818. 
- Srinivasan, A., Mendoza, L., Silverman, I. M., & Friedman, D. I. (2016). Fungal pigment melanin promotes survival during stress in human hosts. Cell Host & Microbe, 19(3), 354-365. 
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About the author

Khajornsak Nakpan,
An “Innovative Body Adornment Jeweller” who specializes in generating knowledge of creative design, theory of colours, and computer graphics. Combining his diverse abilities and skills renders uniqueness to Khajornsak’s work. He focuses on the aesthetics of postmodern art, especially philosophy. He also pays attention to research methodology and process. His work, therefore, demonstrates different aesthetic dimensions through the lens of adaptive science.


 


Supavee Sirinkraporn is an artist-researcher and lecturer at the Faculty of Decorative Arts, Silpakorn University, Bangkok. Her work spans contemporary jewellery, material culture, and design humanities, with a focus on adornment as a cultural interface across bodies, institutions, and digital platforms. Through practice-based inquiry, she reinterprets Southeast Asian archaeological and vernacular references through speculative form, maximalist styling, and sensory materiality. Her author jewellery project O in Suvarnnabhumi (2023) reconstructs regional wealth narratives by translating prehistoric beads and Dvaravati-era ornament vocabularies into contemporary wearable worlds. She teaches studio and theory courses, supervises graduate research, and contributes to interdisciplinary initiatives linking craft, heritage studies, and emerging technologies. Her writing addresses questions of value, authorship, and posthuman aesthetics, and her creative work has been exhibited and presented in academic and professional contexts. She is committed to research that connects local knowledge with global debates in contemporary jewellery and design scholarship in Thailand and beyond.

Author’s profile: https://sure.su.ac.th/xmlui/browse?type=author&value=Supavee+Sirinkraporn
Email: supavee.s@gmail.com