Microbial melanin-like material: A factor beyond influencing the brown color of activated sludge

Melanin is a group of phenolic-quinone pigments. Natural melanin is nearly ubiquitous; found in all types of living organisms, ranging from mammals to bacteria. However, its presence and biosynthesis genomic potential in activated sludge have not been investigated. To explore this potential, melanin-like material was extracted from activated sludge collected from a municipal wastewater treatment plant. The extracted melanin-like material was characterized through biochemical analyses in comparison to synthetic melanin and humic acids that are commercially available. Metagenomic analysis of microbial community members in activated sludge and detection of tyrosine-derived melanin synthesis genes was performed. Additionally, the potential application of the extracted melanin-like material as a natural pigment was evaluated by testing its ability to color wool yarn. It was found that melanin-like material extracted from activated sludge accounted for around 11% of sludge dry mass. The isolated material displayed intrinsic autofluorescence, strong UV absorption, high oxidative stability, and free radical-rich EPR signal. FTIR analysis indicated a mixed polymer dominated by pyomelanin-like structures with eumelanin features, distinguishing it from synthetic melanin and humic acid. Metagenomic screening of the sludge community revealed widespread genomic potential for pyomelanin monomeric precursors biosynthesis across key functional genera (e.g. genera Zoogloea, Nitrotoga, Nitrosomonas, Ca. Accumulibacter, Azonexus, Ca. Competibacter, Propionivibrio, and Rhodoferax). These results suggest that microbial melanin-like material is an overlooked contributor to sludge coloration. Furthermore, the extracted pigment exhibited high affinity and wash fastness on wool fibers, demonstrating its potential for valorization as a sustainable biobased colorant.