The biodegradation of organic matter by microorganisms is the primary component of the transformation of organic waste during composting. In this study, NGS analysis indicated that the mixture under investigation harbored many different microbial genera, thus proving that compost formed from textile waste is an ideal habitat for various new mesophilic and thermophilic microbes. Our findings are supported by those previously obtained, which demonstrated an increase in the concentration of microorganisms (bacteria, actinomycetes, and fungi) during the composting of textile waste. They also reflect the level of diversity of the microbial community in the feedstock and the high degree of functional redundancy known to exist in these microbial communities. In this regard, different metabolic pathways lead to significant changes in the physical–chemical parameters of the substrate, which in turn lead to community changes and influence microbial abundance and the succession of microorganisms. Several phases occur during the composting of textile waste. At the beginning of composting, temperatures below 45 °C characterize the mesophilic phase (Fig. 1). During the mesophilic phase, the availability of favorable conditions (in terms of nutrition, temperature, moisture, etc.) stimulates the proliferation of mesophilic microorganisms, thus activating their metabolism of easily metabolizable substrates—such as carbohydrates and free amino acids. The rise in temperature and the decrease in moisture observed are mainly due to the evaporation of water. Such temperatures (up to 50 °C or even 75 °C) induce the appearance of thermotolerant and thermophilic microorganisms, characterizing the thermophilic phase. During this phase, a significant proportion of the organic matter is transformed by thermophilic microorganisms mineralizing organic carbon and releasing CO2 and nitrogen compounds, thus decreasing the C/N ratio10,12. A decrease in the C/N ratio to a value below 20 was recorded, thus demonstrating a rise in the humification rate and the degradation of recalcitrant molecules such as lignin, hemicelluloses, and cellulose—a good indicator of compost maturity. At the same time, a decrease in the NH4+/NO3− ratio to a value below 1 was observed, which is a good indicator of compost maturity. This decrease could be attributed to the phenomenon of nitrification and/or the denitrification of inorganic nitrogen. A remarkable initial increase in pH followed by a swift decrease was also observed. The rise in pH could be assigned to the ammonification carried out by microorganisms. In contrast, the subsequent decrease in pH could be attributed to the degradation of organic acids by the microorganisms identified11. Although composting is an ancient treatment, it is a very complex process as a result of the numerous changes in physical–chemical and biological states that occur over its course11. A good understanding of these changes requires careful study of the succession of microbial communities that comprise all of the microorganisms present, including those in tiny proportions13. Such an investigation could establish a microorganism database for each of the composting phases according to physical–chemical parameters14. The use of the NGS approach allows the achievement of this goal. During such an analysis, a huge amount of bacterial, actinomycete, and fungal species were identified. In this respect, bacteria are considered among the most dominant microorganisms in quantity and diversity during composting15,16. The authors’ previous work has demonstrated that bacterial concentration in compost samples is higher than fungal and actinomycete concentrations during textile waste composting11. Such an abundance is because of their small size and their ability to proliferate at a wide range of pH and temperature intervals17. The dominance of the genera belonging to Proteobacteria, Bacteroidetes, Actinobacteria, and Firmicutes phyla is expected; it is in agreement with previous results1,13. This demonstrates that, during composting, the most abundant bacteria are those belonging to the phyla mentioned above, owing to their ubiquitous nature2,7,18,19. Meanwhile, many genera were identified in this study as degrading recalcitrant molecules (lignocellulosic compounds) through their enzymatic system: Devosia, Flavobacterium, Pseudoxanthomonas, Pseudomonas, and Achromobacter11,18,20. Aside from this, the Devosia genus was described as a degrader of organic sulfur, phosphorus, and aromatic compounds according to Chandni et al.21. Several authors have demonstrated that Paenibacillus (belonging to the Firmicutes) could utilize different enzymatic activities to destroy the lignocellulosic compounds such as cellulose, xylose, and hemicelluloses17,22. This is highlighted using the UniProt database (Table 3) and via the analysis of enzymatic activities in a previous study by the authors of this paper, thus revealing a huge concentration of different enzymatic activities during the composting of textile waste11. Such genera could explain the rise in temperature due to active metabolism and, therefore, the decaying organic matter represented by the decrease in the C/N ratio.
Other genera were also identified during this investigation as organic matter degraders, although in minor abundance—namely Chitinophaga, Nitrosomonas, and Nitrobacter. Chitinophaga (belonging to Bacteroidetes) was recently identified as cellulose-degrading species during the mesophilic phase, and was found to be able to grow on solubilized forms of cellulose from green-waste compost2. Interestingly, three genera associated with the nitrification phenomenon—namely Steroidobacter, Nitrosomonas, and Nitrobacter—were identified. The numbers of nitrate- (Nitrobacter) and ammonium-oxidizing bacteria (Nitrosomonas) rose markedly through the composting processes. Steroidobacter is capable of denitrification using only a narrow range of organic substrates and nitrate as the electron acceptor23. All of these genera are nitrogen transformers, and can degrade organic matter into products capable of supporting plant growth by providing the necessary nitrogen. Our findings could be used to explain the changes in the NH4+/NO3− ratio observed during this investigation. Ultimately, using the rarefaction curves (Supplementary Fig. S1), we can state that the NGS performed during the current investigation can describe the majority of bacteria present in the samples analyzed.
Additionally, actinomycetes are multicellular filamentous bacteria acting at the later stages of composting. An increase in the concentration of actinomycetes during the thermophilic phase has previously been reported17. This observation is corroborated in the current study, as a significant increase in actinomycete abundance was recorded during the thermophilic phase. This strengthens previous results, demonstrating that the concentration of actinomycetes in compost samples reached peak values during the thermophilic phase11. Actinomycetes are essential agents that act on lignocellulose during the thermophilic phase, although their ability to degrade cellulose and lignin is not as extensive as that of fungi. Still, they can degrade cellulose, hemicellulose, and lignin10,19,24,25. This finding is consistent with the results obtained during this investigation, which show that genera from the Actinobacteria phyla (Streptomyces and Mycobacterium, etc.) are abundant in the thermophilic stages of composting, and have demonstrated their ability to metabolize complex organic compounds—except in the case of Cellulomonas, which is not conspicuous in the thermophilic phase. This could be assigned to the fact that bacteria of this genus cannot survive at high temperatures. The enzymatic activities (especially of the xylanase) of Cellulomonas were more remarkable in the mesophilic phase (37 °C).
All of the genera detected during both phases possess several enzymes (Table 3) involved directly in the degradation of organic matter—such as cellulase, hemicellulase (β-mannanase), pectin depolymerase, and xylanase (hemicellulase), which strengthens the above explanation. Additionally, several cellulolytic, amylolytic, and proteolytic bacteria appeared in greater concentrations in the thermophilic phase than in the mesophilic phase18. The high concentration of cellulolytic microorganisms during the thermophilic phase could be explained by high temperatures that favor the degradation of cellulose26, thus proving the major role played by thermophilic microorganisms in decaying cellulose during the composting of textile waste. Notably, sequences associated with Escherichia, Salmonella, or Listeria were not detected, which demonstrates that composting is an effective process for the destruction of pathogens and pathogen indicators owing to multiple heating cycles during composting18.
Interestingly, fungi can decay the organic matter not transformed by bacteria. Several authors found that Sordariomycetes and Eurotiomycetes are the most abundant eukaryotes in both phases of the composting process8,27. Variations in fungal communities corresponding to different phases (mesophilic and thermophilic) could be explained by changes in the physical–chemical characteristics of the compost highlighted during this investigation, such as pH, temperature, and moisture content. Temperature is the most critical factor affecting fungal growth. Most fungi are mesophilic, and thrive between 5 and 37 °C, with an optimum temperature of 25–30 °C28,29, but some also grow at higher temperatures. For example, Penicillium and Fusarium, which have been the subject of numerous studies, have demonstrated the ability to produce enzymes active in cellulose and lignin degradation even at high temperatures of 46–49 °C8. The presence of Penicillium, Aspergillus, Acremonium, and Fusarium in compost samples was expected. Penicillium is ubiquitous in the environment, and plays a vital role in decaying organic matter (lignin, cellulose, or hemicelluloses)30,31. Aspergillus, Acremonium, and Fusarium are associated with the degradation of organic matter (lignocellulosic compounds, cellulose, and hemicellulose). These findings are supported by the prediction obtained from the UniProt database (Table 6) and from previous studies11, thus demonstrating the ability of these genera to produce a wide range of enzymatic activities. This allows fungi to degrade organic matter even at high temperatures (in the thermophilic phase), and corroborates the results of the current study regarding the C/N and NH4+/NO3− ratios.
Indeed, the composting process generates high temperatures, demonstrating the crucial role of small groups of thermophilic fungi in the biodegradation of organic matter. Generally, fungi are not detected in the thermophilic phase of composting24,26,29. Remarkably, the added value of this study lies in the presence of a high abundance of thermophilic fungi in this phase (Table 5), suggesting that thermophilic fungi have a significant role in the degradation of organic matter during the thermophilic phase of textile waste composting. This finding could be applied when selecting new thermophilic fungi and reusing them to improve the composting performance of textile waste and produce mature compost. Eventually, analysing the rarefaction curves for fungi, it can be concluded that the description of the fungal diversity provided by NGS was effective (Supplementary Fig. S2).
This investigation depicted a detailed study of the textile waste composting process from a molecular microbiology standpoint. Using the next-generation sequencing approach, the microbiota variation that orchestrates the composting process and the change of enzymatic activities was described. These findings allow for the generation of a list of bacteria and fungi and their enzymes that are responsible for decaying lignocellulosic material. The impressive variety of bacterial and fungal microorganisms and metabolic functions during both the mesophilic and thermophilic phases of composting warrant the results obtained during the composting of the textile waste, and prove that the resulting compost meets the required level of maturity. Ultimately, it can be concluded that compost from textile waste is a treasure trove of new microorganisms and enzymes that are beneficial for the degradation of organic matter. Interestingly, this investigation could be used as a database for the dominant microorganisms involved in the degradation of organic matter. This would allow for the selection of new bacterial and thermophilic fungi genera and their reuse in bioaugmentation to improve the performance and reduce the duration of the composting of textile waste, and to produce mature compost.
