Be it the harsh sun, torrential rain or gusty wind, plants around us are bestowed with the ability to face it all. Ever wondered what the secret of their sturdiness is? It turns out, it's a polymer called lignin, which gives them the mechanical strength and toughness to stand firm in the wake of floods, storms and attacks by pathogens. Lignin is found in the wood and barks of trees. It is recalcitrant, meaning that it resists degradation and poses challenges to the environment. In a recent study, researchers from the Jawaharlal Nehru University, National Institute of Plant Genome Research and CSIR-Indian Institute of Toxicology Research, India, have identified a particular strain of bacteria that can help break up lignin and decompose it.
The inability of lignin to naturally degrade like other organic matter is a significant concern in pulp and paper industries, textile industries and distilleries. Today, hazardous and expensive chemicals are used to destroy lignin. These chemicals are not affordable for small-scale industries and hence, affect their ability to manage waste. The current study addresses the need for an environmentally and socially responsible and sustainable process for lignin degradation.
Studies over the past decade have identified several microbes which have the potential to degrade lignin. While fungus forms the majority of these, recent studies have also identified few bacterial species that produce ligninolytic enzymes and help in the degradation of lignin. The researchers of this study have added a new bacterial strain, called β-proteobacterium Pandoraea, to the list. In a previous study, the authors had isolated this bacterium from the soil and observed that they grow and multiply quickly on lignin as a substrate, producing a significant amount of biomass.
In the current study, the researchers have used a gene sequencing approach to identify the biochemical pathway and the enzymes that catalyse the process of lignin degradation by the bacterium. They have identified around 140 genes in the bacterium that could be potentially manipulated for not only degrading lignin but also compounds like benzene, toluene, xylene, quinate, furfural and steroids, which are discharged through various industries and are tough to remove from the environment.
“The study indicated the pathways and enzymes important for metabolism of lignin or aromatic compounds that can be applied in the future for value addition to lignocellulosics”, say the authors about their findings.
The researchers have identified several classes of enzymes like peroxidases, oxidoreductases, laccases and dehydrogenases, all of which play a crucial role in lignin metabolism. Laccase, a recently isolated enzyme from Pandoraea, was found to be active over a wide range of pH and temperature. Since most effluents from industries are alkaline and have a high temperature, these enzymes can be directly used to treat the waste. Doing so saves energy, chemicals and other resources that would have been needed to bring down the pH and temperature of such effluents.
The study also found that the bacterium has a strong stress and detoxification system, which could help it manage the free radicals generated during the breaking down of lignin. Free radicals are unstable atoms that can damage cells. Due to the ability to remove such free radicals, this proteobacterium can act on a wide range of toxic compounds—a property rarely found in most bacterium species.
The study also reports that the bacterium produces polyhydroxyalkanoates (PHAs) when they grow on lignin. These PHAs have the potential to replace conventional plastics, derived from petroleum products, by bioplastics that are degradable. If we were to substitute petroleum-based plastics with PHAs, we could bring down the greenhouse gas emission by 200% and reduce our dependence on fossil fuel by more than 95%!
With India currently grappling with the massive problem of solid waste management, where out of the 50 million tons of waste produced annually, only 30% is processed, this research could bring in new hopes for the large-scale bio-based treatment of lignin without generating hazardous waste.