Environmental Biodegradation | Zoology Optional Notes for UPSC PDF Download

Biodegradation: Nature's Recycling Mechanism

Biodegradation refers to the natural process through which organic matter or substances are converted into simpler forms, thanks to the catalytic actions of living microorganisms.

A General Term with Diverse Applications: Biodegradation is a broad term used to describe any alteration or breakdown of a substance facilitated by biological means.

Biodegradation in Detail

Biodegradable materials can undergo degradation through microbial action either in the presence of oxygen (aerobically) or its absence (anaerobically). Virtually any form of matter, living or not, can be subject to biodegradation. What truly matters is the timespan involved. The European Union has established a biodegradability standard, requiring 90% of the original substance to transform into water, minerals, and carbon dioxide through biological reactions within six months.

The process hinges on the metabolic and enzymatic activities of microorganisms like bacteria, yeast, and fungi. These microorganisms employ two primary modes for biodegradation, depending on the type of matter and the environmental conditions:

1. Mineralization: This mode leads to the complete degradation of organic pollutants, with microorganisms using the matter as their primary carbon source to produce energy.
2. Cometabolism: In this mode, degradation is initiated through the introduction of a growth substrate, which acts as the primary source of carbon and energy for breaking down the target matter. Some naturally occurring microbes exhibit impressive catabolic activity, allowing them to transform and degrade a wide range of compounds, including polychlorinated biphenyls, hydrocarbons, radionuclides, metals, and more.

Biodegradation's Environmental Significance

Biodegradation is celebrated as a sustainable approach to removing complex organic matter. Its importance extends to ecological, natural environmental, and waste management contexts.

Challenging Biodegradable Pollutants

The industrial revolution and an elevated standard of living have ushered in numerous highly toxic organic compounds. These compounds, which include fuels, polycyclic aromatic hydrocarbons (PAHs), dyes, pesticides, and synthetic chemicals like radionuclides, present challenges for native flora in rapidly degrading them. Here are some examples of such organic pollutants:

1. Polycyclic Aromatic Hydrocarbons (PAHs): PAHs belong to the class of 'Hydrophobic Organic Contaminants (HOCs)' and are commonly found in sediments, soils, and the air.

• They occur naturally in substances like gasoline, crude oil, and coal.
• Formation can result from burning coal, wood, or cooking meat at high temperatures.
• PAHs tend to bind easily, forming minute particles that can accumulate in organisms.
• Some man-made PAHs, like 'Naphthalene,' are used in products such as mothballs and other chemicals.
• Cigarette smoke also releases various PAHs into the environment.
• These compounds can accumulate in aquatic organisms and may be transferred to humans through seafood consumption.
• High exposure to PAHs, including Naphthalene, in the air can irritate the eyes, nasal passages, and even lead to liver issues.

2. Polychlorinated Biphenyls (PCBs): PCBs, with their stable and non-flammable properties, are used in hydraulic and electrical equipment, rubber products, and dyes. They easily disperse in the environment, making them persistent and a cause for concern.

3. Pesticides: Pesticides, which include herbicides, fungicides, rodenticides, and insecticides, are designed to enhance crop productivity and prevent diseases. However, their use poses serious health risks to non-target organisms, given their fat solubility and potential for bioaccumulation.

4. Dyes: Dyes, widely used across industries, particularly azo dyes, contain complex xenobiotic compounds. Microorganisms, equipped with oxidoreductive enzymes and dynamic metabolic abilities, play a crucial role in the decolorization of polluted sites.

5. Radionuclides and Heavy Metals: Microbial degradation transforms radionuclides into less toxic and stable compounds. When it comes to heavy metals like arsenic, cadmium, and chromium, microbes employ processes such as bioleaching, biosorption, enzyme-catalyzed redox reactions, and intracellular accumulation to convert them into more benign forms. Heavy metal extraction can be achieved by recovering metal precipitates from microbial samples.

Microorganisms in Biodegradation and Their Impact

Microorganisms, including bacteria, fungi, yeast, and genetically modified microorganisms (GMM), play a crucial role in biodegradation processes, helping to break down complex and toxic organic compounds into simpler forms that can be reused as food sources by other organisms. Here's a closer look at the impact of various microorganisms in biodegradation:

Bacterial Strains:

• Various bacterial strains are involved in biodegradation, making them easily accessible for isolating.
• Bacterial strains from genera such as Klebsiella, Enterobacter, Bacillus, Staphylococcus, Acinetobacter, and more are known for hydrocarbon degradation.
• For PCB degradation, both Gram-positive (e.g., Rhodococcus, Bacillus, Microbacterium) and Gram-negative strains (e.g., Pseudomonas, Sphingomonas, Ralstonia) are involved, utilizing redox and dehalogenation reactions.
• Bacteria also play a role in the degradation of pesticides, such as DDT (Staphylococcus and Stenotrophomonas bacteria) and 'Chlorpyrifos' (Providencia stuartii).
• Bacteria like Shewanella decolorans are efficient in removing azo dyes.

Plant Growth Promoting Bacteria (PDPB and PGBR):

• These bacteria naturally reside in or near plant roots and promote plant growth.
• They help metabolize toxic substances that can be used by plants, forming mutualistic relationships with the plants.
• Examples include Pseudomonas species and Lysinibacillus for PAH hydrocarbon degradation.

Fungal Species:

• Fungi are essential for carbon decomposition and can thrive in low pH and moisture environments.
• They are equipped with extracellular multienzyme complexes for degrading natural polymeric compounds.
• Filamentous fungi like Cladosporium, Aspergillus, and Penicillium are involved in the degradation of hydrocarbons and PCBs.
• Fungi like Aspergillus niger have shown biodegrading activity towards PCBs.
• Fungi also detoxify metals through various mechanisms.

Yeast:

• Yeast species, such as Candida lipolytica and Saccharomyces cerevisiae, are known to degrade hydrocarbons and other chemicals.
• They are also capable of transforming plasticizers, insecticides, fungicides, and PCBs.

Genetically-Modified Microorganisms (GMM):

• GMMs have been engineered through genetic modifications to enhance their biodegradation capabilities for various chemical contaminants.
• They require plasmids specifically designed for degrading certain toxic compounds.
• Examples of GMMs include Pseudomonas putida, Alcaligenes eutrophus AE104, Rhodopseudomonas palustris, and others, used for the degradation of various pollutants.
• GMMs must be carefully monitored and tested for their environmental impact to prevent potential harm to ecosystems.

In summary, microorganisms play a vital role in biodegradation processes, offering a sustainable and natural way to break down complex and toxic substances in the environment. These microorganisms have a diverse range of capabilities and can be harnessed for bioremediation and waste management. Properly engineered GMMs can further enhance biodegradation efforts, but their release into the environment should be carefully managed to minimize potential ecological risks.

Stages of Biodegradation

Biodegradation is a complex process that can be divided into three key stages: biodeterioration, bio-fragmentation, and assimilation:

1. Biodeterioration: This stage involves the mechanical, physical, and chemical weakening of the compound's structure. Abiotic factors such as light, temperature, and environmental chemicals initiate these changes. Biodeterioration represents the initial phase of breaking down complex structures, making them more accessible to microorganisms.

2. Bio-fragmentation: Once the structure is weakened, microorganisms start to break down the complex compounds further. During this stage, polymeric bonds are cleaved, resulting in the transformation of complex substances into simpler oligomers and monomers. Bio-fragmentation can occur under both aerobic (with oxygen) and anaerobic (without oxygen) conditions. In aerobic digestion, the compounds are converted into water, carbon dioxide, and simple molecules, which serve as nutrient sources for microorganisms. In anaerobic digestion, the mass and volume of complex materials are reduced, leading to the production of natural gases. Anaerobic reactions are widely used in waste management facilities to generate renewable energy.

3. Assimilation: In the assimilation stage, microorganisms take up the newly formed molecules through membrane carriers. These molecules serve as an energy source in the form of ATP (Adenosine Triphosphate) or as structural elements for the microorganisms' growth and reproduction.

Factors Affecting Biodegradation

Several factors can influence the biodegradation of materials by microorganisms:

1. Environmental Factors: Environmental factors play a significant role in biodegradation. Soil type, organic matter content, and factors affecting fluid movement in groundwater, like porosity and water saturation, can impact the adsorption and absorption of pollutants in the environment. Soil conditions also determine the availability of gases such as CO2, methane, and oxygen, affecting the speed of biodegradation. Different soil conditions can lead to aerobic or anaerobic biodegradation processes.

2. Biological Factors: Biological factors involve the metabolic potential of microorganisms. Inhibition of enzymatic activity can occur due to competition for limited carbon sources between microorganisms, predation by bacteriophages and protozoa, and the concentration of the contaminant present in the surroundings. The affinity of the contaminant to specific enzymes in microorganisms can influence the rate of degradation. Biological enzyme-catalyzed reactions are influenced by factors like pH, temperature, moisture, and osmotic pressure.

In conclusion, biodegradation is a critical natural process that helps clean the environment and contribute to sustainable development goals. It is a response to the release of toxic chemicals into the environment as a result of industrialization and technological advancements. Microorganisms, through their unique digestive machinery and metabolic abilities, play a vital role in transforming synthetic compounds into stable forms, using pollutants as a carbon source for energy. This process not only benefits microorganisms but also helps restore the balance of nature and reduce the impact of hazardous pollutants on human health and ecosystems.