Biodegradation

Biodegradation is a naturally occurring process in which microorganisms break down organic substances into individual elements and smaller chemical compounds. Microorganisms, such as bacteria or fungi, alter the chemical makeup of a material through metabolic or enzymatic processes. Metabolic processes are chemical reactions that occur within a living organism; enzymatic processes use proteins called enzymes as catalysts to fuel the chemical reaction. All organic substances are biodegradable, although the rate at which the process occurs varies by the type of material, the type of microorganism, and environmental factors such as temperature and moisture.

The process of biodegradation is easily observed in rotting fruits or vegetables left outside in the trash for a few days. Refrigerating some foods or storing others in a dry place can help slow the process. In most cases, biodegradation is a beneficial process that helps recycle organic material, breaking it down into its basic chemical components that can then be used as nutrients for future growth. Biodegradation can also be used as a disposal method for some organic chemical compounds—such as crude oil—that could pose a potential problem for the environment. Some human-made substances are not biodegradable or are only broken down over the course of thousands of years. If not recycled manually, these substances must be specially modified to allow biodegradation to occur and avoid becoming a source of pollution for future generations.

Background

Microorganisms, or microbes, are extremely small living organisms that can only be seen with the aid of a microscope. In general, the term microorganism refers to a variety of life forms such as bacteria, viruses, fungi, and microscopic plants and animals. A virus is a particle made up of strands of genetic material such as DNA or RNA surrounded by a protein shell. Viruses can only reproduce by infecting other cells and using the host’s cellular chemistry against it.

Bacteria and fungi are the primary microorganisms responsible for the process of biodegradation. Bacteria are single-celled organisms that have a cellular wall but no central nucleus. Genetic material such as DNA floats freely inside a bacterium. Scientists have classified bacteria in a number of different forms based on their shape, the composition of their cell wall, how they move, and the environments in which they thrive.

Like all living creatures, bacteria must consume and process food to create the energy needed for survival. Some bacteria act as autotrophs, meaning they can process food directly from the environment in the form of sunlight, water, or carbon dioxide from the atmosphere. For example, cyanobacteria—also known as blue-green algae—transform energy from sunlight into oxygen through the process of photosynthesis. Other bacteria are heterotrophs, organisms that cannot manufacture their own food but must create energy by consuming other organic material.

Bacteria are found in every environment on Earth. A handful of soil can contain billions of bacteria, while the average human body is estimated to be home to a little more than thirty trillion. Some bacteria are harmful, causing illnesses such as pneumonia or food poisoning; however, most are beneficial to humans, aiding with digestion or turning products such as milk into cheese or yogurt.

Fungi are microorganisms that feed on organic material and reproduce through the dispersal of spores. Fungi can be single-celled such as yeasts, or multicelled such as mold, mildew, or mushrooms. Fungi are microscopic, but some grow into long microscopic strands called hyphae that can clump together to be seen with the unaided eye. Mushrooms are the fruiting body of a fungus made up of densely packed hyphae that grow above ground. A fruiting body is the section of the fungus that produces spores. Some fungi can act as parasites that are harmful to other living organisms. For example, certain types of mildew can attack and feed off plants. Other fungi are known as decomposers, which live in soil and feed off dead plant material.

Overview

Biodegradation is a relatively modern term first used about 1960 to describe the breaking down of organic substances by living organisms. Organic substances are compounds in which carbon atoms are chemically bonded to other atoms, most notably hydrogen, oxygen, or nitrogen. Carbon atoms are structured in such a way that they can form stable bonds with many other elements. As a result, they are the primary building blocks of life on Earth. Most often, organic material originates from living organisms or the remains of those organisms; however, organic material can also be made by humans to mimic the carbon-based structures of such matter.

In the process of biodegradation, the chemical bonds between atoms in organic substances are broken down by microorganisms, converting the material into less complex chemical compounds and base elements. For example, a leaf can be broken down into water, carbon dioxide, phosphate, and nitrogen. A phosphate is a chemical compound made up of phosphorus and oxygen. Nitrogen and phosphorus are natural fertilizers and some of the main ingredients in many human-made fertilizers. When the component elements of an organic substance are completely broken down into inorganic materials, the process is known as ultimate degradation or mineralization.

Microorganisms biodegrade organic substances through one of two processes: metabolic or enzymatic. The metabolic process occurs within a living organism. It includes all the biochemical reactions needed to break down food and convert it into energy. Enzymatic processes rely on the production of an enzyme to facilitate biodegradation. Enzymes are special proteins that act as catalysts to speed up the rate of chemical reactions at the molecular level. For example, some fungi produce a digestive enzyme from the tips of their hyphae that breaks down organic material in the soil. The material is then consumed by the fungi as food.

Biodegradation can occur either with the presence of oxygen in an environment or without; however, the byproducts produced in the process can vary. Aerobic biodegradation occurs in an environment where oxygen is present, such as in soil or open water. Organisms with an oxygen-based metabolism can speed up the breakdown of organic materials under aerobic conditions. Microorganisms use this oxygen to break down natural sugars in the organic material into smaller molecules. They use additional chemical reactions to further break down the molecules into water, carbon dioxide, and other compounds. These reactions produce energy, which is consumed by the microorganisms. Excess energy is released in the form of heat. This effect can be observed in decaying organic material called compost piles. Compost is often used by gardeners as a form of homemade fertilizer. Compost piles can build up heat very quickly. Under the proper conditions, compost temperatures can rise about 100–120 degrees Fahrenheit (40–50 degrees Celsius) in the span of a few days.

Biodegradation that occurs without the presence of oxygen is known as anaerobic biodegradation. This process is common in landfills and in water trapped below ground. Most organic materials biodegrade more slowly in an anaerobic environment than in an oxygen-rich environment. Anaerobic biodegradation also produces larger amounts of methane gas, a highly flammable substance that acts as a greenhouse gas. Greenhouse gases absorb infrared radiation and trap heat in Earth’s atmosphere. Excess greenhouse gases are considered one of the primary causes of global warming. Methane is a highly potent greenhouse gas and is more damaging to the environment than carbon dioxide emissions.

While unchecked methane emissions from landfills can be environmentally hazardous, gases produced through anaerobic biodegradation have proven beneficial when properly utilized. The anaerobic process is often used to biodegrade sewage sludge and wastewater. The methane gas created by the process can in turn be used as a fuel source. Unlike fossil fuels, methane is considered a renewable resource. In addition to methane, anaerobic biodegradation leaves behind solid organic compounds that can be used as fertilizer.

Anaerobic biodegradation occurs in four distinct stages. In the first stage, known as hydrolysis, microorganisms break down complex organic molecules into simpler sugars, amino acids, and fatty acids. Amino acids are simple organic compounds that contain carbon, hydrogen, oxygen, and nitrogen. Amino acids combine to form proteins. A fatty acid is a chemical compound made up of long chains of hydrogen and carbon atoms.

The next step, acidogenesis, occurs when acid-producing organisms turn some of the sugars and amino acids into a simpler type of fatty acid. Acidogenesis is also known as fermentation and is the same process that causes milk to sour. Acetogenesis occurs when organisms that produce a byproduct of acetic acid further break down the molecules created through acidogenesis. Acetic acid is a colorless liquid that is one of the main substances found in vinegar. The final stage of anaerobic biodegradation is methanogenesis. This occurs when methane-producing organisms convert the byproducts of the previous three stages into methane, carbon dioxide, and water.

In addition to oxygen in an environment, the rate at which a substance biodegrades is also affected by factors such as temperature and moisture. Biodegradation occurs more readily at a temperature range between 50–95 degrees Fahrenheit (10–35 Celsius). Higher temperatures not only accelerate the growth of microorganisms, but also speed up the rate of chemical reactions. Microorganisms grow much slower at temperatures below 40 degrees Fahrenheit (4.4 Celsius). For this reason, most refrigerators operate at a temperature of about 35–38 degrees Fahrenheit (1.7–3.3 Celsius). Biodegradation is also accelerated by the presence of water, so organic substances left in an environment with more moisture will degrade at a faster rate.

By studying the byproducts given off in the process of biodegradation, scientists have been able to estimate the amount of time it will take for an object to completely biodegrade in certain environments. For example, in a water-based environment, a newspaper can biodegrade in six weeks; on land, it would take about two to five months. Objects such as metal and glass also break down over a period of time but are not considered biodegradable because they are not consumed by microorganisms as a source of energy. In comparison to organic material, metal and glass can remain intact for centuries or millennia. For example, an aluminum can deteriorates in about eighty to one hundred years on land, while a glass bottle can take more than a million years to completely break down.

While the effects of biodegradation can readily be seen on organic material such as discarded food products and yard waste, most substances made from natural sources will easily biodegrade over time. For example, cotton-based clothing is made from the natural fibers of a cotton plant and is biodegradable; however, the amount of time cotton-based clothing takes to biodegrade can be lengthened during the manufacturing process. Crude oil, which is the source of many petroleum-based products such as gasoline and heating oil, is the decayed remains of prehistoric plants and animals. Crude oil is a hydrocarbon, a compound made up of hydrogen and carbon atoms, and as such, can be consumed and broken down by certain bacteria. As a result, microorganisms have been used to clean up oil spills and fuel contamination both on water and land.

Plastics are human-made substances that are manufactured from natural materials such as petroleum-based products, minerals, and plants. However, in the process of making plastics, the chemical properties of the substance change at the molecular level to form more complex molecules with stronger chemical bonds. Microorganisms have a difficult time breaking down such complex molecules and may take decades or centuries to accomplish the task. For example, plastic bags in a water environment may biodegrade in ten to twenty years; on land, the process may take five hundred years or more. Recycling is one way to prevent pollution caused by plastics in the environment, but it is not the only option. Scientists have developed certain plastics that are designed to biodegrade more easily. They have also created additives that can be introduced into an environment that will allow microorganisms to imitate the chemical reactions needed to break down the plastics.

Bibliography

“Aerobic and Anaerobic Biodegradation.” PolimerNet, www.polimernet.com/Docs/Aerobic%20-%20Anaerobic%20Biodegredation%20en.pdf. Accessed 20 Jan. 2020.

“Biodegradation, Explained.” Green Dot Bioplastics, 2020, www.greendotbioplastics.com/biodegradation-explained/. Accessed 20 Jan. 2020.

Chandler, Nathan. “How Biodegradation Additives Work.” HowStuffWorks, 2020, science.howstuffworks.com/environmental/green-science/biodegradation-additives.htm. Accessed 20 Jan. 2020.

DeBruyn, Jennifer, et al. “Biodegradation—Putting Biology to Work.” University of Tennessee Institute of Agriculture, October 2015, ag.tennessee.edu/biodegradablemulch/Documents/biodegradation‗factsheet.pdf. Accessed 20 Jan. 2020.

Dussud, Claire, and Jean-François Ghiglione. “Bacterial Degradation of Synthetic Plastics.” Tara Expeditions, 2020, oceans.taraexpeditions.org/en/m/science/news/bacterial-degradation-of-synthetic-plastics/. Accessed 20 Jan. 2020.

“Fungi.” Microbiology Society, 2020, microbiologysociety.org/why-microbiology-matters/what-is-microbiology/fungi.html. Accessed 20 Jan. 2020.

Joutey, Nezha Tahri, et al. “Biodegradation: Involved Microorganisms and Genetically Engineered Microorganisms.” IntechOpen, 14 June 2013, www.intechopen.com/books/biodegradation-life-of-science/biodegradation-involved-microorganisms-and-genetically-engineered-microorganisms. Accessed 20 Jan. 2020.

Lindsey, Blaise. Biodegradation and Bioremediation. ED-Tech P, 2020.

Samir, Aya, et al. "Recent Advances in Biodegradable Polymers for Sustainable Applications." npi Materials Degradation, vol. 6, no. 68, 19 Aug. 2022, doi.org/10.1038/s41529-022-00277-7. Accessed 18 Nov. 2024.

“What are Microbes?” Genetics Science Learning Center, learn.genetics.utah.edu/content/microbiome/intro/. Accessed 20 Jan. 2020.