What is Biodegradation? Types and Examples

What is Biodegradation?

A leaf at an advanced stage of biodegradation
A leaf at an advanced stage of biodegradation

Biodegradation is the systematic and complete mechanical, physical, and chemical alteration by decomposition of organic material and other matter by biotic microorganisms like bacteria and fungi aided by abiotic factors of an ecosystem like climate and environmental conditions like acidity and topography.

Definition

It is defined as the breakdown of materials into environmentally processable and absorbable products such as water, carbon dioxide, and biomass (the renewable organic material derived from plants and animals) by the action of naturally available microorganisms under normal environmental conditions.

Another definition describes biodegradation as the biologically catalyzed (accelerated reaction) reduction in complexity of chemical compounds and the process by which organic substances are broken down into smaller compounds by living microbial organisms.

It is generally an entirely natural process that occurs without any artificial input. However, when applied to an agronomical human-initiated, contained, and controlled biodegradation process such as for the production of fertilizer, it is termed composting.

Stages of Biodegradation

Organic and inorganic materials undergoing biodegradation in an aquatic biome
Organic and inorganic materials undergoing biodegradation in an aquatic biome

Biodegradation is a systematic process that takes a significant amount of time that can be longer depending on the material involved. This process is carried out in phases and can be categorized into three main stages namely;

  • Biodeterioration
  • Biofragmentation
  • Bioassimilation

Biodeterioration

A fruit undergoing biodegradation
A fruit undergoing biodegradation

Biodeterioration is chemical, physical, or physicochemical (involving both physical and chemical factors) alteration of a product or entity that reduces its inherent quality, cohesive strength, and consequently its overall value.

In spite of the decrease in the overall value of a material or substance undergoing biodeterioration in terms of its utility as a whole, it decomposes thoroughly enough to increase its reabsorption value.  This alteration is caused by micro or macro conditions and organisms or their enzymes.

It is also defined as the undesirable change in the properties of materials brought about by the vital activities of organisms.

Biodeterioration is primarily caused or accelerated by the abiotic factors in an ecosystem. The prevailing climatic or environmental conditions have the effect of weakening the structural integrity of the material.

Some of the abiotic factors that cause the biodeterioration of substances include chemicals found in the environment, mechanical compression from atmospheric pressure that occurs in an ecosystem, the effects of light and temperature.

Biodeterioration typically occurs at the beginning of biodegradation, however, it is also known to occur simultaneously with biofragmentation in peculiar instances.

Biofragmentation

Biofragmentation is the second stage of biodegradation and is defined as the lytic (involving lysis: the decomposition of cell membranes by enzymic, viral, or osmotic mechanisms that weaken their bonds) process that involves the splitting or breaking of the bonds of a polymer (a macromolecular substance or material consisting of several repeating subunits).

The decomposition of these polymer bonds generates products that are termed oligomers (a molecule that contains a set of identical or similar subunits) and monomers (a molecule that is capable of polymerization [bonding in a series of identical links] with other monomers to form chains or linked networks).

When biofragmentation takes place in the presence of oxygen it is termed aerobic digestion and called anaerobic digestion when it occurs in the absence of oxygen. Both reactions produce resultant biomass, residual material, carbon dioxide, and water. However, anaerobic digestion yields methane gas in addition to the listed products.

Bioassimilation

Assimilation or bioassimilation is the last stage of biodegradation and is the process of the products of the concluded preceding biofragmentation being absorbed by the surrounding medium (soil) on a microbial cell level.

Most of the products of the biodeterioration and biofragmentation stages of biodegradation that have just been completed are transported throughout the cells by a set of biological mechanisms (membrane carriers) that regulate the movement of solutes like small molecules and ions through biological membranes.

Other derivative products of biodeterioration and biofragmentation that have not been sufficiently decomposed and conditioned to an easily assimilatable degree will have to undergo further decomposition and bioconditioning to break down and simplify them enough to be diffused through microbial cell membranes.

Adenosine triphosphate ATP (an organic compound and hydrotrope that provides the energy that drives several cell processes) or elements of anabolism (also called biosynthesis is the sequence of enzyme-catalyzed reactions through which relatively complex molecules are formed in living cells from nutrients) is produced by the products of biodegradation after they pass through catabolic pathways (a sequence of intercellular metabolic reactions that decomposes large molecules into smaller units)

Biodegradability

A biodegradable cup produced from organic materials
A biodegradable cup produced from organic materials

Biodegradability is the susceptibility or confirmed tendency of a material or substance to systematically decay and decompose overtime after a period of exposure to the biological elements.

Although it is believed that technically every type of material and substance decomposes over time, the distinction between them is based on their individual rates of decomposition. Organic material generally has a high biodegradability and leaves less residue than synthetic materials.

Materials that have higher biodegradability have weaker cohesive bonds and will generally decompose relatively quickly with minimal environmental exertions. These materials are mostly made up of organic substances that generally have a higher moisture content. Composting is a pertinent example of this.

Some synthetic-based materials like certain plastics, other petrochemically produced materials, and some types of carbon fiber are being increasingly designed with infused substances like organic materials to increase their biodegradability with the purpose of reducing harm from toxic pollution to the environment.

Highly stable compounds decompose at a slower rate and have lower biodegradability. They are sometimes referred to as non-degradables due to the length of time it takes them to decompose.  Thus, the biodegradability of a material depends on its rate of decomposition.

Factors affecting biodegradation

An old typewriter undergoing decomposition despite having a low biodegradability
An old typewriter undergoing decomposition despite having a low biodegradability

The rate of decomposition of substances undergoing biodegradation is determined by certain influencing factors including:

  • The presence of substrates (a substance on which enzymes act to produce a chemical reaction) and their levels of concentration.
  • A redox environment. (An atmosphere suitable for reduction and oxidation reactions.)
  • Availability of inorganic nutrients. (Nutrients that lack carbon such as phosphorus, magnesium, potassium, zinc, iron, and selenium, etc.)
  • The adaptive response of microorganisms.
  • The prevailing temperature of the location.
  • The level of exposure and intensity of light.
  • Water and moisture availability and activity.
  • The availability of oxygen.

Below is the approximated rate of decomposition of compounds in a marine environment.

A table showing how long it takes for the decomposition of compounds in a marine environment
    Item   Approximated Decomposition Time
    Paper towel     Two to four (2-4) weeks
   Newspaper    Six (6) weeks
   Fruit core    Two (2) months
  Cardboard material   Two (2) months
 Wax coated carton   Three (3) months
 Cotton material   One to five (1-5) months
  Wool material   One (1) year
 Plywood  One to three (1-3) years
 Painted wood  Thirteen (13) years
 Plastic bags   Ten to twenty (10-20) years
Tin Fifty (50) years
Disposable diapers Fifty to one hundred (50-100) years
Plastic bottles One hundred (100) years
Aluminum Two hundred (200) years
Glass Yet to be determined

Below is the estimated rate of the decomposition of compounds in a terrestrial environment.

A table showing how long it takes for the decomposition of compounds in a terrestrial environment
   Item   Estimated Decomposition Time
  Vegetables   Five (5) days to one (1) month
 Paper  Two (2) to five (5) months
 Cotton material  Approximately six (6) months
 Rind from oranges  Approximately six (6) months
 Leaves from trees  Twelve (12) months
 Woolen materials  One to five (1-5) years
Carton material coated with plastic  Five (5) years
 Treated leather materials  Twenty-five to forty (25-40) years
 Nylon Fabrics  Thirty to forty (30-40) years
 The metallic element tin (Sn)  Fifty to one hundred (50-100) years
  Aluminum material and products  Eighty to one hundred (80-100) years
Glass material and products Approximately one million (1,000,000) years
Styrofoam materials and products Five hundred (500-∞) years and to infinity
Certain plastics and their products Approximately five hundred (500-∞) years and to infinity

Measurement of Biodegradation through Respirometry

Respirometry is a developed technology used to measure and interpret the biological exchange of gases including oxygen consumption and carbon dioxide production through the application of calorimetry (the science of measuring changes and variables in the state of an entity to derive data on the transfer of heat resulting from the alteration of its state due to physical changes, chemical reactions, or phase transitions under given conditions).

Respirometry tests can be used to determine the rate of decomposition in biodegradation only for instances of aerobic decomposition as they cannot be carried out in the absence of oxygen.

Biodegradation of Plastics

Severe plastic pollution
Severe plastic pollution

The accumulation of plastics and the extensive long-term damage they do to wildlife and the environment at large has been a major threat to the health of biological life and the entire ecosystem since the use of plastics became common.

There have been multiple substances, materials, and gases that have contributed to the devastating environmental pollution being suffered all over the world. Plastics constitute perhaps the largest percentage of this global contamination. Much of this plastic contamination ends up in the oceans doing serious damage to marine life and important ecological processes like the production of oxygen.

Plastic pollution in a marine habitat
Plastic pollution in a marine habitat

Non-degradable plastics, in particular, have been determined to accumulate at the alarming rate of twenty-five million tonnes per annum (25,000,000t/12 months) globally. This figure also includes plastic-based non-woven materials.

This is why there have been conscious efforts to reduce and control the massive and increasing plastic pollution especially in developed societies and the world over in general.  The natural decomposition process from environmental exposure and the action of micro and macro-organisms has been shown to biodegrade not only organic matter but also inorganic materials like plastic.

Certain types of microorganisms have been found to break down plastic materials more successfully and more efficiently than others. Modern scientific and agronomical research is exploring practical ways of applying this knowledge to help control plastic pollution in an eco-friendly way by using natural processes like bioremediation through biodegradation.

Efforts are also being made to lessen the negative impact of excessive global production, usage, and pollution of plastics by taking certain preventive measures including:

  • The development of alternative materials to plastic disposable products that biodegrade easier and faster.
  • Stricter laws and regulation of the disposal of industrial plastic waste.
  • Improvement of disposal practices like the attachment of mesh filters to drainage channels.
  • The infusion of organic and easily degradable materials (biodegradable polymers) into plastic manufacturing to enhance their decomposition. (Example: The addition of starch to plastic.)
  • The use of organic organisms to decompose discarded plastic. (Bioremediation)

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