Pollution is a growing problem around the globe and only increasing as industries rise to face the challenge of a booming population. Pollution from factories, oil spills, and heavy metal contamination can linger in the environment, leading to potentially catastrophic effects. These contaminants can leach into ground water and contaminate agricultural soil. Degradation of agricultural soil in turn can lead to a reduced crop yield, which can exacerbate existing social
problems of food insecurity and poverty that are rising with our growing population. However, there is a potential solution to the growing epidemic of man’s trash: bioremediation.
Bioremediation involves the use of organisms to clean contaminated soil and groundwater. These organisms are often microbes that have a remarkable ability to utilize the most improbable of resources in their environment to synthesize components essential to their survival. Bioremediation takes advantage of indigenous (naturally-occurring) or engineered microbes that are capable of consuming and processing environmental contaminants into a form that is no longer toxic to humans, animals, or agriculture. However, to effectively metabolize environmental contaminants, microbes require additional nutrients that are often scarce in the polluted area. In this scenario, a technique called biostimulation is used, where limiting nutrients such as nitrogen and phosphorus, are added to the environment to stimulate the metabolic pathways in microbes that are needed for decontamination. Biostimulation is sufficient for bioremediation if indigenous microbes are capable of processing the contaminant. However, if indigenous microbes are not present in the contaminated environment, the alternative is bioaugmentation, where microorganisms capable of degrading the contaminant are introduced to the site. It is important to understand the metabolic pathway used by a microbe to transform the contaminant to ensure this microbe (engineered or indigenous) will produce a less harmful by-product than the original contaminant. Additionally, a better understanding of the metabolic pathway will shed light on what environmental conditions will favour degradation, which can then allow for manipulation of this environment to favour bioremediation. Such factors include temperature, water potential, pH, and other nutrients.
Bioremediation has been used in several practical field tests to great effect. A notable example was after the Exxon Valdez oil tanker accident in 1989, where 40.8 million litres of crude oil were released into the Gulf of Alaska. A portion of the contaminated shoreline was used in a biostimulation test to determine if bioremediation could successfully clean up the area. Since petroleum is high in carbon but low in nitrogen and phosphorus, fertilizer containing these components was sprayed along the contaminated shoreline to increase the activity of indigenous hydrocarbon-degrading bacteria. This process had great effect on the four beaches that the fertilizer was applied to from 2011 to 2012. Biostimulation increased the natural degradation rate by 12- to 60- fold. However, more than 30 years after the oil spill, harmful hydrocarbons still remain along the shoreline and maintain their long-term negative effects on the ecosystem.
Although this case study shows the power of bioremediation, it also demonstrates some potential drawbacks. Microbes take time to perform the degradation process. It is not an ideal technique if an immediate solution to the problem is required. Unfortunately, the faster alternative is to haul the contaminated material away to be incinerated or dumped into a landfill. Comparatively, bioremediation is a publicly-accepted, greener technique that involves directly treating the contaminated site. While promising, more work needs to be done to improve this technique for wider field applications.
Recently, much interest has gone toward the use of bioremediation to decontaminate agricultural soil from heavy metal and metalloid contamination. Build up of metals (such as cadmium, arsenic, copper, mercury, lead or chromium) in the soil can be due to factory runoff and industrial pollution. Heavy metal contamination in the soil can lead to loss of crop yield which poses a threat to global food security. While some plants, such as most crops, fare poorly in soil contaminated with heavy metals, other plants are specially adapted to grow in the presence of these contaminants. Such plants make up another type of bioremediation, referred to as phytoremediation, which can be employed to protect crop yields.
Phytoremediation takes advantage of plant species that have adaptations to neutralize the toxicity of heavy metals through a variety of means. One neutralization method used by plants is restricting heavy metals only to their roots by converting them to a non-toxic form that will not move to the rest of the plant. Additionally, some plants can trap metals in their cell wall to prevent these contaminants from affecting the health inside the cell. Finally, certain plants can detoxify metals within the cell by modifying their structure. When metals are moved above the root system, plants that are repulsive to herbivores should be selected to prevent the metals from entering the food chain. In any of these cases, metals are taken out of the soil and groundwater where they are stored in a form that should not harm the environment. Plant species need to be carefully selected for optimal growth in the region of interest, where they can often be grown alongside crops to increase the productivity of the agricultural region.
As a phytoremediation trial, two plant species, Pteris vittata and Sedum alfredii, were planted in southwestern China in an area contaminated with lead, cadmium and arsenic. Over the course of two years, the cultivation of these plants in the area led to a significant reduction of metal contamination. Therefore, using plants for bioremediation is both cost-effective and feasible in cleaning contaminated soils.
Microbial bioremediation can also be performed in soils contaminated by heavy metals. Some bacteria can release small molecules called siderophores which bind heavy metals and change them into a form that is not harmful to the environment. Additionally, some bacteria can take up and metabolize heavy metals into a non-toxic form. Microbial bioremediation and phytoremediation can be used in conjunction or to replace one another where the environment is more suited to a particular technique.
Overall, bioremediation is a slow but growing solution to help eliminate pollution from contaminated soils and groundwater. It takes advantage of the ability of plants and microbes to accumulate or break down contaminants to eliminate pollutants. Ongoing research into improving growth, survival, and ability to break down contaminants will help to improve the practicality of bioremediation in the field. Understanding these organisms and harnessing their potential to reduce pollutants can help to create a cleaner and greener world.
References
1. Barbato, R. A. & Mike Reynolds, C. 22 – Bioremediation of contaminated soils. in Principles and
Applications of Soil Microbiology (Third Edition) (eds. Gentry, T. J., Fuhrmann, J. J. & Zuberer, D. A.)
607–631 (Elsevier, 2021). doi:10.1016/B978-0-12-820202-9.00022-8.
2. Hou, D. et al. Metal contamination and bioremediation of agricultural soils for food safety and
sustainability. Nat. Rev. Earth Environ. 1, 366–381 (2020).
3. Chandran, H., Meena, M. & Sharma, K. Microbial Biodiversity and Bioremediation Assessment
Through Omics Approaches. Front. Environ. Chem. 1, (2020).
4. Office of Land and Emergency Management. Community Guide to Bioremediation. (2021).
Meghan Kates
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