Plant and pollution bioindication: A game of actions and reactions

With the industrial revolution came a major problem to the environment in form of pollution. Either from the industries themselves or from the products of the industries which are being used by humans in their day to day activities, pollutants are being released into the environment. As I am composing this article, somewhere around the world, the atmospheric air is becoming deadlier by the day, aquatic lives are being lost, underground water is being poisoned and human lives and properties continue to be under threat. While it is easy to identify the source of some of the substances responsible for the pollution, it is not as easy to know the source of others. However, it is quite pertinent to monitor the type and the extent of pollution in our environment so as to safeguard human lives and properties.

^{Better life for man, more pollutants in the environment. By Alfred T. Palmer Public Domain}

There are several devices that have been manufactured in order to monitor environmental pollution, especially the atmospheric ones. The devices are usually equipped with sensors.

These sensors work on the principle of measuring the attenuation of infrared radiation (with a specific wavelength) in the air. The sensors consist of an infrared radiation source, a light-water pipe and the infrared detector with the appropriate filter. The signal from the infrared detector is further amplified and then using other electronics evaluates the attenuation of the radiation, which is caused by carbon dioxide, and on this basis, the actual concentration of CO2 in the air is calculated. For simplification – the more CO2, the more the infrared radiation is attenuated, and electronics in the sensor can evaluate it. ^source

Aside from innovative devices, pollution or the presence of pollutants in the environment can be monitored using a living organism; a process known as biomonitoring. Using living organisms (especially plants) to monitor pollution has been reported to be cheaper, more convenient and more credible than other methods.

When it comes to using living organisms in the assessment of pollution in the environment, plants have and continue to be a major candidate. This is perhaps largely due to the fact that a lot of plant species from the different plant groups are able to tick all the right boxes. Cosmopolitancy, totipotency and ability to bioaccumulate pollutants are some of the attributes necessary for an organism to be considered a candidate of bioindication. The organism either show symptoms of being poisoned with the pollutant or analyzed in the laboratory for asymptomatic accumulation.

The different groups of plant can be utilized depending on whether the pollution type is aquatic, dryland or atmospheric. The algae are generally known to be aquatic and as such are mostly relevant in the assessment of aquatic pollutions. Comparison of species richness and diversity of algal assemblages in a water body suspected to be polluted with a reference water body can give an indication of the level of pollution and the type of pollutant.

^{Algae growing in an aquatic environment. public domain picture.}

Either independently or in combination with species richness/diversity comparison, the algal assemblages can be sampled using the appropriate method and taken to the laboratory for analysis. Structural or functional attributes such as biomass, chemical composition, and net primary productivity can be compared with reference site.

When it comes to using the application of plants specifically in the assessment of atmospheric pollutions, bryophytes (mosses and liverworts) have no known rivalry with any other plant group. They possess all the necessary attributes to the finest details and have a history of been used to monitor pollution even in the urban areas of advanced countries. They lack cuticles both on their leaves and other morphological structures and as such, obtain their nutrients and water requirements from the atmosphere. Their root-like rhizoid is merely utilised in the anchorage of the organism to substrates on which they grow.

Bryophytes are found growing almost anywhere moisture can be found. The group has been investigated and while some species were found to possess the capacity to bioaccumulate pollutants present in the atmosphere without showing any physical symptoms, a host of others show different degrees of reaction to the presence of pollutants. Depending on the nature of the pollutant, some species of the plant get their reproduction inhibited while the effect is exhibited by a reduction in photosynthesis in some.

^{A bryophyte growing on a tree. By GT1976 - Own work, CC BY-SA 4.0}

Over the years, scientists have devised a variety of methods through which bryophyte can be used in the monitoring of pollutants in the atmosphere. Some species are known to show symptoms relative to the concentration of pollutants. These symptoms are sometimes studied and analysed among different communities with varying levels of pollutants. In another method, species of bryophytes are removed along with their substrate from a non-polluted area to a polluted area and then studied for injury or any other symptoms which could be attributed to the presence of pollutants. This transplantation, as it is favouritely called can be done in the soil using a moss bag or the bryometer method. Lastly, the plant can be used in pollution assessment using ecophysiological and phytosociological methods.

The advanced plants (the gymnosperms and angiosperms of this world) are not left out in pollution biomonitoring. Higher plants have been studied and discovered to respond to the presence of pollutants in the atmosphere with different species showing different characteristic symptoms to different pollutants. Air pollutants such as sulphur dioxide, ozone, Peroxyacetyl Nitrate, hydrogen fluorides, chlorine, ammonia, ethylene and particulate matters inflict injury symptoms on plants, ranging from death of portions of plants (necrosis), premature dropping of leaves (abscission), down-curving of plant leaves (epinasty) to chlorophyll destruction or degeneration (chlorosis).

Recent research findings have shown that not only could some plant species be utilized in monitoring pollution but can also be used to trace the history or the evolution of environmental pollution in a particular area. In one of the researches by some Brazilian workers, a tree species, Tipuana tipu was found to have a high tolerance for heavy metals absorbed by its root from the surrounding soil as well as other pollutants in the surrounding atmosphere which are usually washed down by rainwater and absorbed by the plant's roots. These pollutants get translocated by the xylem into the body of the tree where they become accumulated and stored in the growth rings of the tree. Analysis of the chemical composition of the annual growth rings of the tree in the laboratory showed a varied concentration of heavy metals such as Cadmium, Copper, Nickel and Lead. Each annual growth ring gave an indication of the extent of the pollutants in the environment for past years.

^{Tipuana tipu, a tree that has been proven to reveal spatial and temporal concentration of heavy metal pollutants in the environment. By Felagund - Own work, CC BY-SA 3.0}

The study of Tipuana tipu revealed that it can be used to study temporal changes in the environmental concentration of heavy metal pollutants. It would be interesting to know if other tree species would show a similar pattern of variation according to the ambient concentration of pollutants. I am quite sure attention would be given to this area of research and more information would emerge as times go by.

Thank you all for reading.

References

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