Indian coffee plantations grow some of the world’s finest quality Arabica and Robusta Coffee. In recent years a vast majority of coffee gardens are subjected to various stress due to the impact of climate change, global warming, and indiscriminate use of synthetic fertilizers, pesticides, and herbicides. Unfortunately, heavy metals like copper, zinc, molybdenum, cobalt, and iron form the base of the formulations. As such, coffee soils are subjected to an alarming increase in heavy metal toxicity. The pollution level has reached significant levels with increasing metal levels and deterioration of agricultural soil quality.
The idea behind writing this article is to create awareness among the coffee community to be sensitized to the way they use chemicals containing heavy metals and how Bioremediation has become an accepted remediation technology and is continually evolving.
Heavy metals are evaluated in coffee because it is exported to other countries, especially the most advanced countries, which have stringent quality control measures in place. In fact, in the recent past, a few containers of coffee exported to France and the U.S. was rejected because of high concentrations of certain chemicals, above the permissible limits. More importantly, as coffee moves higher on the value chain, single-origin coffees are making huge inroads into the specialty coffee sector. Indian coffee which used to get huge premiums in the International market, for its superior quality, cannot afford to be blacklisted due to heavy metal toxicity.
Heavy metals are toxic even in very small amounts, such as arsenic, cadmium, chromium, nickel, and mercury which are classified as category one carcinogens, as they increase cancer risk in humans even with mild to moderate exposure. Unlike contaminants that can be degraded to harmless chemicals, heavy metals cannot be destroyed.
What are Heavy Metals
Scientific Literature clearly spells out that Heavy metals by definition are metallic elements that have a high atomic weight and a much higher density at least five times that of water. They are stable elements, i.e., they cannot be metabolized by the body, and are bio-accumulative, i.e., passed up the food chain to humans. Even at very low concentrations, they are highly toxic and can cause damaging effects.
Heavy Metals In Plants
One of the characteristic features of most heavy metals is that they can be absorbed and stored by coffee plants in the roots, shoots, or berries, but the contents are usually much higher in the vegetative components than in the fruit. Basharat Ali and Rafaqat A. Gill, in their research paper “ Heavy metal toxicity in plants: recent insights on physiological and molecular aspects, Volume11, state that at higher levels, heavy metals have abilities to create severe toxic symptoms in crop plants, and therefore, their utilization and uptake are greatly controlled by the plant cells. To combat metal stress, plants have evolved different defense modifications viz., less uptake of metals, phytochelatins binding, and the activation of numerous antioxidants. The first strategy to combat heavy metals in plants is to activate their enzymatic (POD, SOD, APX, CAT, GR) and non-enzymatic (ASA, GSH) antioxidants. The second strategy is to activate their root exudates and cell wall. The third strategy is to utilize plants (phytoremediation) which can absorb the metal content from the soil at higher rates. Lastly, exogenous applications of growth regulators, nutrients, and organic amendments such as ALA, GABA, MeJA, AsA, NPs, MLE, and Se provide some kind of relief to crop plants to combat the hazardous effects of heavy metals.
Heavy Metals in Soils
Soil heavy metals are divided into two categories from the biochemical characteristics: one is harmful to crops and humans and animals, such as Pb, Cd, and Hg; the other benefits on the biological when in a constant, but when excessive, it will damage to biological, such as Cu, Zn, Mn and so on.
The bioavailability and occurrence of heavy metals, however, are influenced by other factors such as type of soil, clay, sand. Laterite, texture, water holding capacity, porosity, pH, cation-exchange capacity, organic-matter content, soil texture, and interactions among elements. The stability of soil and water is impacted by pollution from heavy metals.
Heavy Metals and Microorganisms
A review of the literature points out that Soil microorganisms, being the maintainers of the structure and function of the ecosystem, are a key factor in determining soil quality. Soil microorganisms can regulate soil properties, but they are also readily affected by the physicochemical properties of the soils. Studies have shown that microorganisms can significantly promote the circulation of soil nutrients, maintain soil fertility, and improve crop health. It is known that toxic stress induced by heavy metals/metalloids can seriously affect the abundance and structure of the soil microbial communities and diversity.
Studies have shown that long-term heavy metal contamination of soils has harmful effects on soil microbial activity, especially microbial respiration. Aside from long-term metal-mediated changes in soil enzyme activities, many reports have shown large reductions in microbial activity due to short-term exposure to toxic metals. Moreover, habitats that have high levels of metal contamination show lower numbers of microbes than uncontaminated habitats.
Flavobacterium, Pseudomonas, Bacillus, Arthrobacter, Corynebacterium, Methosinus, Rhodococcus, Mycobacterium, Stereum hirsutum, Nocardia, Methanogens, Aspergillus niger, Pleurotus ostreatus, Rhizopus arrhizus, Azotobacter, Alcaligenes, Phormidium valderium, Ganoderma applicants are some microbial species that help in bioremediation of heavy metals.
Several fungi have also been found to be good accumulators of heavy metals. Rhizopus arrhizus.
Bioremediation has been considered a promising eco-friendly and sustainable method for heavy metal pollution remediation. Microorganisms play an essential part in the bioremediation of heavy metals. Bioremediation is a biotechnical process, which abates or cleans up contamination. It is a type of waste management technique that involves the use of organisms to remove or utilize the pollutants from a polluted area. Bioremediation employs the use of living organisms, like microbes and bacteria, in the removal of contaminants, pollutants, and toxins from soil, water, and other environments.
Types of Bioremediation
Microbes are stimulated to begin the remediation process via chemicals or nutrients that activate them.
Used mainly in cleaning up soil contamination, this process adds bacteria to the surface of the affected area, where they are then allowed to grow.
Converts toxic materials into inert ones using the native microbiome of the affected area.
Insitu And Exsitu Bioremediation
Bioremediation can either be done “in situ”, which is at the site of the contamination itself, or “ex-situ,” which is a location away from the site. Ex-situ bioremediation may be necessary if the climate is too cold to sustain microbe activity, or if the soil is too dense for nutrients to distribute evenly.
Advantages of Bioremediation
The most significant advantage of adopting bioremediation technologies is the positive impact on the environment. Nature is used to fix nature in bioremediation. Bioremediation offers numerous advantages over other clean-up methods. By relying solely on natural processes, it minimizes damage to ecosystems. Bioremediation often takes place underground, where amendments and microbes can be pumped in order to clean up contaminants in groundwater and soil. Consequently, bioremediation does not disrupt nearby communities as much as other clean-up methodologies. The bioremediation process creates relatively few harmful by-products (mainly due to the fact that contaminants and pollutants are converted into water and harmless gases like carbon dioxide). Finally, bioremediation is cheaper than most clean-up methods
Limitations for Bioremediation
Not all soil microbes can act on heavy metals. It depends on heavy metal toxicity levels, type of heavy metals, and other soil factors like ph. There could be another problem associated with the breakdown of heavy metals due to microbial action. The intermediary product formed during mineralization may be more toxic than the original one, causing irreparable damage to the ecosystem. Only when environmental circumstances allow for microbial growth and activity can bioremediation be effective. In some cases, the introduced organism will have to compete with indigenous microbes which may favor or be antagonistic. As bioremediation procedures involve microbial action, it is time-consuming.
Repeated use of herbicides, insecticides, and chemicals laced with heavy metals like mercury, lead, cobalt, zinc, cadmium, and iron in coffee soils is a cause for concern. Care should be taken to apply these chemicals Only in concentrations, not higher than those permitted for agricultural soil recommended by the WHO and FAO. Even though the situation has not reached alarming proportions across the length and breadth of the coffee belt, due to detrimental and trace heavy metals, an awareness, needs to be created on the regulated use of such chemicals. At present, the data on the relationships between the structure of the soil microbial communities, diversity, and function with respect to total/bioavailable heavy metal/metalloid species are limited in Indian Coffee Plantations. An effort could be made in isolating microbes that can tackle heavy metal toxicity in coffee based on various Agro Climatic Conditions.
Anand T Pereira and Geeta N Pereira. 2009. Shade Grown Ecofriendly Indian Coffee. Volume-1.
Anand Titus Pereira & Gowda. T.K.S. 1991. Occurrence and distribution of hydrogen-dependent chemolithotrophic nitrogen-fixing bacteria in the endo rhizosphere of wetland rice varieties grown under different Agro-climatic Regions of Karnataka. (Eds. Dutta. S. K. and Charles Sloger. U.S.A.) In Biological Nitrogen Fixation Associated with Rice Production. Oxford and I.B.H. Publishing. Co. Pvt. Ltd. India.
Subba Rao. N. S. 1998. Soil Microorganisms And Plant Growth. Oxford and IBH Publishing Co.
Bopanna, P.T. 2011.The Romance of Indian Coffee. Prism Books Ltd.