Role of Rhizosphere Microbiota of Plants Growing in Heavy Metal Contaminated Environments as Ecofriendly Decontamination Bio-Tools and their Role in the Context of Human Health – A Short Review

Introduction

During the last century, the Earth’s spheres (Biosphere, Lithosphere, Hydrosphere, and Atmosphere) are being contaminated at an alarming rate by several contaminants such as organic and organometallic contaminants, radioactive isotopes, gaseous pollutants and nanomaterials, etc. due to man’s mining, agriculture, industrial practices and processes activities, in addition to the natural anthropogenic sources [1]. Among the pollutants, the heavy metals (HMs) such as chromium, nickel, cadmium, lead, etc. and metalloids such as arsenic are the major cause of threats to our environments due to them being non-biodegradable, persistent, and accumulative [2]. These elements are naturally present in the earth’s crust as well but, due to human activities, their concentrations in all kind of environments have increased to alarming levels and upon their entry into food chain these heavy metals affect the metabolic processes of organs and organelles like mitochondria, nuclei, endoplasmic reticulum, etc. of living organisms [2]. However, a few of these elements such as iron, zinc, cobalt, copper, etc. also act as micronutrients for animals and plants and their deficiencies may cause diseases in plants and animals. On the contrary, an access of these elements may have toxic effects.in living organisms.

Roots of plants growing in agricultural, industrial, polluted, aquatic (creeks, river "banks" in stationary or slow flowing fresh or brackish waters in swamps, creaks, drains and channels, and in seeping areas), derelict terrestrial habitats (industrial sites, deserts, aquatic habitats, etc) are found to be occupied by universal and ubiquitous symbiotic arbuscular mycorrhizal fungi (AMF) and AMF spores were recovered from the sediments and soil [3, 4]. These beneficial Plant Growth Promoting Rhizobia (PGPR, and AMF rhizosphere microbes), effect plant growth directly by N-fixation, P-solubilization, siderphores / phytohormones production or indirectly by biocontrol activities and tolerance to abiotic stresses. PGPR and AMF are well researched by scientists over the last few decades and their role as biological fertilizers in reducing the utilization of chemical fertilisers and as bioinoculants, and their effectiveness in improving plant production and yield are now well established. This minireview aims at exploring [1] the potential sources of HMs pollutants in the environment [2]. This mini review aims at (1) exploring the potential sources of HMs pollutants in the environment; (2) the role of the rhizosphere microbiota of the plants growing on these HM-contaminated sites in human health; and (3) various biotechnology strategies involved in decontamination of such polluted ecosystems.

Potential Sources of HM-Pollutants

HM-pollution in the air, water, and soil environments is caused by both natural (volcanic activity, erosion and weathering of minerals, forest fire, etc) and man-initiated anthropogenic activities such as industrial, mining and smelting of metalliferous ores, and using HM-containing compound for domestic, agricultural and horticultural applications such as pesticides, paints and pigments, tanneries, coal combustion, domestic, municipal and industrial effluents, aerial emissions from combustion of leaded fuels, batteries waste, etc.) . Human population growth in the world demanded more food which was met by intensive use of chemical fertiliser and protection of horticultural and agricultural products from pathogens by use of chemicals such as pesticides worldwide. These practices resulted in agrochemical residues in soil, water and air environments, which had toxic effects on human and non- human biota [5]. HMs can enter the ecosystem by deliberate/ unintended anthropogenic actions such as oil spills, mining and smelting, fires, application of sewage sludge, coal combustion, etc. Various physical and chemical factors such as pH, temperature, speciation, temperature, movement, speed and direction of wind or waters, solubility, availability, mobility, etc., influence the mobility and distribution of HMs [6]. These contaminants are a significant threat to all aquatic, terrestrial and aerial environments and affect the overall dynamics of the atmosphere.

HMs and Human Health

HMs are non-biodegradable and their abundance play an important but on binding with the active element within a cellular protein, they bioaccumulate in human systems and become toxic in large amounts causing physiological complications and damage to nucleic acids (i.e. neurotoxicity of nervous system damaging neurons and inhibiting neurotransmitter), and their repair mechanisms, misfolding or aggregation of proteins (enzymes), and damaging cell membrane lipids, i.e. resulting in cell damage, cellular function loss, and even carcinogenesis [7, 8]. These authors have tabulated the classification, uses, properties of various HMs and their thresholds and provided a detailed description of the toxicology and pharmacokinetic processes of various HMs such as Aluminium, Vanadium, Chromium, Manganese, Cobalt, Nickle, Zinc, Copper, Molybdenum, Arsenic, Silver, Cadmium, Selenium, Mercury, etc [7]. The chemistry of HMs contributes important implications within the human- ecological context. Recently, Nkwunonwo, et al. [9] reviewed the health implications of HMs in food chain in Nigeria and reported that the major staple foods are the major host of carcinogenic and mutagenic components of HMs. Recently, Ali, et al. [6] reviewed the sources of HMs in the environments and stated that the rising degree of environmental pollution with HMs raises a concern and need to be recognised and addressed by public authorities as prolonged exposure and increased accumulation of toxic HMs in our environments is a Public Health issue.

Rhizosphere Microbiota of Plants Growing on HM-Contaminated Sites

In addition to providing nutrients to plants, rhizospheres of plants growing on contaminated sites, the microbes such as AM Fungi and Plant Growth Promoting Rhizobia (PGPR), can improve the soil structure, increase decompositions rates, act as bio-protectant against phytopathogens by producing cell wall lysing enzymes and antibiotics. Extrametrical fungal hyphae of AMF also play a role in absorption, sequestration, and translocation of substrate contaminants such as Heavy Metals (HMs) in mycorrhizal plants [10]. Besides various free- living N-fixing bacteria (Pseudomonas, Bacillus spp.), many symbiotic bacteria (Rhizobium, Frankia spp.),P-solubilizing/ mobilising bacteria (Pseudomonas, Bacillus sp.), and fungi (AMF, ectomycorrhizal fungi, Orchid fungi), act as beneficial rhizosphere microbiota capable of converting unavailable forms of N and P into available forms of nutrients, and acting as biofertilizers [5, 11]. They occur in the rhizoplanes and rhizospheres, or even in the roots and exert beneficial effects on plant growth, and cause N-fixing root nodules in legumes where symbiotic biological N-fixation occurs via their enzyme reducing N-gas into nutrients. In addition to N-fixation, these microbes cause P-solubilisation, produce phytohormones and siderophores. Indirectly, these rhizobia microbiota assist their associated plants in reducing abiotic stresses, controlling soil/root pathogens by producing antibiotics.

Although rhizobia commonly existing within the root-nodules of the leguminous plants growing on HM contaminated sites is a common phenomenon, but it is also now becoming evident that, besides Rhizobium, there are many other bacteria co-existing within the nodules, but their significance or role in biological N-fixation is still unclear [12] and papers cited therein. These non-pathogenic microbes within leguminous root nodules as endophytes could be enhancing the N-fixing processes in legumes [13].

HMs Decontamination Strategies

HM contaminants are notoriously persistent in the environment and various physio-chemical and bio-remedial strategies are employed to decontaminate as below [14]:

Microbe-assisted Phyto-management Strategies

Phytoremediation and the microbes associated with roots, either symbiotically or free-living in the rhizospheres of the plants used for this purpose, has emerged an alternate strategy. 'Khan and Mohammad [18] reviewed the principles and application of this strategy, and provided an overview of the use of fast growing, non- food bioenergy plants, like hemp and vetiver grass, and their associated microbiota that can both tolerate and immobilize HMs in the roots, i.e. sequestrate contaminant HMs thereby protecting plants from metal toxicity. Various microbe-assisted Phyto management strategies, such as Crop Rotation, Co Cropping or Mixed Cropping, etc., have been proposed to employ rhizosphere microbiota for enhancing growth of plants used for nano- phytoremediation, increasing their biomass for bioenergy production purposes and thereby stabilizing heavy metal polluted terrestrial and aquatic ecosystems [18].

Conclusion

• Globally, our air, water, and soil are subject to increasing heavy metal contamination due to natural and man’s anthropogenic activities. Although some of the HMs are essential elements, their deficiency in the ecosystem may cause diseases in plants and animal, including man. On the contrary, an access of these elements may have toxic effects in living organisms.
• Roots of plants growing in HM-contaminated terrestrial and aquatic habitats are found to be occupied by universal and ubiquitous endosymbiotic fungi and plant growth promoting bacteria which play an important role in directly or indirectly plant growth. These rhizosphere microbes of plants growing on HM-contaminated substrates received much attention by researchers during the last two decades and their effectiveness in improving plant growth and yield is now well established.
• This mini review discusses various physio-chemical and biological decontamination strategies and the role these microbes play in plant and animal health. Among the various biotechnological strategies involved in decontamination of such polluted ecosystems, special attention is given to phytoremediation as the environmentally safe plant-based techniques.
• It is proposed that the integration of high biomass producing non-food bioenergy crops such as cannabis and HM-adapted site-specific rhizosphere microbiota will synergistically further improve the process of phytoremediation. As pointed out by Khan [5], the role of indigenous microbiota in the biodiversity and sustainability of terrestrial and aquatic ecosystems is not fully explored and exploited for their enhanced phytoremediation property.