Muhammad Yahya Khan

Petroleum Hydrocarbons-Contaminated Soils: Remediation Approaches

Petroleum, the backbone of today’s mechanized society, now has become a threat to environment due to extraction and transportation. Accidental oil spills occur regularly at many locations throughout the world. Contamination of soil and water resources with petroleum oil and its products has become a serious problem due to carcinogenic and mutagenic compounds. Efforts are now focused on seeking potential remediation techniques for cleanup of petroleum hydrocarbons-contaminated soils in a cost effective and eco-friendly way. Various physical, chemical and biological remediation strategies have been used to restore contaminated soils. However, plant assisted bioremediation of petroleum hydrocarbons-contaminated soil is getting more attention as compared to sole use of either microorganisms or plants. The challenging task for such efforts to be successful is not only the survival of microorganisms upon their inoculation into hostile contaminated environment but also positive plant-microbe interactions. Bacteria having ACC-deaminase enzymes are considered helpful for plants in stressed environment. We have discussed that use of bacteria equipped with dual traits of bioremediation potential and ACC-deaminase activity in association with plants can be a good approach for remediation of petroleum hydrocarbons-contaminated soil.

Phytoremediation of Light Crude Oil by Maize (Zea mays L.) Bio-Augmented with Plant Growth Promoting Bacteria

ABSTRACT The aim of this study was to evaluate the converged effect of maize and plant growth promoting bacteria on degradation of petroleum hydrocarbons under axenic conditions. Artificially spiked sand with 10 g kg−1 light crude oil was planted with maize alone and in combination with eight bacterial isolates having plant growth promotion and bioremediation potential to observe the dissipation of petroleum hydrocarbons. Results showed remarkable suppression of maize growth and biomass production due to phytotoxicity of the crude oil contamination. However, bio-augmentation of plants with bacteria having ACC-deaminase activity significantly compensated the reduction in plant growth compared to uninoculated plants. The results revealed that plants bio-augmented with PM32Y exhibited significant increase in root length (75%), plant height (74%), and biomass (67%) as compared to uninoculated plants after 60 days of planting. The same bacterium in convergence with maize caused 43% degradation of petroleum hydrocarbons as compared to the unplanted and uninoculated control. Amplification, sequencing and phylogenetic analysis of 16S rRNA gene sequence identified PM32Y bacterium as Bacillus subtilis strain. It is concluded that bio-augmentation of plants with plant growth promoting bacteria having bioremediation potential and ACC-deaminase activity can successfully be used in phytoremediation of petroleum hydrocarbons.

Prospects of Bacterial-Assisted Remediation of Metal-Contaminated Soils

Industrial revolution resulted in plenty of contaminants in the environment. Several organic and inorganic pollutants have adversely affected soils and water resources, causing serious health issues in humans. Among inorganic contaminants heavy metals are of prime importance as they are nondegradable in the environment. Arsenic, cadmium, chromium, cobalt, copper, lead, mercury, selenium, zinc, and other metals originating from various point and nonpoint sources are contaminating natural resources. Elevated concentrations of poisonous metals are not only disturbing soil health and microbial ecology but also decreasing crop production and global food security. Entry of metal pollutants into the food chain is dangerous for human health. Serious efforts are needed to mitigate rising threats of metal contamination. Physical, chemical, and biological approaches can be used to remediate such type of pollutants. However, bioremediation is considered as a promising technique, being cost effective and environment friendly with minimum adverse effects, esthetic advantages, and long-term applicability. Phytoremediation is a type of bioremediation to remove toxic metals from soil through hyperaccumulation or phytostabilization in plant cells. Generally, higher contents of toxic metals in soil and water result in more uptake by roots and more translocation toward shoots, causing interference in metabolism and reduced growth. Successful phytoremediation is limited to the plant types, tolerance to the high metal concentrations, accumulation rate, growth rate, adaptability, and biomass production. Metal-tolerant bacteria can help plant to tolerate metal stress via different mechanisms involved including production of different hormones such as auxins, cytokinin, and gibberellic acid or suppressing stress-induced enzymes such as plant ethylene level. This chapter reviews possible interactions between plant and bacteria to make situations more conducive for remediation of metal-contaminated soil. The chapter also covers different strategies/mechanisms adopted by plants and bacteria to mitigate toxic effect of metals on plant growth in metal-contaminated soils.

Arbuscular Mycorrhizas: An Overview

Almost every plant in natural ecosystem forms association with fungi either intracellularly as in arbuscular mycorrhizal fungi (AMF), or extracellularly as in ectomycorrhizal fungi. Arbuscular mycorrhizas (AMs) represent the most widespread symbiosis with land plants. The associated fungi colonize the plant roots and reside in the internal tissues of their host plant. This mutualistic association not only plays key role for enhancing plant growth by facilitating the uptake of water and essential nutrients but also protects the plant from adverse soil conditions. The application of mycorrhizal fungi is a promising alternative strategy for sustainable crop production under normal as well as biotic and abiotic stress conditions. The mycorrhizal plants have an improved ability for nutrient uptake and have ability to tolerate stress environments. There are increasing interest for the application of AM fungus for improving plant growth and enhancing crop production. The AM fungus also has positive impact on crop growth by improving soil quality by increasing water infiltration and retention and therefore reducing soil erosion. This review chapter epitomizes the current knowledge on the significance of AM fungus for improving crop production and maintaining agriculture sustainability.

Ameliorating Salt Stress in Crops Through Plant Growth-Promoting Bacteria

Abiotic stresses are emerging vicious environmental factors limiting agricultural productivity around the world, while food demand is increasing with growing population. Among these abiotic stresses, salt stress is a serious threat to put down crop production especially in arid and semiarid regions of the world. Therefore, some serious steps are required to stop or slow down the lethal effects of salinity for ensuring food security. Various strategies are adopted to tackle the deleterious impacts of salinity to crops including breeding techniques and genetic engineering, but these techniques have their level of significance and cannot satisfy the whole demand. However, some biological strategies are cost-effective, environment friendly, and easy to adopt/operate. In this scenario, the use of various microorganisms (bacteria, fungi, algae) to enhance salinity resilience in crops is encouraged due to their vital interactions with each other and crop plants. Bacteria are widely used to mitigate deleterious impacts of high salinity on crop plants because they possess various direct and indirect plant beneficial characteristics including exopolysaccharide and siderophore production, biofilm formation, phosphate solubilization, induced systemic resistance, and enhanced nutrient uptake, and they act as biocontrol agents to protect crop plants from many diseases by killing pathogens. This chapter focuses on the negative effects of high salinity on plants, bacterial survival in salt stress, and their mechanisms to mitigate salinity stress and the role of beneficial microbes to enhance crop tolerance against salinity stress.