ABSTRACT
While many plants and bacteria have their own mechanisms for dealing with heavy metal or organic contaminants, the interaction of plants and microorganisms may be the primary method of organic contaminant removal, and these interactions can increase or decrease heavy metal accumulation in plants, depending on the nature of the plant-microbe interaction. Because phytoremediation is a relatively new technology, understanding mechanisms of plant-microbe interactions in removing contaminants from the environment is still not completely characterized. There have, however, been advances in our understanding of how plant-associated microorganisms play a role in contaminant removal from the environment as the technology matures. A set of terms has emerged to re¬ect our understanding that the process often does not involve plants alone, but a reliance on microorganisms or the symbiosis of plants and microorganisms. Phytoremediation was previously considered to be a separate technology from bioremediation, which refers to contaminant-degrading or removing microorganisms that can be found in many soils, sediments, and water. The application of nonnative microbes to contaminated sites has not been entirely successful for several reasons, including the inability of introduced microorganisms to compete with indigenous soil microbes, inability of introduced microbes to grow at depths or in conditions where the contaminant is located, low nutrient availability for microbial growth, poorly bioavailable contaminants, use of carbon sources other than the contaminant by the microorganisms, and high concentrations of toxic compounds that will inhibit microbial growth (for a detailed review, see Gerhardt et al. 2009). Phytoremediation, the use
17.1 Introduction: Plant-Microbe-Contaminant Interactions in the Rhizosphere ...................... 327 17.2 Plant-Microbial Organic Contaminant Degradation ........................................................... 328 17.3 A Special Concern: Emerging Organic Contaminants and their Removal .......................... 333 17.4 Plants and Microbes in Heavy Metals Removal: Plant Growth Promotion and Root
Elongation ............................................................................................................................. 334 17.5 Microbial Extracellular Polymeric Substances and Chelators for Metal Contaminants
Removal ................................................................................................................................ 336 17.6 Microbial Metal Resistance and Metal Contaminants Removal .......................................... 337 17.7 Microbial Alteration of Rhizosphere pH and Metal Contaminants Removal ...................... 338 17.8 Genetically Modi—ed Plant-Microbe Phytotechnologies ..................................................... 338 17.9 Conclusions and Perspectives ............................................................................................... 339 References ......................................................................................................................................340
