https://population.un.org/wup/
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[2]
[3]
p
[4][5]. Removal of some of these atmospheric toxic compounds through associations between plants and bacteria, in the context of Green Architecture, is discussed in this work.
[8]
. Chlorinated dioxins are generated as unwanted by-products in the chemical syntheses of several pesticides, disinfectants, wood preservatives and in the incineration of PCB-containing plastic insulators[9]
. Both chlorodioxins and co-planar PCBs cause immunotoxic and endocrine effects, as well as the induction of malign tumours, neurotoxic and/or immunotoxic effects[4]
.
Figure 1. Chemical structures of different polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), dioxin and dioxin-like molecules;
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to initiate the enzymatic attack on the aromatic rings of PAHs by a dioxygenase that catalyses the dihydroxylation of PAHs. These dihydroxylated intermediates are then cleaved by ring-cleaving dioxygenases, leading to different compounds that are further converted to tricarboxylic acid (TCA) cycle intermediates, used for anabolic biomass formation and mineralized to CO2 [14]
. Only a single specialized bacterium,Sphingomonas wittichii
strain RW1, can use the non-chlorinated dibenzo-p
-dioxin as its sole carbon- and energy source and co-metabolize several of its chlorinated derivatives[15]
. Other bacterial isolates, with highly similar catabolic genes to that of theS. wittichii
strain RW1 are capable of growing using PCBs as the sole carbon source[16].
.[17]
; they can also be mobilized from soil to leaves by evaporation or wind, or be transported from roots to leaves[18]
. The amount of contaminants which finally accumulates in vegetation depends on the physico-chemical properties of the particular contaminant, and also on the characteristics of leaf surfaces and root architecture, as well as on many other environment-related parameters such as wind, rain, temperature, sorption to soils, organic content of soils, and composition of root exudates[19]
. After deposition, it is accepted that mobilization of contaminants through the plant is a consequence of two different processes: i) the accumulation of contaminants in plant tissues mainly correlated to their hydrophobicity and plant lipid contents, and ii) the transfer between plant tissues driven mainly via xylema[20]
. Once located in the plant interior, contaminants are mainly activated by cytochrome P450 monooxygenases (CYP). The resulting compounds are later conjugated with glucose, glucuronic acid, or glutathione moieties. These conjugates are then sequestered in the cell wall or in vacuoles[21]
. Despite evidence for the accumulation and subsequent biotransformation of organic contaminants in plants, it is believed that the contribution of plant uptake for their removal from the environment is very low. Soil-bound contaminants, such as PAHs, are strongly associated with soil organic matter and poorly transferred to plant roots. Furthermore, contaminants or derived products accumulating in plant cell walls or vacuoles may return to the environment after plant decay. However, plants may stimulate organic contaminant degradation through several processes such as by increasing the bioavailability of the contaminants, influencing desorption from soil particles, and stimulating the biodegrading microbiota in the rhizosphere .
[24]
. Therefore, the combination of both organisms could be a good solution for the elimination of contaminants[25]
. Two different strategies can be used:[26]
, and ii) the contaminant has to be bioavailable in a form that it can be taken up by the bacterial cell[27]
. The elimination of contaminants by bacteria decreases their concentration in the rhizosphere and, therefore, improves plant growth[28]
.[29]
; therefore, it is an important environment for the elimination of atmospheric contaminants. The accumulation of air pollutants and airborne PMs on leaf surfaces is dependent on the plant species, leaf size and structure, but is also affected by the types of waxes which make up the cuticle, the hairs covering the leaf, and leaf smoothness . Although this environment is a hostile habitat for microorganisms (exposition at high doses of UV radiation, temperature variations, climatic factors, low nutrient content and pollution[31]
, it can support bacterial populations of up to 108
bacteria g−1
leaf together with smaller fungal populations[32]
. It has been demonstrated that natural populations of phyllospheric bacteria can remove hydrocarbon-related air pollutants[33]
; in some cases this capacity was improved by spraying bacteria able to degrade the pollutants[34].
.[35]
. Some architectural elements to mitigate air pollution are already being developed such as green wall biofilters in buildings, or green belts established around industrial areas . In these elements, plants are normally considered passive accumulators of contaminants. However, improving rhizosphere and phyllosphere bacterial communities in these architectural elements with tailor-made bacterial consortia, which attack diverse organic contaminants, could be considered a way of improving the capacity of plants to remove air pollutants[25].
. In order to successfully implement the utilization of plant-bacteria combinations as a strategy to ameliorate air pollution in cities, there are still a number of issues that need to be solved: i) Selection of the best bacterial consortia, using endophytic, phyllospheric and/or rhizospheric bacteria, taking into consideration all environmental requirements; ii) because of the close proximity of green structures to citizens, detailed safety analyses of degrading microorganisms and studies about the accumulation of possible toxic intermediates from degradative pathways should be investigated ; and iii) the plant and its associated microbiome have co-evolved throughout time, establishing complex interrelationships to function almost as a single supra-organism (holobiont)[40]
. During bioremediation, the presence of pollutants and exogenous bacteria can affect the functioning of the holobiont[41]. The study of these new interactions and how the degradation potential of contaminants could be altered by the complex signalization existing in these niches is a novel research field to be explored.
. The study of these new interactions and how the degradation potential of contaminants could be altered by the complex signalization existing in these niches is a novel research field to be explored.