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Metal

Complexation and Precipitation

Chelator-M+ Complex

Fig. 11.9 Mycorrhizal-induced metal biosorption and metal complexation (adapted from Apak 2002, Gadd 1993, Galli et al. 1994, González-Chavez et al. 2002, and González-Guerrero et al. 2008)

Alternatively, under such conditions, bulk soils tend to have a comparatively lower particulate binding capacity and subsequently lower aggregation potential resulting in the relatively more rapid collapse of their matrix structure. In addition to the physical entanglement of soil aggregates by the mycorrhizosphere, mycorrhizal-induced soil aggregation can also be attributed to the exudation of organic acids by extraradical hyphae (Bais et al. 2006; Bertin et al. 2003; Rovira 1969). Fundamentally, these mucilage exudates - typically consisting in polysaccharides and other extra-cellular polymeric compounds - contribute to nutrient chelation for mineral solubilization within the mycorrhizosphere as well as protection of the extraradical mycelium from desiccation. Further to these essential roles, such organic exudates also adhere to soil particles and permit the physical entanglement of micro- and macro-aggregates leading to the development of soil clusters within the mycorrhizosphere. Glomalin and glomalin-related soil proteins, considered to be effective biochemical markers of AM fungal growth and mycorrhizosphere development, are also believed to bind soil particles in the same manner leading to increased structure stability (Driver et al. 2005; Purin and Rillig 2007). Altogether, this improved potential for soil cluster formation can increase the incidence of micropores within the soil architecture leading to an overall increased colloidal surface area. As a result, the mycorrhizosphere can have a higher affinity for retaining water molecules as well as metal and nonmetal ions compared to bulk soil. Taking into account the processes of hyphal proliferation and chelator exudation, Auge (2004) and Rillig and Mummey (2006) have likened the mycorrhizospheric network to an essential skeletal structure and the production of mycorrhizal-derived organic compounds as the "glue" which, together, contribute in holding together the soil matrix. Consequently, from a biogeochemical perspective, the mycorrhizospheric network should play central role in enhancing soil water and nutrient retention. When subjected to environmentally stressful conditions, these enhanced soil stabilization properties can significantly increase the soil's resilience to then buffer the growth environment for plants and associated soil microorganisms. Notably, under drought stress, the development of soil aggregates increases water and nutrient retention to delay the effects of soil drying (Auge 2004; Rillig and Mummey 2006) ; meanwhile, these aggregates also increase water infiltration during drought recovery due to the more hydratable (or water stable) soil matrix (Rillig et al. 2010). This myc-orrhizal-induced structural advantage benefits plant stress tolerance by increasing the soil's water storage capacity and increasing its resilience. Likewise, the increased water retention capacity within the mycorrhizosphere can also impact plant stress tolerance in relation to nutrient stress (e.g., reciprocal ion antagonisms leading to deficiency) since soil nutrient bioavailability is closely correlated with soil water potential. As a result, nutrient bioavailability may be increased within the mycorrhizosphere due to a greater retention capacity; meanwhile nutrient losses are decreased due to a reduction in leaching.

3.1.2 Metal Biosorption

The metal-binding capacity of soil is primarily dictated by its essential composition, whereby soils having a higher proportion of organic matter (e.g., humic and fluvic acids) typically tend to have a greater retention capacity and redox potential than other soil types (Bohn 1971; McBride 1994). Further to the role of the mycorrhizosphere in stabilizing the soil's structural matrix, the extraradical hyphae have also been shown to increase the biosorption potential of soils. This is attributed primarily to the preferential binding of metal ions to negatively charged mycelial and root surface constituents (Fig. 11.9) , such as carboxyl, hydroxide, oxy-hydroxide, and sulfhydryl groups (Apak 2002; Gadd 1993; Galli et al. 1994; Gonzalez-Chavez et al. 2002, 2008). Similar analyses of non-mycorrhizal fungi suggest that phenolic polymers and melanins should also be effective metal binding sites even among AM fungi (Baldrian 2003 , Fogarty and Tobin 1996). Likewise, the exudation of organic chelators within the mycorrhizosphere (described above) has been shown to result in an enhanced binding capacity due to the formation of metal-ligand complexes and precipitates in the soil solution. For these reasons, the general processes of metal biosorption, including ion-exchange (i.e., CEC), metal complexation, and metal-ligand precipitation and crystallization occurring on and within the fungal cell wall (Gadd 1993; Galli et al. 1994), represent significant mechanisms regarding the modulation of metal bioavailability within the mycorrhizosphere. As in the case of mycorrhizal-induced soil aggregation, these enhanced metal biosorption properties can improve the soil's resilience by increasing its nutrient retention capacity, while reducing nutrient losses due to leaching (Giller et al. 1998, Leyval et al. 1997). Notably, there is considerable evidence suggesting that, when essential and nonessential metals occur at exceedingly high exposure levels representing potentially toxic metal conditions, such metal biosorption properties can significantly reduce the bioavailability of metals in the soil solution to reduce plant metal uptake and then delay the onset of metal phytotoxicity (Audet and Charest 2007b). In this regard, a wide array of plant species (refer to Audet and Charest 2007b for broad list plant species) subjected to increasingly high metal concentrations, both essential (e.g., Cu, Fe, Mn, Ni, and Zn) and nonessential elements (e.g., Cd, Co, Cr, and Pb), have repeatedly been shown to incur considerably lower (up to 50%) metal uptake among AM (Gl. caledonium, Gl. intraradices, Gl. mosseae, and a consortium of unidentified Glomus species) than non-AM plants; an effect often coinciding with an increased plant growth and (or) health status. As proposed by Leyval et al. (1997), and later Audet and Charest (2006, 2007a, b, 2008, 2009), Hildebrandt et al. (2007), and Giasson et al. 2008, these findings suggest that mycorrhizal-induced metal biosorption could represent a significant extrinsic plant stress avoidance strategy, whereby excess soil metals are bound and precipitated in the soil solution as well as sequestered in fungal tissues instead of being transferred to host roots. As such, plant investment in this extrinsic stress avoidance mechanism could complement known intrinsic plant detoxification mechanisms, for instance, metallothienin and phytochelatin metabolisms (Cobbett 2000; Cobbett and Goldsbrough 2002) by reducing

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