Synergistic effects of arbuscular mycorrhizal fungi and phosphate rock on heavy metal uptake and accumulation by an arsenic hyperaccumulator

Abstract

The effects of arbuscular mycorrhizal (AM) fungi and phosphate rock on the phytorextraction efficiency of a hyperaccumulator (Pteris vittata) and a non-hyperaccumulator (Cynodon dactylon) plant were studied. Both seedlings were planted in As contaminated soil under different treatments {(1) control (contaminated soil only), (2) indigenous mycorrhizas (IM), (3) mixed AM inoculum [indigenous mycorrhiza + Glomus mosseae (IM/Gm)] and (4) IM/Gm + phosphate rock (P rock)} with varying intensities (40%, 70% and 100%) of water moisture content (WMC). Significant As reduction in soil (23.8% of soil As reduction), increase in plant biomass (17.8 g/pot) and As accumulation (2054 mg/kg DW) were observed for P. vittata treated with IM/Gm + PR at 100% WMC level. The overall results indicated that the synergistic effect of mycorrhiza and P rock affected As subcellular distribution of the hyperaccumulator and thereby altered its As removal efficiency under well-watered conditions.

Introduction

Long-term mining activities have produced large areas of tailings, causing pollution and land degradation. Amongst all mining methods, flotation has been the most widespread method of metal ore enrichment [1], involving both mechanical and chemical processing of metalliferous minerals. However, such a method may give rise to unfavourable air–water conditions for plant growth by restricting water infiltration during rainfall and decreasing water recharge by capillary rise from deeper layers during dry periods [2]. Thus, variation of water content in soil is considered to be one of the most important abiotic factors limiting plant growth and yield in the field [3]. Consequently, most metals in contaminated soil are less available to plants, e.g., in waterlogged soils, the solubility of some metalloids such as As is generally reduced due to the low redox potential [4] and formation of sparingly soluble sulfides [5]. Such phenomenon may also affect the phytoextraction efficiency of metal hyperaccumulators in the field.

Arsenic is a crystalline metalloid that exists in several forms and states. Its toxicity and mobility in the environment is dependent on both its chemical form and species [6]. Total As concentration alone is insufficient to assess its environmental impact in contaminated soils. Henceforth, measuring available form of As in soil is a better indicator to reflect the phytotoxicity of As [7]. Sequential extraction [8] has been commonly used to assess both As availability and its mobility in soils. However, for soil As, due to its chemical similarity to P, the method used for P fractionation has also been used specifically for As fractionation [9]. Soil As is operationally separated into four fractions: water-soluble plus exchangeable As (We–As, using NH₄Cl), Al-bound As (Al–As, using NH₄F), Fe-bound As (Fe–As, using NaOH) and Ca-bound As (Ca–As, using sulphuric acid) because As retention in soil is related to the content of extractable amorphous and crytostalline hydrous oxides of Fe and Al and exchangeable Ca in soil [10]. Based on the sequential extraction, information about the retention and partitioning of As in soils can be estimated. Even though sequential extraction suffers from a lack of specificity during chemical fractionation and the resorption of dissolved metals by soils during the extraction, it is still a useful tool to evaluate metal bioavailability in soils [9].

Mine wastes usually contain high levels of toxic metals and low levels of nutrients unsuitable for plant growth. It has been indicated that AM fungi can colonize plant roots in metal contaminated soil [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23] and it has been commonly observed that AM fungi increases shoot uptake of metals [24], [25] in severely contaminated soils. In addition, AM fungi could protect plants against harmful effects of metals [26] and respond to water deficit at morphological, anatomical and cellular levels with modifications. This will allow the plant to avoid stress or to increase tolerance [27] so as to alleviate the stress symptoms. There are a number of principle strategies used by mycorrhizal plants to deal with toxic metal cations in contaminated soil: (1) Compartmentalization strategies: As is translocated to subcellular compartments (invariably the vacuole) where they can be stored in places such as the vacuole and the cell wall away from the cytoplasm [28], [29]. In addition, under the influence of AM fungi, more compounds (metal chelators such as amino acids, phytochelatins and metallothioneins) present in the cell may be stimulated for transporting metals within the plant [30]; (2) Avoidance strategies: plants and mycorrhizas may inhibit certain metals entering their cells and thereby reduce the host exposure to toxic metals [30]. Therefore, understanding the subcellular distribution of As is essential in elucidating the mechanisms involved in metal tolerance and metal hyperaccumulation of these plants. It will help to explain the effects of soil moisture regime on cellular activities as well as define the possible roles of mycorrhiza in aiding its host with regards to As accumulation in adverse environments. Mycorrhizas colonized in the plant can be effective in ameliorating metal toxicity on the host plant [31] by changing the distribution of As in the cells of the plant. The mechanisms employed by the higher plant at the cellular level to accumulate heavy metals may probably be similar to some of the strategies employed by fungi, namely binding to extracellular materials or sequestration in the vacuole compartment [32]. It was also claimed that indigenous AM fungi are able to protect host plants under adverse environmental conditions by enhancing water exchange between soil and plant [33], [34]. Furthermore, arbuscular found in AM fungi can increase water uptake and assist plants to tolerate water stress conditions [35] and also increase the adhesion capacity of the soil aggregates, thereby contributing towards soil stability for plant growth [2]. Therefore, mycorrhizal hyperaccumulating plants are able to adapt to such adverse soil environments by altering redox potential and releasing metal chelating compounds into the rhizosphere, and efficiently absorbing them into the root and translocating them to the shoot [36], [37]. In addition, mycorrhizal symbiosis may also assist host plants in using essential elements that are unavailable to non-mycorrhizal plants and therefore contributing to plant growth under water deficit conditions [38]. However, the fungal strains isolated from highly contaminated sites may grow slowly, with a long lag phase, lower efficiency in utilizing nutrients, and with diversed nutritional requirements or altered pH of the growth media (potentially changing metal ion speciation and toxicity) [30]. It was revealed that both fungal isolates and plants may be varied in their metal accumulation ability under different soil conditions. Thus, the use of AM fungi to enhance plant growth in metal contaminated soils may require careful selection of specific fungal isolates. The skillful use of a suitable amendment to maximize plant growth and to capitalize on interactions or competitions between metals and elements such as P is needed in order to increase the adaptability of plants in field sites.

There has been a growing interest in the role of mycorrhizas in affecting biological regulation of plant production and phytoextraction efficiency. Recently, the role of mycorrhizal fungi in soil C and N dynamics has been extensively studied [39], [40], [41]. However, the role of mycorrhizas in passive processes, such as water moisture content in contaminated soil is less well understood. The objectives of the experiment were therefore to (1) determine whether mycorrhizal inoculum and phosphate rock amended to As contaminated soil could enhance plant accumulation of As under different soil moisture contents and (2) assess the strategies of mycorrhizal hyperaccumulator plant for As accumulation compared with a non-hyperaccumulator plant, when grown in As contaminated soil.