Assessing the contribution of the rhizosphere microbiome in arsenic biogeochemical cycle in rice fields
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Data di Pubblicazione:
2016
Citazione:
Assessing the contribution of the rhizosphere microbiome in arsenic biogeochemical cycle in rice fields / S. Zecchin. ((Intervento presentato al 21. convegno Research on Food Science Technology and Biotechnology tenutosi a Napoli nel 2016.
Abstract:
1. Introduction
In order to develop this PhD project, two main parameters affecting arsenic (As) biogeochemistry in rice fields are analyzed: water management and sulfate (SO42-) amendment. The purposes of the activities are to:
A) Investigate the role of water management on the composition of rice rhizospheric microbial populations involved in As cycle
B) Identify the microbial populations responsible of the decrease of As bioavailability as a consequence of SO42- amendment in rice cultivated under continuous flooding
2. Arsenic contamination of rice grains
Arsenic contamination of rice is an issue of global concern (EFSA 2009). Recent studies revealed that Italy, together with Asian countries like Bangladesh, is one of the countries mostly affected by As contamination of rice grain in Europe (Meharg et al. 2009). Furthermore, from January 2016 specific limits for As concentration in rice have been established (Commission regulation (EU) 2015/1006). Since Italy is the European leader of rice production, it is important to find effective solutions to produce rice with As contents below these limits.
Arsenic contamination affects rice more than other crops as a consequence of agronomic practices used in rice cultivation (Spanu et al. 2012). All over the world, rice is mostly cultivated under continuous flooding. In this condition, oxygen is rapidly depleted in the soil, with the reduction of the redox potential in the system. At highly reduced redox potentials, As is rapidly released into the porewater as a consequence of two main processes: the dissolution of FeIII-minerals, where As is firmly bound, and the reduction of arsenate (AsV) to arsenite (AsIII), a highly soluble and toxic oxidation state of As that can be rapidly taken up by the plants and transferred to the grain (Yamaguchi et al. 2014). Changes in the water management of rice paddies have been proposed as possible options to reduce As uptake by rice plants (Spanu et al. 2012). Another important aspect is that, in the presence of sulfide, AsIII co-precipitate with the formation of AsnSn minerals (Fisher et al. 2008). Previous studies indicated that the addition of a sulfur source in soil promotes the decrease of As content in rice grains (Hu et al. 2007), suggesting sulfur amendment as another hypothetical mechanism to reduce As bioavailability in rice paddies.
3. Microbial metabolisms able to influence As biogeochemistry
Since the early stages of evolution, microbes have evolved different strategies to survive in the presence of different As forms (Slyemi and Bonnefoy 2012). The highly toxic AsIII can be oxidized to the less toxic AsV by AsIII-oxidizing bacteria (AOB), with AsV-oxidases encoded by the aio operon (Cavalca et al. 2013). In other microrganisms AsV is reduced to AsIII by enzymes encoded by the ars operon. AsIII is then extruded from the cells by specific AsIII-efflux membrane pumps encoded either by the ars or the ACR operons (Andres et al. 2016). Arsenic can also be used by some microorganisms in their energy metabolism. AsV can be used as electron acceptor by anaerobic AsV-respiring bacteria carrying arr genes (dissimilatory AsV-reducing bacteria, DARB). On the other hand AsIII can be used as electron donor by chemolitotrophic aerobic or anaerobic bacteria (Zhu et al. 2014).
Arsenic is also influenced by the presence of Fe and sulfur minerals. Therefore, microorganisms that use Fe or sulfur for their metabolic activities can indirectly influence As biogeochemistry in the environment. FeOB and SO42--reducing bacteria (SRB) promote the co-precipitation of As with FeIII and sulfide minerals, whereas FeIII-reducing bacteria (FeRB) and sulfide-oxidizing bacteria (SOB) contribute to the release of As in anoxic environments.
Although some work ha
Tipologia IRIS:
14 - Intervento a convegno non pubblicato
Keywords:
Arsenic; rice; rhizosphere; FeII-oxidizing bacteria; FeIII-reducing bacteria; SO42--reducing bacteria; sulfur-oxidizing bacteria
Elenco autori:
S. Zecchin
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