Fitoestabilización de metales pesados en sedimentos costeros asistida por bacterias rizosféricas

Abstract

The problem of environmental pollution by heavy metals is increasingly becoming a matter of global concern due to increased industrialization and alteration of natural biogeochemical cycles. Estuaries and coasts are particularly threatened ecosystems. In the estuary of Odiel (SW Spain), both natural currents and anthropogenic causes are responsible for high metal contents in the sediments. The high concentration of salt in the estuary make it suitable for the growth of halophilic plants, such as species of the genus Spartina, which possess a high capacity for the accumulation of metals and it is considered as an appropriate candidate for phytostabilization processes. The main objective of this work was the isolation and characterization of bacteria from the rhizosphere of Spartina plants, in order to propose a suitable native inoculant with potential for the phytoremediation of contaminated marshes of Odiel. The Gram-negative strains Pantoea agglomerans RSO6 and RSO7 together with the Gram-positive strain Bacillus aryabhattai RSO25 were previously selected from 25 autochthonous bacteria isolated from the rhizosphere of Spartina maritima. The selection criterion was based on characteristics such as: heavy metal resistance, bioaccumulation, plant growth promoting properties (PGP) and ability to form biofilms. In order to test the plant growth promoter properties, Spartina densiflora was used as a model plant, which, although it is an invasive plant, is widely naturalized in the Tinto-Odiel marshes and shows a great capacity to accumulate metals in the root and the rhizosediment. RSO6 and RSO7 strains improved seed germination of Spartina densiflora by 250% in contaminated sediments, while RSO25 increased by 300%, compared to uninoculated controls. The bacterial consortium did not achieve further improvement. The three strains, particularly Gram-negative, promoted plant growth and mitigated metal stress as could be seen in physiological parameters such as functionality of the photosynthetic apparatus (PSII) and maintenance of nutrient balance. Regarding the uptake of metals, while Gram-negative bacteria did not significantly affect metal accumulation in plant tissues, the Gram-positive strain increased metal accumulation only in the roots, with no additional loading to the shoots. Our results confirm the possibility of modulating plant growth and accumulation of metals in contaminated sediments after inoculation with selected bacteria, as well as the suitability of halophilic-rhizobacterial interactions as a biotechnological tool for phytostabilization purposes, thus avoiding the risk of translocation of metals to the aerial part of the plants and their entry into the food chain.