Abstract
Heavy metal pollution has become an environmental and health problem worldwide. With the aim of finding novel strategies for metal bioremediation, endophytic fungi from the heavy metal hyperaccumulator plant Vachellia farnesiana were isolated and characterized. The plants were growing in mine tailings, rich in Zn, Pb, and Cu. Morphological and phylogenetic analyses indicated that the fungal strains belonged to Neocosmospora and Aspergillus genera. The Neocosmospora isolate belongs to the Fusarium solani species complex (FSSC) that groups phytopathogen species. However, in this case the plants from which it was isolated did not show any signs of disease. Both fungal strains were able to remove significant amounts of heavy metals from liquid cultures, either in a mixture of the three metals or each metal in a single culture. In response to lead exposure, the Neocosmospora sp. strain secreted specific novel phenolic compounds other than anthraquinones or naphtoquinones, which have been described in similar situations. The Aspergillus sp. dropped the pH in the medium. High-performance liquid chromatography determinations indicated that this strain secreted mainly glutamic acid in response to lead, a novel mechanism, which has not been reported elsewhere. Malic and succinic acids were also produced in response to lead exposure. Possibly, glutamic and succinic acids (synthesized in the Krebs cycle) can be used to cope with metal toxicity due to the plant providing photosynthates to the fungus. These fungi showed the potential to be used for bioremediation or restoration of metal-polluted environments.
Highlights
Mining is a necessary but very polluting activity
Its target consists of recovering a single type of metal from mines, leaving behind rocks with a relative high content of other heavy metals, which pose a threat to living organisms and overall environmental health [1]
As a consequence of mining activities, millions of tons of mine wastes are left in the environment, which are known as mine tailings and may contain different heavy metals or semimetals [2]
Summary
Mining is a necessary but very polluting activity. Usually, its target consists of recovering a single type of metal from mines, leaving behind rocks with a relative high content of other heavy metals, which pose a threat to living organisms and overall environmental health [1]. One mechanism consists of rendering the metal insoluble, so it is not bioavailable In this case the metals are not extracted and eventually can change their redox state to become again soluble and, toxic for living organisms. Fungi have vacuoles very similar to those of plants in which they can accumulate metals, preventing them from interfering with the cell’s cytoplasmic metabolism [12] They have biochemical mechanisms to cope with the oxidative stress generated by heavy metals, such as the synthesis of glutathione, thioredoxins, and metallothioneins, all of which are capable of sequestering heavy metals, making them innocuous to the cell [13,14]. We decided to look for the presence of fungal endophytes in V. farnesiana roots growing in the mine tailing and explore their bioremediation capabilities with regard to these three heavy metals
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