Evaluating metal phytorremediation and biondication potential of Spartina alterniflora in a South American estuary

Abstract

Soil metal pollution has been widely studied in salt marshes but mainly regarding non-essential metals. The aim of this study was to assess the levels of two essential metals (Fe and Mn) and one non-essential one (Cd) in Spartina alterniflora salt marshes in a South American estuary in order to evaluate the potential of this species as a phytoremediator and/or bioindicator of Fe, Mn and Cd and to analyze the distribution of these metals according to the edaphic conditions. The metals present in the soils varied among the three sites studied according to the content of organic matter and fine sediments. In comparison with other Spartina-dominated salt marshes worldwide, in this study Fe and Mn were approximately in the same range, whereas Cd levels were always lower, with a high number of samples below the method detection limit (MDL). All metals were highly correlated with each other suggesting an association of Cd with Mn and Fe oxides/hydroxides or sulfides and/or a common anthropogenic source. Metals in plant tissues also varied from site to site and between the aboveground and belowground tissues. Compared to the metal levels in Spartina tissues in other salt marshes, our levels of Fe and Mn were in the same range, whereas the Cd levels were lower, among most samples, especially those from aboveground tissues that were below the MDL. The bioconcentration factor (metal in belowground tissues/metal in soil) was always lower than one for Fe and Mn meaning that there is no accumulation of these metals in belowground tissues, but this factor for Cd was sometimes higher than one, even as hig h as 3.45, implying that S. alterniflora can accumulate this metal in its tissues, pointing to a potential role of this species in Cd phytor- emediation. Translocation factors (metal in aboveground tissues/metal in belowground tissues) were always lower than one for Fe and could not be calculated for Cd but were usually higher than one for Mn, showing the role of this element in photosynthetic tissues and a possible function of this species for phytoextraction of Mn. In most samples the Fe levels in plant tissues were higher than the permissible levels reported in the literature, suggesting a potential role of S. alterniflora in Fe phytoremediation. No correlation was observed between metal concentrations in soils and aboveground tissues; therefore, S.alterniflora is not a good bioindicator for the metals studied. Although our results are not conclusive, they reinforce the importance of local edaphic conditions on the behavior of metals in salt marshes and shed light on the potential role of S. alterniflora in the phytoremediation of highly toxic metals such as Cd or poorly studied metals such as Fe and Mn.