Ecological Risk Assessment of Potential Toxic Elements in Salt Marshes on the East Coast of the Red Sea: Differential Physiological Responses and Adaptation Capacities of Dominant Halophytes

Abstract

The impact of the mutual interactions between salinity and the phytoavailability of potential toxic elements (PTEs) on the adaptation of halophytes in their natural habitat is complex and far from clear. Herein, we aimed to evaluate salinity- and PTE-induced oxidative stress in selected halophytes and the antioxidant responses of these plants. For that, five salt marshes were selected, and the physiological responses of dominant halophytes (Tamarix nilotica, Heliotropium crispum, Zygophyllum coccineum, Halopeplus perfoliata, and Avicennia marina) were evaluated against the physicochemical features of their rhizosediments. The tested locations varied in their physicochemical properties and showed various levels of salinity stress and a low fertility status. Distinct variations in ten PTE concentrations were recognized among locations and within plants, with Cr and Co showing the highest ecological risk indices. The high levels of salinity and PTEs were associated with higher foliar levels of malondialdehyde, particularly in A. marina and Z. coccineum. The bio-concentration ratio revealed hyperaccumulating potentials of PTEs by the tested halophytes. Z. coccineum showed effective accumulation of Co, Fe, and Pb, while T. nilotica exhibited effective accumulation of Cu, Cd, and Zn. H. perfoliate had higher accumulation of Cr and Hg, whereas A. marina accumulated a significant amount of Hg, Cd, Zn, and Mn. H. crispum leaves accumulated the highest Ni levels among the tested halophytes. Altogether, our results highlight the potential risk of pollution of the tested areas with PTEs and the efficient physiological adaptation of each of the tested halophytes as a unique biological system. They also reflect the high capabilities of the tested halophytes as phytoextractors of their corresponding PTEs and their potential as efficient tools for phytoremediation of salt- and PTE-affected lands.