Increased metal tolerance and bioaccumulation of zinc and cadmium in Chlamydomonas reinhardtii expressing a AtHMA4 C-terminal domain protein

Abstract

The use of microalgal biomass for metal pollutant bioremediation might be improved by genetic engineering to modify the selectivity or capacity of metal biosorption. A plant cadmium (Cd) and zinc (Zn) transporter (AtHMA4) was used as a transgene to increase the ability of Chlamydomonas reinhardtii to tolerate 0.2 mM Cd and 0.3 mM Zn exposure. The transgenic cells showed increased accumulation and internalisation of both metals compared to wild type. AtHMA4 was expressed either as the full-length protein or just the Cterminal tail, which is known to have metal binding sites. Similar Cd and Zn tolerance and accumulation was observed with expression of either the full-length protein or C-terminal domain, suggesting that enhanced metal tolerance was mainly due to increased metal binding rather than metal transport. The effectiveness of the transgenic cells was further examined by immobilisation in calcium alginate to generate microalgal beads that could be added to a metal contaminated solution. Immobilisation maintained metal tolerance, while AtHMA4-expressing cells in alginate showed a concentration-dependent increase in metal biosorption that was significantly greater than alginate beads composed of wild type cells.This demonstrates that expressing AtHMA4 full-length or C-terminus has great potential as a strategy for bioremediation using microalgal biomass.

Emerging role of microalgae in heavy metal bioremediation

“…Fortunately, cationic diffusion-stressed EST databases also expressed MTP1's upregulation, which confirmed its role in the detoxification of Cd [61]. MTP roles have also been shown to improve cadmium resistance and sequestration of C. reinahrdtii acidic vacuoles [63,64].…”

“…T A B L E 1 The capability of several microalgae/cyanobacteria to remove heavy metals (reproduced from Kumar et al [38] In the same way, Ibuot et al [64] reported that expression of either the FL protein or the CT domain of a plant Cd and Zn transporter (AtHMA4) known to bind metals in C. reinhardtii with the increment of the bioaccumulation and internalization of both Cd(II) and Zn(II) ions. This phenomenon elucidated increased metal binding, rather than metal transit, was the primary cause of increased metal bioaccumulation.…”

Comparison of cell size, chlorophyll fluorescence and cadmium (Cd2+) bioaccumulation between wild-type and mutant strains of Chlamydomonas reinhardtii upon exposure to Cd2

“…The generation and characterization of Cd-resistant and hypersensitive C. reinhardtii mutants have also been reported in studies of heavy metal tolerance (Samadani et al, 2018;Hanikenne, 2003;Hu et al, 2001). In particular, the overexpression of metal transporters in C. reinhardtii for bioremediation is an attractive approach; it may allow both increased metal accumulation into the cell and increased metal tolerance, such as by transfer of a toxic metal out of the cytosol and into an internal compartment (Ibuot et al, 2020;Ibuot et al, 2017). The genetic manipulation of C. reinhardtii has been shown to be successful in creating stable, inherited phenotypes (Pollock et al, 2017).…”

“…The genetic manipulation of C. reinhardtii has been shown to be successful in creating stable, inherited phenotypes (Pollock et al, 2017). Changes in the heavy metal biosorption capacity of C. reinhardtii have been reported in genetically modified strains (Ibuot et al, 2020;Cheng et al, 2019;Aksmann et al, 2014;Lin et al, 2013;Mayfield et al, 2007;Shimogawara et al, 1998).…”