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 internalization of both metals compared to wild-type. AtHMA4 was expressed either as the full-length (FL) protein or just the C-terminal (CT) tail, which is known to have metal-binding sites. Similar Cd and Zn tolerance and accumulation was observed with expression of either the FL protein or CT 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 immobilization in calcium alginate to generate microalgal beads that could be added to a metal contaminated solution. Immobilization 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 FL or CT has great potential as a strategy for bioremediation using microalgal biomass.
Highlights
Yañez‐Mansilla, & Jeison, 2019; Zeraatkar, Ahmadzadeh, Talebi, Moheimani, & McHenry, 2016). While such studies have demonstrated that the use of microalgal biomass for metal bioremediation is technically feasible, further improvements could be made by enhancing the selectivity and capacity of metal binding and accumulation by microalgae, which could be achieved through genetic engineering (Cheng, Show, Lau, Chang, & Ling, 2019)
This study aimed to examine the ability of FL AtHMA4 expression or AtHMA4 CT domain expression to provide increased tolerance and accumulation of Cd and Zn to the microalga C. reinhardtii
The A. thaliana heavy metal transporter AtHMA4 has been previously considered as an attractive gene for genetically engineering plants for bioremediation (Siemianowski et al, 2011; Verret et al, 2004), and this gene has been used to increase metal tolerance in yeast (Mills et al, 2005, 2010), it has never been expressed in microalgae
Summary
Biota (Järup & Åkesson, 2009; van Straalen, 2002). Microalgae are a potential biomass source for metal pollutant bioremediation within. Yañez‐Mansilla, & Jeison, 2019; Zeraatkar, Ahmadzadeh, Talebi, Moheimani, & McHenry, 2016) While such studies have demonstrated that the use of microalgal biomass for metal bioremediation is technically feasible, further improvements could be made by enhancing the selectivity and capacity of metal binding and accumulation by microalgae, which could be achieved through genetic engineering (Cheng, Show, Lau, Chang, & Ling, 2019). An alternative genetic engineering approach aimed to enhance metal accumulation and tolerance in microalgae by overexpression of a metal transporter (Ibuot, Dean, McIntosh, & Pittman, 2017) In this case, a transport protein called CrMTP4 was overexpressed in Chlamydomonas reinhardtii and shown to enhance Cd tolerance and total Cd uptake into the microalgal cell. The efficiency of metal biosorption in the transgenic C. reinhardtii following alginate immobilization was examined
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