Phycoremediation mechanisms of heavy metals using living green microalgae: physicochemical and molecular approaches for enhancing selectivity and removal capacity

Abstract

Heavy metal (HM) contamination of water bodies is a serious global environmental problem. Because they are not biodegradable, they can accumulate in food chains, causing various signs of toxicity to exposed organisms, including humans. Due to its effectiveness, low cost, and ecological aspect, phycoremediation, or the use of microalgae's ecological functions in the treatment of HMs contaminated wastewater, is one of the most recommended processes. This study aims to examine in depth the mechanisms involved in the phycoremediation of HMs by microalgae, it also provides an overview of the prospects for improving the productivity, selectivity, and cost-effectiveness of this bioprocess through physicochemical and genetic engineering applications. Firstly, this review proposes a detailed examination of the biosorption interactions between cell wall functional groups and HMs, and their complexation with extracellular polymeric substances released by microalgae in the extracellular environment under stress conditions. Subsequently, the metal transporters involved in the intracellular bioaccumulation of HMs as well as the main intracellular mechanisms including compartmentalization in cell organelles, enzymatic biotransformation, or photoreduction of HMs were also extensively reviewed. In the last section, future perspectives of physicochemical and genetic approaches that could be used to improve the phytoremediation process in terms of removal efficiency, selectivity for a targeted metal, or reduction of treatment time and cost are discussed, which paves the way for large-scale application of phytoremediation processes.

Keywords: Bioengineering; Heavy metal; Mechanisms; Microalgae; Phycoremediation.

Figure 1

HMs-phycoremediation mechanisms (modified from (García-García et al., 2016; Kumar et al., 2015)).

Figure 2

Schematic view of cell wall structures of some microalgae species (modified from (Baudelet et al., 2017; Carvalho et al., 2020), For better interpretation of the color figure, the reader could refer the web version of this article).

Figure 3

Extracellular bio-removal mechanisms of HMs using living cells of microalgae, modified from (Navakoudis and Ververidis, 2018).

Figure 4

General scheme of intracellular detoxification of HMs in microalgae cells adapted from (Blaby-Haas and Merchant, 2012; Torres et al., 2008).

Figure 5

Proposed biotransformation pathways of Cr(VI) developed from the finding of (Deng et al., 2006; Lee et al., 2017; Rahman and Thomas, 2021; Yen et al., 2017).

Figure 6

Proposed biotransformation pathways of As (V and III) developed from (Arora et al., 2018; Garbinski et al., 2019; Wang et al., 2015b).

Figure 7

Procedure of cell surface engineering for a target metal biosorption.