# Mycoremediation as a Potentially Promising Technology: Current Status and Prospects--A Review
## 6 Mechanisms of Mycoremediation  
The synthesis and secretion of biological compounds, mode of action, metabolites, and growth needs are essential characteristics of fungal species that are known pathways and mechanisms for mycoremediation (Table 6). Figure 3 depicts multiple mechanistic paths for mycoremediation. The mechanisms of mycoremediation can be categorised as follows:  
* Avoidance reduces metal accumulation via absorption, precipitation, and biosorption, which lowers metal toxicity. * Extrusion is the process of transporting contaminants out of the fungal biomass. * Sequestration mechanisms involve synthesising intracellular chelating compounds and subsequent chelation in the fungal cells to dilute the contaminants. * Biotransformation includes the reduction, oxidation, demethylation, methylation, and evaporation processes which convert toxic compounds and heavy metals (HMs) into less harmful forms. Various microorganisms, including fungi, algae, and bacteria, can break down PAHs. However, the most significant part of mineralisation is by far played by bacteria. On the other hand, fungi primarily biotransformation PAHs, which involves detoxification to less toxic or nontoxic metabolites that can be utilized by other organisms [175]. Most bacteria that can break down high molecular weight PAHs are Actinomycetes from the genera Mycobacterium, Rhodococcus, and Gordonia [176]. The PAH is converted to a dihydrodiol by a multi-enzyme system as the first step in the aerobic catabolism of an aromatic molecule by bacteria. This hydroxylated intermediate can undergo further processing to become a central intermediate catechol or protocatechuate cleaved through ortho or meta cleavage pathways (Figure 4). In polynuclear aromatic compounds like PAHs, the rings are sequentially broken down through dihydroylation and cleavage, producing more intermediates before being converted into tricarboxylic acid cycle intermediates [177; 178]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Pros** & **Cons** & **Remarks** & **Ref. ** \\ \hline \multirow{4}{*}{**Biotransformation**} & A faster fungal growth rate & \multirow{4}{*}{Biocatalyst operating parameters} & \multirow{4}{*}{Biocatalyst operating parameters} & \multirow{4}{*}{Biological catalysts are used in} \\  & shortens the time required for transformation. & & & \\  & The biocatalyst operates between & & Enzymes are an expensive & \\  & 20 and 40 \({}^{\circ}\)C and has a pH range & & system. & \\  & of 5.0 to 8.0 at ambient & & A high concentration of product & \\  & conditions. & & or substrate can inhibit certain & \\  & It requires minimal operational & & biocatalytic reactions, thus & \\  & control and & & halting biotransformation. & \\  & time-saving technology. & & & \\ \hline \multirow{4}{*}{**Biosorption**} & Cost-efficient production of & \multirow{4}{*}{Expensive regeneration.} & Multiple metals can \\  & biomass. Simple and & & Numerous kinds of adsorbents & \\  & customisable method to & & are required. & \\  & remediate a wide range of & & & \\  & contaminants. Simultaneous & Reactor saturation and clogging. & & \\  & removal of many HMs. & & & \\ \hline \multirow{4}{*}{**Precipitation**} & Removal of high pollution loads & \multirow{4}{*}{Unfeasible for the removal of small amounts of pollutants.} & \multirow{4}{*}{Pollutants are eliminated, and the end product} \\  & is mainly achievable. & & & \\  & Excellent at removing metal & & & \\  & sulphide. & & & \\  & The simplest and least expensive & & & \\  & wastewater treatment system. & & & \\ \hline \multirow{4}{*}{**Natural attenuation**} & Economically feasible.