21

Soil Microorganisms and Nematodes for Bioremediation and Amelioration

bacteria, for example, express the MerB gene, which encodes organo-mercu­

riallyase, which converts organomercurials to mercuric ion (Hg²⁺) (Brown et

al., 2003). Luo et al. (2011) reported that the cadmium resistant endophytic

bacterium Serratia sp. LRE07 was able to absorb approximately 65% of Cd

and 35% of Zn in bacterial cells from a single metal solution, considerably

lowering the phytotoxic effects of the metals by sharing the metal burden.

The metal biosorption by bacteria can be facilitated passively or actively.

In passive uptake metal binds with functional groups on the cell surface which

could be by precipitation, chelation, or ion exchange (Schiewer & Volesky,

2014). While in active biosorption or bioaccumulation, the metals can be

metabolically taken up inside the bacterial cell. Endophytes are reported to

alter the bioavailability and uptake of metals into plant by secreting a variety

of metabolites, including siderophores, organic acids (e.g., citric, oxalic, and

acetic acids), etc., thus avoiding phytotoxicity (Visioli et al., 2014; Tiwari

et al., 2021; Lal et al., 2021). Barzanti et al. (2007) reported that 81% of

bacterial isolates recovered from Alyssum bertolonii were shown to produce

siderophores and to promote plant growth under Ni stress.

When copper-resistant endophytic bacteria were introduced into a host

plant, they showed strong Cu translocation from the root to the shoot of

Brassica napus, resulting in a reduction in Cu hyperaccumulation overall

(Sun et al., 2010). Toxic metals and metalloids are immobilized by fungal

endophytes, which create metal oxalate or trigger chelation onto melanin-like

polymers. By over-expressing As translocation factor, vesicular-arbuscular

mycorrhiza (VAM) can boost arsenic (As) absorption in the hyperaccumu­

lating fern Pteris vittata L. (Trotta et al., 2006). Mucor sp. and endophytic

yeasts (Cryptococcus sp. CBSB78 and Rhodotorula sp. CBSB79) were

identified from Brassica chinensis L. growing in metal-rich soil, with the

potential to increase Pb, Zn, copper (Cu), and cadmium (Cd) bioaccumula­

tion (Deng et al., 2011; Wang et al., 2013). Table 1.4 lists several commonly

identified endophytes that helped with heavy metal phytoremediation.

1.5.2 ENDOPHYTE AIDED PHYTOREMEDIATION OF ORGANIC

POLLUTANTS

Numerous research has shown the efficacy of endophyte-assisted phytore­

mediation in the removal of organic contaminants from soil. Even at

extremely low concentrations, the presence of stubborn organic pollutants in

the environment limits plant and microorganism development and metabolic