INTRODUCTION
The rapid economic development unintentionally added a challenge for
mankind to manage and clean up the environmental pollution piling up as
a severe threat to environmental sustainability. One of the major
environmental contaminants are the heavy metals including arsenic (As),
cadmium (Cd), Lead (Pb), Nickel (Ni), Chromium (Cr), Cobalt (Co), Copper
(Cu) and Zinc (Zn). Among this As in the last few decades has been found
to be frequently discharged in groundwater possibly due to geogenic
activity in Bay of Bengal. Studies suggests that a very minute quantity
of As can cause serious health issues in human, animal and even in
plants system. Arsenic is a group 1 carcinogen and thus chronic exposure
may lead to neurodegenerative diseases, type 2 diabetes mellitus and
even cancer (Tam et al. 2020). World Health Organization (WHO) has set
10µg L-1 as the permissible limit of As in potable
water and up to 20 mg Kg-1 in agricultural soils.
Recently Shaji et al. (2021) concluded that >100 countries
are affected with arsenic contamination, among which Bangladesh and
India from Asia are highly exposed to this metal (Chakrobarty et al.
2018; Bhowmick et al. 2018). Surprisingly, in India around 20 states
along with four union territories are listed as arsenic affected area. A
recent comprehensive study stated the threat of As toxicity to around 94
to 220 million population of which almost 94% is only from Asia
(Podgorski and Berg, 2020). The methods of management of arsenic
pollution in environment are only limited. The physico-chemical
procedures that included soil washing with nitric acid, sulfuric acid or
adsorption by specific media, coagulation, precipitation, immobilization
etc. are expensive and generate secondary pollutants. Among the
biological methods, the phytoremediation was used either by free
floating plants or rooted plants (Yan et al. 2020). Microorganisms with
their inherent capabilities to tolerate heavy metals are the alternative
simple, viable, cost effective and sustainable remediation tools for
these contaminants. Biosorption by the bacterial cells is well known and
simplest mechanisms in which heavy metals are adsorbed on the cell
surface through different functional moieties, and accumulated at the
cell membrane (Giri et al. 2013; Titah et al. 2018). This kind of
interaction has been recognized through Fourier Transform Infra-Red
Spectroscopy (FTIR) in various studies where amide, halide, nitro and
hydroxyl chemical functional groups play an active role in metal
sorption (Tian et al. 2012; Prasad et al. 2013; Singh et al. 2013;
Altowayti et al. 2022). Molecular mechanisms pertaining to As tolerance
have been considered as arsenate reduction (ars ), arsenate
respiratory reductase (arr ), arsenite extrusion genes
(acr ), arsenite oxidation (aio ) and methylation (Renu et
al. 2020). Studies indicated that the bacteria followed above-mentioned
strategies to combat arsenic toxicity. Dey et al. (2016) reported
arsenite oxidation in Bacillus sp. and Aneurinibacillus
aneurinilyticus isolated from As contaminated region in Burdwan
district of West Bengal. Similarly, arsenic extraction efficiency of
lower plants has also been increased by the inoculation of arsenic
tolerant bacteria (Lampis et al. 2015). On the other hand, the
importance of arsenate reductase gene (arsC ) has been reported in
majority of bacteria possessing arsenic tolerance (Sun et al. 2004;
Suhadolnik et al. 2017). In addition to this, arsenic removal in few
studies suggested mediation of arsenite extrusion genes (acr ) (Gu
et al. 2019) and arsenic respiratory reductase (Mirza et al. 2017). It
has been established that the redox cycling of arsenic by microbes plays
a crucial role in arsenic ion mobility in the environment (Stolz, 2006).
In the present study, we have reported As-tolerant bacterial strainBacillus mycoides NR5 isolated from presumptive As contaminated
site for its bioremediation potential and evaluated PGP ability in
spinach. The study focusses on mitigation of As toxicity in spinach
plants in presence of the selected bacterial strain. The study reports
in-depth mechanism of As tolerance in the bacterium and its role in
alleviation of As toxicity in spinach.