- Soil clinging to roots of the mangrove species Goran harbours two bacterial species that love arsenic and can tolerate salt.
- The bacteria can find use in bioremediation technologies and even promote plant growth.
- Bioremediation strategies using these bacteria could be used in countries such as Bangladesh, Vietnam, China, Thailand, Argentina, Greece, Chile and Spain, which are facing similar problems as is seen in West Bengal in India.
In the Sundarbans, the largest expanse of contiguous mangrove forests in the world, researchers have stumbled upon two bacterial species in the soil clinging to roots of the Goran tree. These species can thrive in the salt-stressed mangrove soil, and also trap and reduce arsenic uptake by plants, such as paddy, in these conditions.
Moreover, these microbial minions also churn out chemicals that promote plant growth in these high salt environments. Letting these arsenic-loving microbes loose in soil could become an effective bioremediation strategy to combat arsenic contamination and salt stress in areas embattled with loss of soil productivity, say researchers in a new study.
Goran (Ceriops decandra) is a near-threatened, salt-tolerant species which grows as a small tree or shrub and is highly valued as firewood.
Digging around in the plant’s rhizosphere – the micro-ecological zone in direct contact with plant roots – Bose Institute’s Abhrajyoti Ghosh and colleagues isolated the arsenic-tolerant strains Kocuria flava and Bacillus vietnamensis, that could potentially remedy dietary arsenic exposure.
“The major health hazards associated with intake of arsenic and its expensive remediation cost necessitates the development of cost-effective technologies to restore the polluted agricultural land. Among the various cost-effective technologies, microbe-assisted bioremediation has attracted immense attention in recent years,” said Ghosh, of the department of biochemistry, Bose Institute and lead researcher of the study.
When either of the strains was applied on rice seedlings as inoculants, the researchers discovered that it “protected the plants significantly from the toxic effects of arsenic and salt stress, and enhanced different growth parameters.”
The study notes inoculation of the rice seedlings “improved the growth parameters of plants under arsenic and salt stress conditions”, probably by the synthesis of growth promoting factors. More importantly, the microbes reduced the arsenic uptake in plants under salt stress by absorbing and accumulating arsenic themselves.
Ghosh stressed that “taken together, the present study showed that arsenic resistant isolates have a potential for bioremediation of arsenic in the saline rhizosphere.”
The globally significant Sundarbans ecosystem
Spanning 10,000 sq km along the coast of India and Bangladesh, the Sundarbans represent the largest expanse of contiguous mangrove forests in the world. This globally significant ecosystem is situated in the Bay of Bengal, within the delta of the Ganges, Brahmaputra, and Meghna rivers.
A network of water courses intersects with a highly variable landscape, including sand bars, mud flats and mangrove islands. The reserve forests are shared roughly 60-40 between Bangladesh and India. It is home to the iconic Royal Bengal Tiger. Another 5400 sq km of non-forested, inhabited region in India, along the north and north-western fringe of the mangrove forest, is also known as Sundarbans region in India.
The Indian Sundarbans archipelago with 102 islands is administered by two separate districts of West Bengal state in east India: the South 24 Parganas and the North 24 Parganas district.
Contiguous to the district of Kolkata, the South 24-Parganas district has a unique urban-rural continuum in its human settlements.
For the study, sampling was carried out at spots such as Godkhali, Bonnie Camp and Kalas among others in the southern end of the South 24-Parganas district which forms the Sundarbans. Incidentally, this district was the worst hit in the 2009 cyclone Aila. The South 24-Parganas district is also one of the nine districts in the state hit by groundwater arsenic contamination, an issue that is also of global concern.
During their investigation, the scientists discovered that the isolates, K. flava and B. vietnamensis, could tolerate 35 mM and 20 mM of arsenite (the ion form of arsenic), respectively. “Arsenic resistance of these strains has never been described before. Because arsenic is present in irrigation water, it gets bioaccumulated in plant tissues and eventually in seeds. In South 24 Parganas, we have a considerable amount of arsenic in irrigation water which eventually pollutes the soil,” explained Ghosh.
Salinity and agriculture
In 2009, when Aila ripped through the Sundarbans mangroves in India, it brought with it storm surges that flooded agricultural areas with saline water, which eventually led to a decline in productivity in stretches. Also, a considerable part of West Bengal’s agriculture takes place in the coastal zones (0.82 million ha) where elevated salinity is documented in the soil. Salinity is one of the major abiotic stresses that influences modern agricultural practice, says Ghosh.
In general, about six per cent of the world’s total land area is salt-affected and unfavourable for agricultural practice.
Together, salinity and high levels of arsenic make these soil areas unfavourable and demand technologies that allow sustainable agricultural practices at elevated salinity and its effective bioremediation.
“Though the arsenic remains in the soil, application of these microbes could lower the quantity of arsenic that piles up in the plant tissues because the arsenic will be immobilised on the cell surface of the bacterial species,” Ghosh said.
This strategy (arsenic immobilizing and salt tolerant microbes) could be used in countries such as Bangladesh, Vietnam, China, Thailand, Argentina, Greece, Chile and Spain, which are facing similar problems as is seen in West Bengal in India, he proposed.
The study also draws attention to the plant-microbe interaction occurring at the rhizosphere level. The decrease in arsenic uptake significantly by plant parts is a consequence of successful colonisation of either of the isolates in the rhizospheric environment of rice plants.
“Since we have targeted Sundarbans and the agricultural soils around it and because these organisms are naturally present in those areas, they will not change the natural soil microbial consortia upon application. You are basically adding a bit extra to facilitate arsenic reduction and plant growth promotion,” Ghosh notes.
Ecosystem engineering and biodiversity
Dubbing the study as “significant”, M.N.V. Prasad, professor, Department of Plant Sciences at University of Hyderabad and an expert in bioremediation, advocates the use of an ecosystem engineering approach wherein a bioremediation technology can be developed for practical application by “designing sustainable ecosystems” that integrate human society with its natural environment for the benefit of both.
“Arsenic-enriched groundwater from the vast alluvial aquifer in the Bengal Delta Plain is a subject of global concern. Any perturbed or constrained ecosystem can be rehabilitated by ecological and ecosystem engineering coupled with appropriate biodiversity. Bacteria and bio-augmentation approach can be economically feasible,” Prasad adds.
A 2011 ‘State-of-the-Art report on Bioremediation, its Applications to Contaminated Sites in India’ report authored by Prasad states “arsenic is one of the most toxic elements present in soils and water.”
“Over the years, arsenic has been widely used in agriculture and industrial practices such as pesticides, fertilisers, wood preservatives, smelter wastes and coal combustion ash, which are of great environmental concern, apart from the natural sources.”
“Arsenic contamination affects biological activities as a teratogen, carcinogen and mutagen as well as having detrimental effects on the immune system. The most common manifestations are skin melanosis and keratosis,” the report said.
According to the social worker and environmentalist Tushar Kanjilal, known for his pioneering work in the Sundarbans, executing bioremediation procedures in the region is not an overnight task.
“To ensure the success of scientific innovations, these ideas must be consistently applied on a pilot basis in plots of land in the Sundarbans. There are a lot of factors that come into play in such fragile and remote ecosystems. In addition, problems like embankment breaches and livelihoods are of bigger concern to the residents of the islands than arsenic which may not be present in significant amounts in the mangroves,” Kanjilal added.
According to Manturam Pakhira, minister for Sundarban Affairs, the West Bengal government has taken up planting of mangroves across sandbars in the region on a large scale.
“There will be no sandbar that is devoid of mangroves. The survivability of the entire Sundarbans ecosystem, right from microbe to man, depends on these species, so we are ensuring these threatened and near-threatened species of mangrove trees can flourish,” Pakhira added.
Citation:
- Mallick, I., Bhattacharyya, C., Mukherji, S., Dey, D., Sarkar, S. C., Mukhopadhyay, U. K., & Ghosh, A. (2018). Effective rhizoinoculation and biofilm formation by arsenic immobilizing halophilic plant growth promoting bacteria (PGPB) isolated from mangrove rhizosphere: A step towards arsenic rhizoremediation. Science of the Total Environment, 610, 1239-1250.
