Iron occupies a paradoxical place in biology. It is the fourth most abundant element in Earth’s crust, and virtually every living organism depends on it: hemoglobin cannot carry oxygen without it, mitochondria cannot generate energy without it, and DNA replication and repair rely on iron-dependent enzymes. Yet the same redox chemistry that makes iron indispensable also makes it dangerous. In aerobic conditions, iron readily catalyzes the production of hydroxyl and other reactive radicals through Fenton and Haber-Weiss chemistry, attacking proteins, peroxidizing membrane lipids, and damaging nucleic acids. Excess iron in the human body can cause corrosive injury to the gastrointestinal tract, hemorrhagic necrosis, and interference with the heart, liver, and central nervous system. The American Association of Poison Control Centres recorded more than 4,000 single-exposure cases of iron poisoning in 2015 alone, underscoring that even an essential nutrient becomes a hazard at the wrong concentration.

The World Health Organization has set a maximum allowable limit of 0.3 milligrams of iron per liter in drinking water, but industrial activity routinely pushes concentrations far higher. Mining, agriculture, and manufacturing discharge iron-laden effluents into waterways, and conventional removal methods carry their own burdens. Chemical oxidation and physical techniques such as adsorption on synthetic media and ion exchange can be effective, but they tend to generate secondary pollutants and hazardous byproducts, and they are often expensive to operate at scale. Iron is also notoriously stable in aqueous solution, making it a stubborn target. Bioremediation, the use of living or dead biological material to transform and detoxify contaminants, has long been touted as a sustainable alternative: it is inexpensive, avoids toxic residues, and can even allow recovery of the captured metal for reuse.

Bacteria are the workhorses of this approach. Their enormous diversity, rapid growth, and ability to thrive in hostile environments make them ideal candidates for metal cleanup. Many species deploy a layered arsenal against toxic metals: biosorption, in which metal ions passively bind to negatively charged carboxyl, phosphoryl, amino, and sulfo groups on the cell wall; bioaccumulation, in which living cells actively internalize metals using exopolymers and polysaccharides; bioprecipitation, where metabolites convert dissolved metals into insoluble solids; and bioleaching. Thermophilic bacteria, adapted to the scalding conditions of hot springs, bring an extra advantage. Their enzymes and cell structures remain stable at temperatures that would denature the proteins of ordinary microbes, which is precisely the kind of resilience needed to treat hot industrial effluents without costly cooling.

To find such a microbe, the researchers collected water from the Gauri Kund spring in Rudraprayag district, a geothermal site with a temperature of about 52 degrees Celsius and a near-neutral pH of 6.3. Samples were transported in sterile thermos flasks to preserve temperature, and bacteria were grown on iron-supplemented agar at 55 degrees Celsius. Three metal-resistant isolates emerged, designated GA4, GA5, and GA6, and all could grow in the presence of iron ions. Genetic fingerprinting of the most promising strain, GA6, using 16S rRNA gene sequencing and BLAST analysis revealed a 98.14 percent similarity to Bacillus thermophilus HS-BTL2, a species previously described from compost and only rarely studied. Notably, the team reports that this is the first time B. thermophilus has been documented in a hot spring in the Indian Himalayas, adding a new entry to the region’s microbial catalogue a decade after the devastating Kedarnath floods.

Laboratory characterization showed that all three isolates grew across a striking range of temperatures, from 45 to 80 degrees Celsius, with optimal growth at 55 degrees Celsius and pH 7.0, conditions that mirror the spring itself. The GA6 strain proved to be a rod-shaped, spore-forming, Gram-positive bacterium, and biochemical profiling with a 35-test carbohydrate kit showed it could metabolize a wide array of sugars, including trehalose, maltose, and glycerol, alongside positive reactions for esculin hydrolysis and citrate utilization. Crucially, the minimum inhibitory concentration test, which measures the highest iron concentration the bacterium can tolerate, came in at 450 micrograms per liter, confirming that GA6 is highly resistant to iron toxicity and well suited to survive in contaminated environments.

The bioremediation experiments delivered the study’s headline numbers. When live B. thermophilus cells were incubated with iron-contaminated medium for 48 hours at 55 degrees Celsius, inductively coupled plasma mass spectrometry showed iron concentrations falling from 41.36 to 5.72 micrograms per liter, a biosorption rate of 86.17 percent. The dead biomass performed even better. Cells grown in bulk, killed by autoclaving, dried, and ground into powder, reduced iron from 22.05 to just 1.81 micrograms per liter, removing 91.79 percent of the metal. This pattern, dead biomass outperforming living cells, is consistent with earlier studies on Bacillus subtilis and Acinetobacter strains, and the researchers attribute it to the larger effective surface area of dead cells and their freedom from the metabolic constraints that govern living organisms.

The most consequential test came when the bacterium was unleashed on real-world pollution. Effluent was collected from the drainage area of the State Infrastructure and Industrial Development Corporation of Uttarakhand Limited (SIDCUL) in Haridwar, adjusted to neutral pH, and inoculated with the bacterium. After 48 hours at 55 degrees Celsius, iron in the effluent had dropped from 58.30 to 21.02 micrograms per liter, a removal rate of 63.94 percent. That figure trails the laboratory results, as expected in a chemically complex industrial matrix, but it still compares favorably with earlier work in which Escherichia coli, Bacillus subtilis, and Pseudomonas putida removed 56, 62, and 80 percent of iron from distillery effluent respectively. The result demonstrates that a thermophile harvested from a Himalayan pilgrimage site can function in the unglamorous reality of industrial wastewater.

The researchers argue that dead biomass offers particular practical advantages for deployment: it requires no nutrients or environmental control, poses minimal risk of introducing living organisms into sensitive ecosystems, avoids competition with native microbes, and can be stored and reused as a stable biosorbent. At the same time, live cells bring metabolic versatility, including the production of siderophores, chelating agents, and extracellular polymeric substances that bind and sequester metals, and the team notes that synthetic biology could eventually engineer B. thermophilus for enhanced degradation of specific contaminants. Either way, the thermophile’s thermostable enzymes, which remain active at temperatures up to 80 degrees Celsius, mean treatment plants could process hot effluent streams directly rather than cooling them first, a significant energy saving.

Beyond the engineering promise, the study carries a conservation message. The hot springs of the Garhwal Himalaya, shaped by the same tectonic forces that raised the mountains, harbor microbial communities that remain largely unexplored, and sites like Gauri Kund face pressure from development, tourism, and natural disasters. The authors frame their work as both a technological proof of concept and a case for protecting these geothermal habitats as reservoirs of biodiversity with direct value to human welfare. If a single spore-forming bacterium scooped from a sacred spring can strip more than nine-tenths of the iron from contaminated water at 55 degrees Celsius, the argument goes, the untapped microbial wealth of the Himalayan geothermal belt may hold solutions to pollution problems that conventional chemistry has struggled to solve, affordably, sustainably, and without creating new hazards in the process.

Subject of Research: Thermophilic bacterial bioremediation of iron from industrial wastewater

Article Title: Thermophilic remediation of iron using Bacillus thermophilus: effect of variables and microbiological characteristics

Article References: Patil, A., Rajamohan, N., Rohilla, V., & Arya, M. (2025). Thermophilic remediation of iron using Bacillus thermophilus: effect of variables and microbiological characteristics. Discover Biotechnology, 2(1), Article 10. https://doi.org/10.1007/s44340-025-00019-w

Image Credits: AI Generated

DOI: 10.1007/s44340-025-00019-w

Keywords: bioremediation, Bacillus thermophilus, thermophiles, heavy metals, iron, wastewater, industrial effluent, hot springs, biosorption, Himalaya, ICP-MS, water treatment