The problem these microbes address is rooted in chemistry. Tea thrives in acidic soil with a pH between 4.5 and 5.5, but acidity comes at a cost. In these conditions, aluminum and iron ions become more soluble and react with phosphate, locking it into insoluble precipitates that plant roots cannot absorb. Although soil typically holds 35 to 70 percent of its phosphorus in some form, only 1 to 2.5 percent exists in accessible states such as H2PO4− and HPO42−. In Darjeeling’s tea gardens, available phosphorus often falls below 10 parts per million, and heavy monsoon rains strip away up to 30 centimeters of fertile topsoil in some estates. Farmers respond with synthetic fertilizers, but the study’s authors note that overreliance on these products further erodes soil fertility, reduces microbial diversity, and disrupts nutrient cycles, creating a downward spiral of declining yields and quality.

Phosphate-solubilizing bacteria offer a biological escape route. These organisms secrete organic acids such as gluconic, citric, and oxalic acids, which lower the local soil pH and chelate metal cations including calcium, iron, and aluminum. This acidolysis releases phosphate into the plant-available forms HPO42− and H2PO4−, effectively mining the soil’s own mineral reserves. Common phosphate solubilizers in tea garden soils include species of Bacillus, Pseudomonas, and Burkholderia. What makes the new Darjeeling isolates exceptional is their combination of traits. The five strains, identified as Brevundimonas diminuta, Bacillus licheniformis, Brucella pseudogrignonense, Achromobacter xylosoxidans, and Rothia kristinae, were previously known to tolerate pesticides, antibiotics, and heavy metals, but their ability to simultaneously degrade cellulose and metabolize caffeine is described as exceptionally rare.

The cellulose-degrading capacity matters because tea gardens are buried in it. Seasonal pruning generates enormous quantities of woody litter rich in cellulose, a polysaccharide of glucose units linked by tough β-1,4-glycosidic bonds that resist physical and chemical breakdown. In acidic, nitrogen-poor tea soils, natural decomposition limps along at only 20 to 40 percent per year, leaving carbon locked away from the nutrient cycle. When the researchers grew their isolates on carboxymethyl cellulose medium and stained the plates with iodine, two strains, PSMR 5.7 and PSMR 2.1, produced clear halos indicating cellulase activity, with a relative enzyme activity of 3, a moderate but meaningful level. These bacteria, adapted to the gardens’ low pH and nitrogen scarcity, hydrolyze litter cellulose into simple sugars, feeding the soil food web and priming the mineralization of other nutrients.

Caffeine presents a subtler threat. Tea leaves contain 2 to 4 percent caffeine by dry weight, and fallen litter loads the soil with this alkaloid year after year. Caffeine acts as an allelochemical, suppressing seed germination and root elongation while inhibiting microbial growth. Over time it contributes to soil sickness, shifting bacterial communities in favor of pathogens over decomposers and reducing the polyphenol and amino acid content of tea leaves, which can cut yields by 20 to 30 percent. Three of the isolates, PSMR 6.2.9, PSMR 2.1, and PSMR 5.3, proved capable of using caffeine as their sole carbon and nitrogen source, tolerating concentrations up to 3 percent. Through the N-demethylation pathway, they break caffeine down into theobromine, paraxanthine, and ultimately xanthine and urea, releasing bioavailable nitrogen back into the soil. In effect, the microbes convert a toxin into fertilizer.

The plant growth-promoting credentials of the strains were equally impressive. On Pikovskaya’s agar, all five produced halo zones around their colonies, with Bacillus licheniformis (PSMR 6.4.3) achieving the highest phosphate solubilization index of 2.75. In liquid culture, the strains produced indole-3-acetic acid, the plant hormone that stimulates root growth, at levels reaching 119.62 micrograms per milliliter for PSMR 5.7 and 110.62 for PSMR 6.4.3, well above the threshold considered excellent. All five produced ammonia, with PSMR 5.3 yielding the most at 1.71 micromoles per milliliter, a significant contribution since tea plants absorb ammonium through specialized root transporters to build theanine and other flavor-defining compounds. Three strains also produced siderophores, iron-chelating molecules that combat chlorosis, and two produced hydrogen cyanide, a natural antimicrobial that suppresses tea pathogens such as Fusarium oxysporum and Alternaria alternata.

To test whether these laboratory traits translated into real growth, the team ran greenhouse pot experiments with mung bean (Vigna radiata) and chickpea (Cicer arietinum), comparing individual isolates against three formulated consortia grouped by Gram staining type. The results were striking. Individual isolates delivered average increases of 87 percent in shoot length, 50 percent in root length, 72 percent in fresh weight, 197 percent in dry weight, and 76 percent in chlorophyll content in mung bean, while chickpea showed gains of 87, 165, 96, 51, and 47 percent respectively. The standout performer was PSMR 2.1, which boosted mung bean shoot length by 129 percent and chickpea fresh weight by a remarkable 485 percent. Correlation analysis revealed a strong link between phosphate solubilization and chlorophyll production, with correlation coefficients of 0.77 in mung bean and 0.85 in chickpea, confirming that phosphorus availability drives photosynthetic capacity.

Perhaps the most surprising finding came from the consortium trials. Although cross-streak tests showed no antagonism among the five strains, suggesting they could coexist peacefully, the mixed formulations consistently underperformed the best individual isolates in the pot experiments. The mixed SET-III consortium did release the most phosphorus in soil incubations, raising available phosphate 60 percent above the control after 30 days, but when it came to actual plant growth, single strains won. The researchers attribute this to competition for nutrients and niche overlap within mixed communities, where certain isolates may dominate or suppress others even without direct antibiosis. The lesson, they argue, is that compatibility testing and mechanistic understanding must precede any attempt to deploy microbial consortia as bioinoculants, and that for tea plantations, a single well-matched strain may be the smarter strategy.

The implications extend well beyond Darjeeling. Tea is grown across millions of hectares of acidic tropical and subtropical soil worldwide, and phosphorus deficiency plus soil sickness are chronic constraints everywhere. A biofertilizer that simultaneously solubilizes phosphorus, decomposes cellulose waste, detoxifies caffeine, fixes nitrogen into ammonium, produces growth hormones, and fights pathogens would address nearly every major soil problem in a tea garden at once. The study’s authors frame their isolates as candidates for sustainable biofertilizer development that could reduce chemical inputs, recycle on-farm waste, and restore long-term soil health. Previous work supports the concept: endophytic phosphate-solubilizing bacteria from tea plants have been shown to increase available phosphorus by 40 percent, and cellulose-degrading bacteria in tea soils have accelerated litter decomposition by 20 to 40 percent.

Challenges remain before these laboratory strains reach the field. The pot experiments used two model plants rather than tea itself, and the caffeine utilization assays showed declining bacterial growth as concentrations rose above 1 percent, indicating limits to detoxification capacity. Formulation, shelf life, and performance under real monsoon conditions all require further testing. Still, the study offers a compelling proof of concept: the solution to tea’s soil crisis may already be living in the soil, waiting to be identified and deployed. For an industry whose most famous product is marketed on purity and terroir, microbes harvested from the gardens themselves may prove the most authentic fertilizer of all.

Subject of Research: Phosphate-solubilizing bacteria from Darjeeling tea garden soils with cellulose-degrading, caffeine-utilizing, and plant growth-promoting traits

Article Title: Phosphate solubilizing microbes from tea garden soil exhibit cellulose degrading and caffeine utilizing potential with high plant growth promoting activity

Article References: Phosphate solubilizing microbes from tea garden soil exhibit cellulose degrading and caffeine utilizing potential with high plant growth promoting activity. (n.d.). https://doi.org/10.1007/s44372-026-00878-5

Image Credits: AI Generated

DOI: 10.1007/s44372-026-00878-5

Keywords: phosphate-solubilizing bacteria, tea garden soil, Darjeeling, cellulose degradation, caffeine utilization, plant growth-promoting rhizobacteria, biofertilizer, soil fertility, soil acidification, indole-3-acetic acid, siderophores, Vigna radiata