The backdrop is one of the planet’s great carbon vaults. Mangrove ecosystems are recognized as major blue-carbon reservoirs, holding between 4.4 and 11.7 petagrams of organic carbon globally — billions of tonnes locked in waterlogged soils where oxygen is scarce and decay is slow. The Sundarban alone, sprawling across 3,629.57 square kilometers of delta, carries a carbon stock estimated at 26.62 teragrams, more than 26 million tonnes. But these reservoirs are under pressure. Rising salinity, shifting hydrology and intensifying human activity are reworking the microbial communities that underpin coastal food webs, including the microalgae and cyanobacteria that, though individually microscopic, collectively fix enormous quantities of carbon in shallow, sunlit waters and export it into the sediments below. As the composition of those communities changes, the researchers argue, identifying taxa that can keep fixing carbon under future conditions becomes a conservation question as much as a biotechnological one.

To find them, the team turned the mangrove inside out, genomically speaking. Rather than cataloguing which microbes were present by microscopy, they performed whole-genome metagenomic profiling of degraded mangrove soils — shotgun-sequencing the collective DNA of the sediment community and mapping the reads against known genes to reconstruct which metabolic machinery the ecosystem was actively deploying. Where older surveys relied on marker genes such as 16S ribosomal DNA to sketch community composition, whole-genome metagenomics captures the entire functional repertoire: every copy of every carbon-metabolism gene present in the sediment’s pooled genome library. The analysis surfaced six dominant microalgal taxa whose abundances tracked the prevailing salinity and nutrient stress, effectively flagging the organisms the environment itself had shortlisted. The logic is elegant: a degraded, saline, nutrient-fluctuating mudflat is a brutal natural selection chamber, and any microalga that thrives there already carries genetic equipment for osmotic tolerance and flexible carbon metabolism that laboratory strains bred in benign conditions may lack.

All six taxa were then isolated into pure cultures from the same habitats and subjected to a 16-day screening under ambient carbon-dioxide levels of roughly 420 parts per million — the concentration of today’s atmosphere. The investigators tracked two deceptively simple metrics: specific growth rate, the exponential pace at which cells divide, and biomass gain, the sheer quantity of organic material accumulated per liter. Three strains pulled clear of the field: Chlorella sp., a spherical green microalga long studied for biofuels; Limnospira platensis, the coiled filamentous cyanobacterium better known as spirulina; and Leptolyngbya boryana, a slender, sheathed cyanobacterium that forms soft mats in the wild. Each represents a different branch of the photosynthetic tree, which made their head-to-head comparison a genuine test of three distinct evolutionary strategies for grabbing carbon.

The finalists were then pushed up a carbon-dioxide ladder designed to mimic present and future atmospheres: 0.04 percent, 0.05 percent, 0.20 percent and 10 percent CO₂. The lower rungs track the world we inhabit and the near-term trajectory of rising emissions; 0.20 percent — twenty times today’s ambient level — probes the physiological limits of acclimation; and the 10 percent tier, roughly 100,000 parts per million, goes far beyond any plausible atmospheric scenario and approaches the CO₂ content of industrial exhaust streams. For microalgae, extra CO₂ is a double-edged gift: it supplies the limiting substrate for photosynthesis, but dissolved as carbonic acid it pushes culture pH downward, forcing cells to spend energy on pH regulation — a stressor documented in other Chlorella studies. The question, therefore, was not simply which microbe survives elevated CO₂, but which one converts the additional carbon into new cells rather than stalling.

Leptolyngbya boryana swept the board. It delivered the highest biomass yield of the three, at 1.31 grams per liter, the richest carbon content at 0.52 grams of carbon per gram of dry weight — meaning carbon comprised more than half of everything it built — and the steepest carbon-dioxide fixation rate, reaching 149 milligrams of CO₂ per liter per day. That last figure comes from the carbon balance of the culture: by measuring how much carbon ends up locked in harvested biomass, researchers back-calculate how much CO₂ must have been drawn from the gas phase to supply it. In practical terms, a cubic meter of dense L. boryana culture could in principle scrub on the order of 149 grams of CO₂ daily before any process optimization — a benchmark that matters enormously when engineers size photobioreactors for emissions treatment.

The deeper explanation lies in the microbe’s genes. Metagenomic analysis showed that L. boryana carried the strongest representation of two canonical carbon-fixation pathways among the isolates: the Calvin–Benson–Bassham, or CBB, cycle and the reductive tricarboxylic acid, or rTCA, cycle. The CBB cycle is photosynthesis’s carbon-grabbing engine: the enzyme RuBisCO attaches CO₂ to a five-carbon sugar, ribulose-1,5-bisphosphate, splitting it into three-carbon molecules that ATP and NADPH then reduce into sugars. The genes cbbL and cbbS encode the large catalytic and small structural subunits of that enzyme; gap2 encodes a glyceraldehyde-3-phosphate dehydrogenase that drives the cycle’s reduction step; and zwf, encoding glucose-6-phosphate dehydrogenase, feeds the oxidative pentose-phosphate pathway, generating reducing power and the sugar skeletons needed to regenerate RuBisCO’s substrate. The gene accC encodes the biotin-dependent carboxylase subunit of acetyl-CoA carboxylase, the committed first step that diverts fixed carbon into fatty-acid synthesis — an essentially irreversible carbon sink. Enrichment of rTCA machinery, a reversal of the Krebs cycle that incorporates CO₂ through reductive carboxylation reactions, suggests the organism carries layered, redundant routes for pulling inorganic carbon into biomass.

Why does a single hardy cyanobacterium warrant this attention? Because the dominant technologies for capturing carbon dioxide — chemical solvents, engineered membranes, solid sorbents — are energy-hungry and costly, and they merely concentrate the gas without converting it. Biological fixation is different: photosynthesis transforms CO₂ into living biomass that can, in principle, be harvested, processed into feeds, fertilizers or biofuel precursors, and removed from the atmospheric ledger. A saline-adapted strain sharpens that proposition. Coastal cultivation of L. boryana could run on seawater rather than scarce freshwater, sidestepping competition with agriculture, and could be co-located with coastal industries whose emissions supply the carbon. There is also a blue-carbon synergy to consider: biomass grown in coastal systems feeds carbon into the same sediment pools that make mangroves such formidable long-term stores. The team’s broader research program, including earlier reviews on harnessing microalgae for net-zero emissions and fieldwork tracking algal diversity as Sundarban mangroves are converted to rice paddies, frames such strains as both climate tools and barometers of ecosystem health.

The researchers are candid about the distance between flask and deployment. Laboratory cultures offer idealized light, temperature and mixing; open ponds and industrial bioreactors do not, and scale-up routinely collides with light limitation in dense cultures, contamination by grazers and rival microbes, and the energy cost of harvesting and dewatering soupy biomass. The study, which rests on Nayak’s doctoral research and was supported by India’s DST-INSPIRE Fellowship, the Department of Biotechnology, the National Innovations in Climate Resilient Agriculture program and an ICAR National Fellow project, provides the starting genotype and the genetic targets. The next steps it implies are familiar to the field: validating performance in outdoor saline cultures, quantifying fixation rates under real flue-gas compositions with their sulfur and nitrogen oxides, and potentially deploying genome-editing tools to push the expression of cbb and accessory genes even higher.

There is a quiet irony in the provenance of this strain. It was not found in a pristine sanctuary but in degraded mangrove soils — ecosystems already bent by salinity intrusion and human pressure. The very stressors that threaten the Sundarban’s carbon vault appear to have forged a microbe exceptionally well equipped to re-carbonize it. Whether Leptolyngbya boryana graduates from a discovery in 3 Biotech to an industrial carbon-capture workhorse remains to be seen, but the study makes a compelling case that the answer to an atmospheric problem may already be growing, patiently and photosynthetically, in the mud.

Soil salinity is a growing constraint in coastal regions and irrigated farmland. When soluble salts accumulate around roots, they initially create osmotic stress, making it harder for plants to extract water even when the soil appears moist. As salt ions enter tissues, they can cause ionic toxicity, disrupt membranes and enzymes, trigger oxidative stress and restrict nutrient acquisition. Young seedlings are particularly vulnerable because their root systems and physiological defences are still developing. Mustard is an important oilseed crop in India, but the CS61 variety, like many crops, can experience reduced productivity when exposed to excessive salt. The researchers turned to mangroves because their rhizospheres are shaped by periodic tidal flooding, high salinity, oxygen-poor sediments and fluctuating chemical conditions. Microorganisms that persist there must maintain cellular function under stress, making the mangrove root zone a promising reservoir of bacteria with salt-tolerance traits. The central idea is not that these microbes remove all salt from soil, but that they can help plants tolerate its effects by changing the biological conditions around the root.

The team collected rhizosphere soil from 11 mangrove sites extending from Koparkhairane to Belapur in Navi Mumbai, Maharashtra. From those samples, they recovered 1,263 bacterial isolates. The initial screening narrowed the collection to organisms able to grow with 5 percent sodium chloride, leaving 168 moderately salt-tolerant isolates. At 10 percent sodium chloride, 97 isolates still grew on solid medium. After repeated purification, 36 were confirmed as pure cultures, and a preliminary blood-agar test excluded isolates showing alpha- or beta-hemolysis, which can indicate potentially undesirable biological activity. Seventeen non-hemolytic isolates remained for detailed testing. In liquid medium, all 17 grew strongly at 2, 4 and 6 percent sodium chloride. At higher concentrations, growth varied, and only some strains maintained moderate growth at 10 percent. None grew at 12, 14 or 16 percent in the tested broth conditions. That decline provided a useful physiological boundary: the bacteria were highly salt tolerant compared with ordinary soil isolates, but they were not immune to extreme osmotic pressure.

The surviving isolates were then examined for functions associated with plant growth promotion. These tests looked for traits that could influence nutrient availability, root development or stress responses. Nitrogen-fixation and phosphate-solubilization activity appeared in 82.4 percent of the isolates, potentially helping convert nutrients into forms plants can access. Every isolate produced indole-3-acetic acid, or IAA, a plant hormone involved in cell expansion and the formation of lateral roots, although production levels differed. Five isolates showed strong IAA activity. All isolates also produced ammonia and biosurfactants, while 64.7 percent produced gibberellin-like compounds and 94.1 percent showed protease activity. Cellulose degradation occurred in 58.8 percent, pectin degradation in 35.3 percent and starch hydrolysis in 23.5 percent. Exopolysaccharide production was less common, detected in 11.8 percent of the isolates. Such secreted polymers can sometimes help bacteria and their plant hosts manage water stress by altering the immediate soil microenvironment, although the present study did not establish the contribution of each mechanism inside living plants.

The researchers also tested whether the bacteria could inhibit Aspergillus niger, a fungus used in the study’s laboratory assay as an indicator of antifungal potential. Sixteen of the 17 isolates produced detectable inhibition zones. Three strains stood out because their activity was consistent and pronounced: BJ2, DJ12 and J4. Genetic identification using nearly complete 16S rRNA gene sequences assigned BJ2 to Micrococcus luteus, while DJ12 and J4 were identified as Microbacterium barkeri. Their sequences were deposited in GenBank under accession numbers PX974661, PX974662 and PX974663, respectively. The three strains also showed no inhibitory interaction with one another in a cross-streak compatibility test, an important preliminary condition for combining them. The proposed consortium therefore brought together a Micrococcus strain and two Microbacterium strains with overlapping but not identical functional profiles. The authors note that M. barkeri has been less explored as a plant-growth-promoting bacterium, so its apparent activity in this work warrants additional investigation rather than immediate agricultural deployment.

In the first plant test, surface-sterilized Mustard CS61 seeds were treated with each bacterial strain separately, with an equal-volume mixture of all three, or with sterile water as a control. The seeds were placed on filter paper moistened with 0.85 percent sodium chloride and observed under controlled laboratory conditions. Every treatment reached 100 percent germination within two days, showing that the applied salt concentration did not prevent germination itself. The differences appeared in the seedlings that followed. Relative to the salt-stressed control, BJ2 increased root length by 98.8 percent and DJ12 by 96.4 percent, whereas J4 produced a much smaller 0.9 percent increase. The consortium generated the largest root response, an increase of 240.7 percent. Shoot length rose by 49.7 percent with BJ2, 23.5 percent with DJ12 and 49.4 percent with J4; the consortium increased it by 60.3 percent. Measures combining seedling size and germination, including the seedling vigour index, also improved most strongly with the combined inoculum. The statistical tests indicated significant treatment effects on root and shoot length.

The pot experiment provided a second test under controlled conditions. Mustard seeds were inoculated with the same individual strains or consortium and planted in soil exposed to salinity equivalent to 1 percent sodium chloride. An untreated control and an autoclaved-soil treatment were included. All treatments again reached 100 percent germination within two days, but bacterial inoculation significantly changed subsequent growth. The untreated plants had an average root length of 35.43 millimetres. DJ12 produced the longest roots, averaging 58.37 millimetres, followed by BJ2 at 57.43 millimetres, J4 at 56.57 millimetres and the consortium at 55.60 millimetres. For shoots, J4 performed best at 107.9 millimetres, followed by the consortium at 105.93 millimetres, BJ2 at 104.3 millimetres and DJ12 at 98.33 millimetres, compared with 96.8 millimetres in the control. The autoclaved treatment produced shorter roots and shoots, suggesting that biologically active cells or their ongoing products, rather than sterilized soil alone, were important to the observed response. The study used triplicate treatments, and analysis of variance with Dunnett’s test found highly significant effects for the principal growth measures.

Leaf chemistry and soil measurements added further clues, although they do not yet demonstrate how the inoculants would perform in a farm field. Total chlorophyll reached 0.365 milligrams per gram of fresh tissue in J4-treated plants, compared with 0.131 milligrams per gram in the control and 0.109 milligrams per gram in autoclaved soil. Higher chlorophyll levels may indicate better maintenance of photosynthetic machinery under salt stress, but the experiment did not directly measure photosynthetic rates or yield. After the pot trial, consortium-treated soil contained more measured carbon and nitrogen than control soil: carbon rose from 0.92 to 1.39 milligrams per litre, while nitrogen increased from 2,199.26 to 2,343.17 milligrams per kilogram. The soil pH increased from 6.85 to 7.55, and electrical conductivity rose from 851 to 948 microsiemens per centimetre. These changes are consistent with microbial effects on nutrient cycling, but their long-term significance remains uncertain. The authors propose metagenomic studies to investigate uncultured mangrove microbes and identify genes associated with salt tolerance and plant growth promotion. Field trials will also be needed to test persistence, colonization, compatibility with native soil communities, crop yield and biosafety before the consortium can be considered a reliable agricultural product.

The study’s strongest implication is methodological as well as agricultural: it demonstrates a pipeline for finding stress-adapted bacteria in an ecosystem that is rarely treated as a source of crop inoculants. Screening began with a large isolate collection and combined salt tolerance, plant-growth traits, safety-related hemolysis testing, molecular identification and compatibility testing. That sequence helps distinguish organisms that merely survive salt from candidates with a plausible capacity to interact beneficially with plants. The reported IAA production and cellulase activity are useful indicators, but they are laboratory traits; they do not by themselves prove that the compounds were produced at effective concentrations in the mustard rhizosphere.

The results should therefore be viewed as proof of potential rather than evidence of a ready-to-use biofertilizer. The experiments were conducted under controlled saline conditions, and the measured outcomes focused mainly on germination and early vegetative growth. Longer trials are needed to determine whether improved roots and shoots persist through flowering and seed production, particularly because salt levels, soil texture, temperature and native microbial communities vary substantially in agricultural fields. Future work should also verify strain identity with more discriminating genomic methods, quantify root colonization, test inoculant shelf life and examine whether the consortium remains stable during storage and after application. Comparing treated and untreated plants across several salinity regimes would help separate general growth promotion from genuinely salt-specific protection. Such validation is essential before introducing mangrove-derived strains beyond experimental settings.

Subject of Research: Mangrove-derived plant growth-promoting bacteria improving mustard growth under salinity stress

Article Title: Effect of plant growth promoting rhizobacteria on the growth promotion of Mustard CS61 (Brassica juncea)

Article References: Bhat, M. R., Auti, M., & Poojary, P. (2026). Effect of plant growth promoting rhizobacteria on the growth promotion of Mustard CS61 (Brassica juncea). Discover Biotechnology, 3(1), Article 12. https://doi.org/10.1007/s44340-026-00059-w

Image Credits: AI Generated

DOI: 10.1007/s44340-026-00059-w

Keywords: plant growth-promoting rhizobacteria, mustard, Brassica juncea, salinity stress, mangrove microbiology, Micrococcus luteus, Microbacterium barkeri, bioinoculants, Effect, plant, growth, promoting