Key genes drive stronger CO2 fixation in mangrove microalgae

A Carbon-Hungry Cyanobacterium From the Sundarban’s Tidal Mud Could Become Climate Tech’s Unlikely New Workhorse

In the salt-laced sediments of India’s Sundarban, one of the world’s largest mangrove-dominated coastal systems, scientists have identified a filamentous cyanobacterium with an outsized appetite for carbon dioxide. Writing in the Springer journal 3 Biotech, researchers at the ICAR-National Rice Research Institute in Cuttack report that Leptolyngbya boryana, a photosynthetic microbe recovered from degraded mangrove soils, fixed up to 149 milligrams of CO₂ per liter of culture per day, built biomass at 1.31 grams per liter, and committed just over half of its dry weight — 0.52 grams of carbon per gram — to organic matter under enriched CO₂. Those three measurements, produced by a screening pipeline that began with environmental DNA extracted from muddy soil and ended in controlled carbon-dioxide enrichment, position the strain among the most capable wild microalgae yet characterized for biological carbon sequestration in saline coastal conditions. The work, led by doctoral researcher Sujit Kumar Nayak with biogeochemist Pratap Bhattacharyya as corresponding author, doubles as a blueprint for how to find climate-ready microbes: let a stressed ecosystem do the natural selection first, then interrogate the survivors’ genomes.

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.