Ancient nitrogen isotopes reveal surge in N2-fixing microbes after end-Triassic extinction

About 201.6 million years ago, the emplacement of the Central Atlantic Magmatic Province triggered the end-Triassic mass extinction, wiping out many ecologically important groups of terrestrial and marine organisms. New research now shows that the nitrogen cycle, a system that underpins marine ecosystems and regulates the carbon cycle, underwent a dramatic reorganization in the extinction’s immediate aftermath. By reading nitrogen isotopes locked inside ancient pigments, scientists have reconstructed, for the first time, how different groups of photosynthetic organisms sourced their nitrogen before, during and after the crisis, and the results point to an unexpected dominance of nitrogen-fixing microbes in coastal waters.

The study, published in Communications Earth & Environment, focused on St. Audrie’s Bay in the United Kingdom, a key reference section for the end-Triassic event that represents a coastal to shelf environment in the northwest Tethys Ocean. Rather than measuring bulk organic nitrogen, which blends signals from every organism in the water column, the team analyzed compound-specific nitrogen and carbon isotopes of sedimentary metalloporphyrins. These molecules are the diagenetic remnants of chlorophylls and bacteriochlorophylls, pigments synthesized exclusively by phototrophs, and their isotope signatures survive alteration largely intact. That means each porphyrin type carries the isotopic fingerprint of a specific group of primary producers living at a particular depth in the ancient water column.

Three porphyrins anchored the reconstruction: nickel DPEP, vanadyl DPEP, and a C35 vanadyl porphyrin. Nickel and vanadyl DPEP are considered to derive primarily from chlorophyll a, chlorophyll c, and bacteriochlorophyll a, but the researchers found they told strikingly different stories. During the extinction interval, recorded in the lower Paper Shales of the Blue Lias Formation, nickel DPEP nitrogen isotope values ranged from 1.0 to 4.1 per mil, while vanadyl DPEP values sat far lower, between −6.3 and −4.9 per mil, and the C35 porphyrin was depleted even further, at −9.9 to −9.4 per mil. A gap of roughly 6 to 8 per mil between nickel and vanadyl DPEP was far too large to be explained by diagenetic metallation, which experiments show produces only small fractionations, such as negligible shifts for nickel insertion and 2.4 per mil of depletion for zinc insertion into a metal-free porphyrin.

Instead, the offset reflects distinct phototrophic communities drawing on isotopically distinct nitrogen pools. In stratified water columns with euxinic bottom waters, ammonium depleted in the heavy isotope nitrogen-15 accumulates below the chemocline, the boundary layer where oxygen gives way to sulfidic conditions, because nitrification is suppressed. When the chemocline sits shallow enough, purple sulfur bacteria that synthesize bacteriochlorophyll a flourish within and below it, assimilating large amounts of that nitrogen-15-depleted ammonium. The team therefore interprets nickel DPEP as predominantly derived from chlorophyll a-producing algae and cyanobacteria in the oxygenated upper photic zone, vanadyl DPEP as primarily from bacteriochlorophyll a-producing purple sulfur bacteria at or below the chemocline, and the C35 porphyrin, whose carbon enrichment fits the reversed tricarboxylic acid cycle used by green sulfur bacteria, as input from deeper-dwelling green sulfur bacteria. Together, the three pigments provide a vertically resolved isotopic profile of end-Triassic stratified waters, from sunlit surface waters down into the dark, sulfidic depths.

Supporting biomarkers from the neighboring Lilstock section, about 7 kilometers away, flesh out the environmental backdrop. Elevated concentrations of okenane from purple sulfur bacteria and isorenieratane from brown-pigmented green sulfur bacteria demonstrate persistent photic zone euxinia extending through much of the water column in the lower Paper Shales, while soil erosion biomarkers such as dibenzofuran and dibenzothiophene point to increased terrestrial input. In the overlying upper Paper Shales, euxinia retreats to deeper waters, the cyanobacterial biomarker renierapurpurane rises, and increasing gammacerane indices signal enhanced water column stratification. The pigments thus track a water column whose chemocline deepened as the extinction interval progressed, squeezing the habitable space for nitrate-assimilating algae.

That deepening is central to the study’s headline finding: a surge in nitrogen fixation. Nitrogen-fixing cyanobacteria convert atmospheric N2, which has an isotope value near zero per mil, into biologically usable nitrogen, and their cellular nitrogen isotope values average around −1 per mil. Accounting for the experimentally constrained difference between cellular nitrogen and chlorophyll in marine cyanobacteria, nickel DPEP values within or close to the expected range for nitrogen fixation emerged throughout the Paper Shales. Using a model built on stratified marine nitrogen cycling, the researchers estimated that nitrogen fixation contributed roughly 20 percent of primary production in the lowermost Paper Shales, where a shallow chemocline still allowed nitrate produced at the boundary to reach the condensed oxygenated photic zone.

In the upper Paper Shales, however, the contribution climbed to roughly 50 to 65 percent of primary production on average, with maximum contributions of about 70 to 90 percent coinciding with rising cyanobacterial biomarker abundance and the deepening chemocline. These values substantially exceed previous estimates for anoxic coastal environments and approach modeled equatorial open-ocean values for oceanic anoxic events of around 70 percent. Because the Paper Shales represent a coastal to shelf setting deposited at a paleolatitude of about 30 degrees north, the results suggest that coastal nitrogen cycling during large igneous province events may have been more strongly perturbed than previously recognized. The discrepancy with bulk nitrogen isotope records likely reflects the fact that bulk measurements integrate nitrogen from photosynthetic and non-photosynthetic organisms throughout the water column, while nickel DPEP isolates a specific subset of phototrophs, and low total nitrogen contents in the shales, between 0.09 and 0.18 percent, mean minor isotopic shifts could disproportionately move bulk values.

The compound-specific carbon isotope records added their own insight. Negative carbon isotope excursions in all three porphyrins occurred synchronously with shifts in other phototroph-derived biomarkers and with the negative excursion in total organic carbon, indicating a carbon-cycle perturbation that altered dissolved inorganic carbon throughout the water column, likely linked to CAMP magmatic activity. That distinguishes the lower Paper Shales excursion from stratigraphically lower carbon isotope excursions at St. Audrie’s Bay, which are instead attributed to the introduction of carbon-13-depleted land plant material and microbial mat development. The lower Paper Shales excursion therefore stands as a globally correlatable chemostratigraphic marker for the extinction, revealed through the novel compound-specific approach.

The authors emphasize that the findings carry weight beyond deep time. The end-Triassic event provides a potential analog for long-term climate perturbations driven by rapid carbon dioxide release, and enhanced nitrogen fixation during the ETE, as well as during other large igneous province-associated extinctions and oceanic anoxic events, suggests a recurring ecosystem response to rapid carbon-cycle disruptions. Integrated compound-specific nitrogen isotope analyses and complementary geochemical proxies indicate that carbon dioxide-driven warming may promote restructuring toward lower-diversity, diazotroph-dominated phototrophic communities, shifts that are predicted in future climate models. Expansion of nitrogen-fixing cyanobacteria during the ETE likely had cascading consequences, including reduced light penetration into deeper waters, decreased oxygen availability, and competitive exclusion of other phytoplankton groups, influencing food-web structure and ecosystem stability.

The researchers note that the mechanisms sustaining stratification during the end-Triassic likely differed from many modern stratified basins, where anoxic bottom waters are often stabilized by freshwater inputs, whereas CAMP-driven greenhouse warming may have enhanced stagnation and stratification. Still, the overall redox architecture appears broadly comparable, underscoring how sensitive nitrogen cycling is to redox structure, as also observed in Mediterranean sapropel formation. Expanded compound-specific nitrogen isotope records across extinction and anoxic intervals, the team concludes, are needed to resolve regional versus global patterns in nitrogen biogeochemical cycles and to better constrain how ecosystems respond to rapid climate forcing, a question with direct relevance to future climate scenarios.

Subject of Research: Reconstruction of nitrogen cycle reorganization and nitrogen fixation during the end-Triassic mass extinction using compound-specific isotope analysis of sedimentary porphyrins

Article Title: Phototroph-specific nitrogen isotopes detail N2-fixation increases at the end-Triassic extinction

Article References: Fox, C. P., Isaji, Y., Ogawa, N. O., Whiteside, J. H., Al-Suwaidi, A., Grice, K., & Ohkouchi, N. (2026). Phototroph-specific nitrogen isotopes detail N2-fixation increases at the end-Triassic extinction. Communications Earth & Environment, 7(1), 839. https://doi.org/10.1038/s43247-026-04159-7

Image Credits: AI Generated

DOI: 10.1038/s43247-026-04159-7

Keywords: end-Triassic extinction, nitrogen cycle, nitrogen fixation, porphyrins, phototrophs, Central Atlantic Magmatic Province, St. Audrie's Bay, cyanobacteria, photic zone euxinia, isotope geochemistry, stratified water column, large igneous province