Ancient cycads survived mass extinction by using nitrogen-fixing bacteria

In the primeval landscapes of the Mesozoic Era, grazing dinosaurs found a culinary preference for cycads, an ancient lineage of plants abundant in the forest understory, sustaining various prehistoric creatures for millions of years. This botanical saga commenced approximately 252 million years ago. Fast forward to the present, and only a handful of palm-like cycad species endure in today’s tropical and subtropical habitats.

Similar to their colossal herbivorous counterparts, the majority of cycads faced extinction. This decline began in the late Mesozoic, persisting into the early Cenozoic Era, marked by the significant K-Pg boundary event 66 million years ago—characterized by an asteroid impact and volcanic upheaval. Remarkably, a select few cycad groups managed to defy extinction, persisting through the ages.

A recent study titled “Nitrogen Isotopes Reveal Independent Origins of N2-Fixing Symbiosis in Extant Cycad Lineages,” published on Nov. 16 in Nature Ecology & Evolution, sheds light on the survival strategy of these resilient cycad species. The research, led by Michael Kipp, proposes that the surviving cycads depended on symbiotic bacteria in their roots. These bacteria facilitated nitrogen fixation, a process where plants exchange sugars with root bacteria for atmospheric nitrogen, akin to the symbiotic relationships seen in modern legumes and other nitrogen-fixing plants.

Michael Kipp’s intrigue in this botanical tale was not solely botanical but extended into the realm of geochemistry and climate history. Kipp, the lead author, merged geochemical analysis with the fossil record, aiming to unravel the Earth’s climatic narrative.

During his Ph.D. work at the University of Washington, Kipp delved into ancient plant fossils, seeking a unique perspective on past atmospheres. The revelation unfolded gradually; most ancient cycads were not nitrogen-fixers, especially those belonging to extinct lineages. This discovery redirected the narrative from atmospheric conditions to an evolutionary ecological tale.

“It’s a story about the ecology of these plants that changed through time,” says Kipp, who devoted almost a decade to this pursuit, first at UW and later as a postdoctoral researcher at CalTech.

Now, as an assistant professor of Earth and Climate Sciences at Duke, Kipp continues his journey, leveraging the fossil record to decipher Earth’s climate history, offering potential insights into its future. Traditional knowledge about ancient atmospheres primarily stems from chemical studies of ancient sea life and sediments. Kipp’s innovative approach applies similar methods to terrestrial plants, introducing a new dimension to climate research.

A fossil of a veined leaf from the same strata as an extinct cycad was used for comparison of nitrogen isotopes. Credit: Michael Kipp—Duke University

“Going into the project, there were no published nitrogen isotope data from fossilized plant foliage,” Kipp notes. Developing and fine-tuning this methodology required time, patience, and access to precious plant fossils—samples that some museum curators were understandably reluctant to see vaporized for data extraction.

In the fossil samples from surviving cycad lineages, aged around 20 to 30 million years, Kipp observed a consistent nitrogen signature, mirroring that of present-day cycads. This implies their reliance on symbiotic bacteria for nitrogen. However, in older and extinct cycad fossils, this nitrogen signature was conspicuously absent.

The intriguing question remains: How did nitrogen fixation contribute to the survival of these cycads? It might have aided them in adapting to drastic climate shifts or in outcompeting faster-growing angiosperm plants that thrived post-extinction. Perhaps, it played a role in both scenarios.

“This is a new technique that we can do a lot more with,” Kipp remarks, emphasizing the potential of this innovative approach to unlock more secrets buried in the ancient plant record.

Source: Duke University

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