New research on cycads, ancient plants from the Mesozoic Era, suggests that they survive into modern times because of a symbiotic relationship with nitrogen-fixing bacteria. This study provides insights into the ecological evolution of cycads and their adaptation to past climate changes.
Cycads are ancient plants that were once prevalent in the Mesozoic Era. Most of them have become extinct, with only a few species surviving in tropical and subtropical regions. The researchers found that these surviving cycads relied on symbiotic bacteria to fix nitrogen, a trait not found in their extinct cousins.
During the Mesozoic Era, which began 252 million years ago, ancient plants like cycads were a favorite of herbivorous dinosaurs. They grew in abundance at the bottom of the forest, helping to maintain the survival of these dinosaurs and other prehistoric animals. Today, only a few palm-like plants survive in tropical and subtropical habitats.
Extinction and survival of cycads
Like their logging herbivorous counterparts, most cycads have become extinct. Their disappearance from their former habitat began in the late Mesozoic and continued into the early Cenozoic, with asteroid impacts and volcanic activity at the K-Pg boundary 66 million years ago. However, unlike the dinosaurs, there are several groups of cycads that somehow survive to this day.
New research findings on cycad survival
A new study published today (November 16) in the journal Nature Ecology & Evolution suggests that the survival of cycad species depends on symbiotic bacteria in their roots, which provide nitrogen for their growth. Like modern legumes and other plants that utilize nitrogen fixation, these cycads exchanged their sugars with bacteria in their roots for nitrogen taken from the atmosphere.
Lead author Michael Kipp was originally interested in the fact that the tissues of nitrogen-fixing plants could provide a record of the atmospheric composition of the environment in which they grow. He combines geochemistry with the fossil record to try to understand Earth's climate history.
Kip already knew that modern cycads were nitrogen-fixing plants, and while working on his doctoral degree at the University of Washington, he began analyzing some very old plant fossils to see if he could get another perspective on the ancient atmosphere. Most ancient cycad fossils show that they were not nitrogen-fixing plants, but these have also been shown to be of extinct species.
"This is not a story about the atmosphere, we realized it's a story about the ecology of these plants changing over time," Kipp said.
Kipp will join Duke this year as an assistant professor of earth and climate sciences in the Nicholas School of the Environment, where he will continue to use the fossil record to understand Earth’s climate history and, therefore, its possible future.
Methods and findings
Much of what we know about the ancient atmosphere comes from chemical studies of ancient marine organisms and sediments, Kipp said. Applying some of these methods to terrestrial plants is a new endeavor. "At the time of this project, there had been no published nitrogen isotope data from fossil plant leaves. He spent some time fine-tuning the method and obtaining samples from rare plant fossils that museum curators were unwilling to see to obtain the data. In the few surviving (cycads) lineages "In stone samples that are not that old -- 20 million or 30 million years old -- we see the same nitrogen signature that we see today. That means their nitrogen came from symbiotic bacteria, but that nitrogen signature is not present in the older, extinct cycad fossils."
Impact and future research
What's less clear is how nitrogen fixation helps surviving cycads. This may have helped them withstand drastic changes in climate, or it may have enabled them to better compete with the faster-growing angiosperms that flourished after the mass extinction, "or possibly both."
"This is a new technology and we can do a lot more with it," Kipp said.
Funding for this research came from: the Paleontological Society, the University of Washington Royal Research Fellowship, and NASA Exobiology Grant NNX16AI37G.