evolution: symbiosis with Cycads (Introduction)

by David Turell , Tuesday, November 21, 2023, 15:59 (158 days ago) @ David Turell

New research:

https://www.nature.com/articles/s41559-023-02251-1?et_rid=825383635&et_cid=4994162

"Cycads are ancient seed plants (gymnosperms) that emerged by the early Permian. Although they were common understory flora and food for dinosaurs in the Mesozoic, their abundance declined markedly in the Cenozoic. Extant cycads persist in restricted populations in tropical and subtropical habitats and, with their conserved morphology, are often called ‘living fossils.’ All surviving taxa receive nitrogen from symbiotic N2-fixing cyanobacteria living in modified roots, suggesting an ancestral origin of this symbiosis. However, such an ancient acquisition is discordant with the abundance of cycads in Mesozoic fossil assemblages, as modern N2-fixing symbioses typically occur only in nutrient-poor habitats where advantageous for survival. Here, we use foliar nitrogen isotope ratios—a proxy for N2 fixation in modern plants—to probe the antiquity of the cycad–cyanobacterial symbiosis. We find that fossilized cycad leaves from two Cenozoic representatives of extant genera have nitrogen isotopic compositions consistent with microbial N2 fixation. In contrast, all extinct cycad genera have nitrogen isotope ratios that are indistinguishable from co-existing non-cycad plants and generally inconsistent with microbial N2 fixation, pointing to nitrogen assimilation from soils and not through symbiosis. This pattern indicates that, rather than being ancestral within cycads, N2-fixing symbiosis arose independently in the lineages leading to living cycads during or after the Jurassic. The preferential survival of these lineages may therefore reflect the effects of competition with angiosperms and Cenozoic climatic change."

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https://mail.google.com/mail/u/0/#inbox/FMfcgzGwHpPGcvZrwjwFXZNBKjFQQMxG

"Modern cycads live in partnership with bacteria: The cycads share the sugars they make, and in exchange, the microbes convert nitrogen from the air into a form that the plants can use. This means that the nitrogen in cycad tissues comes more directly from the atmosphere than it does in other plants—and, therefore, examining it can tell researchers about that air. Kipp had already demonstrated this with living cycads , so he started applying methods to examine nitrogen in ancient foliage samples to cycad fossils. But his analysis didn’t tell him about the Mesozoic air. Instead, it revealed that the ancient cycads didn’t have microbial partners.

“'Instead of being a story about the atmosphere, we realized this was a story about the ecology of these plants that changed through time,” Kipp says.

"How nitrogen-fixing bacteria helped cycads survive the mass extinction isn’t clear. Maybe the nitro-boost helped them stay competitive in the face of rapidly diversifying flowering plants, Kipp suggests. The methods he helped develop should let researchers dig deeper. And hopefully, they’ll prove useful for studying ancient atmospheres, too."

Comment: we can't live without good bacteria. A major defense in theodicy essays.