In an enlightening study published in Nature Ecology & Evolution, researchers led by Michael A. Kipp and colleagues have delved into the evolutionary history of cycads, ancient seed plants known for their resilience and endurance since the early Permian period. Cycads, often termed ‘living fossils’, have persisted in restricted tropical and subtropical habitats, relying on nitrogen from symbiotic N2-fixing cyanobacteria in their roots. This unique symbiosis, however, raised questions about its origins, given the historical abundance of cycads in nutrient-rich Mesozoic ecosystems where such symbiosis is less advantageous​​.
Nitrogen, a vital nutrient for ecosystem productivity, is supplied mainly to the biosphere by a small fraction of prokaryotic microbes capable of N2 fixation. This process converts atmospheric nitrogen into a bioavailable form, a capability absent in eukaryotes. The research team employed foliar nitrogen isotope ratios, a well-studied proxy in modern plants, to investigate the history of N2 fixation in cycads. The foliage of plants with N2-fixing symbionts typically exhibits δ15N values near atmospheric N2 levels, starkly contrasting plants assimilating nitrogen from soil​​.
The study revealed fossilized cycad leaves from Cenozoic representatives of extant genera showed nitrogen isotopic compositions consistent with microbial N2 fixation. Conversely, all extinct cycad genera displayed nitrogen isotope ratios indistinguishable from co-existing non-cycad plants, suggesting nitrogen assimilation from soils rather than through symbiosis. This pattern indicates that N2-fixing symbiosis in cycads is not ancestral but arose independently in lineages, leading to living cycads during or after the Jurassic. The survival of these lineages may be attributed to competition with angiosperms and climatic changes in the Cenozoic era​​.
The rise of angiosperms to ecological dominance from the Mesozoic into the Cenozoic marked a significant shift in terrestrial ecosystems. This expansion likely pressured cycads, once widespread and diverse, to adapt by forging symbiotic relationships with N2-fixing prokaryotes. Modern cycads, now rare, have survived this ecological transformation, suggesting that the ability to form N2-fixing symbioses played a crucial role in their endurance through Cenozoic times​​.
In conclusion, the study offers a mechanistic explanation for the survival of specific cycad lineages that acquired symbiotic N2 fixation. In contrast, lineages without this strategy perished, possibly due to competition with angiosperms and changing climatic conditions. This research sheds light on the evolutionary journey of cycads and highlights the significance of nitrogen fixation in the survival of plant species in an ever-changing world​​.
References:
- Kipp, M.A., Stüeken, E.E., Strömberg, C.A.E., et al. (2023). Nitrogen isotopes reveal independent origins of N2-fixing symbiosis in extant cycad lineages. Nature Ecology & Evolution. https://doi.org/10.1038/s41559-023-02206-6
