Abstract
Fossil evidence from the Rhynie chert indicates that early land plants, which evolved in a high-CO2 atmosphere during the Palaeozoic Era, hosted diverse fungal symbionts. It is hypothesized that the rise of early non-vascular land plants, and the later evolution of roots and vasculature, drove the long-term shift towards a high-oxygen, low CO2 climate that eventually permitted the evolution of mammals and, ultimately, humans. However, very little is known about the productivity of the early terrestrial biosphere, which depended on the acquisition of the limiting nutrient phosphorus via fungal symbiosis. Recent laboratory experiments have shown that plant–fungal symbiotic function is specific to fungal identity, with carbon-for-phosphorus exchange being either enhanced or suppressed under superambient CO2. By incorporating these experimental findings into a biogeochemical model, we show that the differences in these symbiotic nutrient acquisition strategies could greatly alter the plant-driven changes to climate, allowing drawdown of CO2 to glacial levels, and altering the nature of the rise of oxygen. We conclude that an accurate depiction of plant–fungal symbiotic systems, informed by high-CO2 experiments, is key to resolving the question of how the first terrestrial ecosystems altered our planet.This article is part of a discussion meeting issue ‘The Rhynie cherts: our earliest terrestrial ecosystem revisited’.
Highlights
The first plants to colonize the Earth’s land surface faced an entirely different climate to today, with atmospheric CO2 concentrations being greater than 1000 ppm [5]
While modern plants flourish under elevated CO2, access to mineral nutrients likely posed a problem for the early terrestrial biosphere—the earliest land-colonizing plant species lacked roots, being non-vascular and liverwort-like [8,9,10,11], and the substrate onto which they emerged was a skeletal mineral soil, largely lacking in organic matter [12]
It is likely that the nutrient acquisition strategies via fungal symbiosis had a significant effect on global primary productivity, and climate, but these aspects have not been explored in detail
Summary
The first plants to colonize the Earth’s land surface (during the Palaeozoic Era, 541 –250 Ma) faced an entirely different climate to today (figure 1), with atmospheric CO2 concentrations being greater than 1000 ppm [5]. Fossil beds at the Rhynie chert provide evidence that these early plants formed symbioses with fungi [13,14,15], which are likely to have facilitated mineral nutrient acquisition, in particular phosphorus [16,17]. Plants play a key role in the biogeochemical cycles of carbon, phosphorus and oxygen, and this role is amplified by symbiosis with fungi (figure 2). In exchange for photosynthetically fixed carbon, mycorrhizal fungi provide plants with mineral nutrients that would be otherwise inaccessible [17]. Plants and mycorrhizal fungi drive additional drawdown of atmospheric CO2 through their enhancement of silicate weathering [19], which
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