Earth’s climate has undergone a number of major shifts throughout it’s 4 billion year history, yet it is unclear the degree to which these changes were due to the evolution and spread of plants in symbiosis with microorganisms. This PhD project will use a combination of lab-based experimentation, targeted field sampling and mathematical models to provide a first evaluation of the role of plant symbioses in global biogeochemical cycles and climate throughout earth’s history and their potential impacts on future climate change.
One of the most significant climate shifts in Earth’s history was the substantial increase in oxygen to ‘breathable’ levels around 420-400 million years ago (Ma). Coincident with this2¬, atmospheric CO2 concentrations underwent a dramatic decline, falling >90% from 410–340 Ma. This decline is thought to have been driven, in large part, by geochemical weathering processes and by the increasing plant demand for CO2 for photosynthesis, and appears to have lowered global surface temperatures sufficiently to cause a multimillion year ‘ice-age’ during the Permo-Carboniferous. During the Palaeozoic, the terrestrial flora underwent rapid expansion and diversification, with plants evolving in structural complexity and stature. The co-evolution of plant rooting systems and symbiotic microbes are likely to have played a crucial role in providing the expanding biomass of terrestrial ecosystems with soil nutrients. This would have become particularly critical as phosphours (P) became increasingly limited due to depletion of mineral P sources through biological weathering.
Differences in microbial symbiont CO2 responsiveness in provisioning plant hosts with N and P suggests that plant symbiont identity may have played a greater role in climate modulation that previously thought. Currently, our knowledge of the drivers of change in Earth’s past climates are based on dynamic global vegetation and climate models. These have previously been developed and parameterised using data from a variety of sources, including extant land plant responses to simulated past climates. However, none of the existing models take into account the effect of the co-evolution of plants with a variety of microbial symbionts and the critical role they are likely to have played in nutrient cycling and biological weathering throughout the evolution of the terrestrial biosphere. This project will address this fundamental knowledge gap using a multi-scale approach, using lab experiments, field studies and the development of improved vegetation and climate models to test the following key questions:
→ How did land plants respond to global cooling and CO2 changes in terms of symbiotic function and palaeoecology? What was the resulting feedback on global climate?
→ To what degree was diversification of land plants responsible for the increases in atmospheric O2 to current, breathable levels?
→ What can we learn from plant evolution and their feedbacks with global climate to inform future climate change mitigation strategies?
Under the supervision of Dr Katie Field, Dr Sarah Batterman and Dr Benjamin Mills, carbon and nutrient fluxes in plants representative of key clades in the land plant phylogeny will be studied. The species selected will encompass changes in plant structural complexity, nutrient acquisition strategy and choice of symbiotic partner(s). Carbon and nutrient fluxes will be examined using microcosm and isotope tracer-based approaches in both the lab and field, with potential field sites in Panama and New Zealand. Together, these data will allow development of dynamic global vegetation models (DGVMs) to include nutrient cycling and carbon-for-nutrient exchange. Such models can then be linked with existing earth system models (ESMs) to explore the relationships and feedbacks between terrestrial biosphere evolution and the Earth’s climate
Batterman, S.A., Hedin, L.O., van Breugel, M., Ransijn, J., Craven, D.J., Hall, J.S. (2013a) Key role of symbiotic N2 fixation in tropical forest secondary succession. Nature 502: 224-227
Batterman, S.A., Wurzburger, N., Hedin, L.O. (2013b) Nitrogen and phosphorus interact to control tropical symbiotic N2 fixation: A test in Inga punctata. Journal of Ecology 101: 1400-1408
Field KJ, Beerling DJ, Bidartondo MI, Rimington WR, Allinson KE, Cameron DD, Duckett JG, Leake JR, Pressel S. (2016) Functional analysis of liverworts in dual symbiosis with Glomeromycota and Mucoromycotina fungi under a simulated Palaeozoic CO2 decline. The ISME Journal. 10: 1514-1526
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Field KJ, Rimington, WR, Bidartondo, MI, Allinson, KE, Beerling, DJ, Cameron, DD, Duckett, JG, Leake, JR & Pressel, S. (2015) First evidence of CO2 responsive mutualisms between ancient lineages of plants and fungi of the Mucoromycotina. New Phytologist 205(2): 743-756
Field KJ, Cameron DD, Leake, JR, Tille, S and Beerling, DJ. (2012) Contrasting arbuscular mycorrhizal responses of vascular and non-vascular plants to a simulated Palaeozoic CO2 decline. Nature Communications 3:835 doi: 10.1038/ncomms1831
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Lenton, T. M., Dahl, T. W., Daines, S. J., Mills, B. J. W., Ozaki, K., Saltzman, M. R. & Porada, P. (2016) Earliest land plants created modern levels of atmospheric oxygen. Proceedings of the National Academy of Sciences of the United States of America 113: 9704-9709
Mills, B. J. W., Belcher, C. M., Lenton, T. M. & Newton, R. J. (accepted) A modelling case for high atmospheric oxygen concentrations during the Mesozoic and Cenozoic. Geology.
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