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Genetic Record of Atmospheric Carbon dioxidE (GRACE)

Final Report Summary - GRACE (Genetic Record of Atmospheric Carbon dioxidE (GRACE))

The reconstruction of past atmospheric composition of carbon dioxide is notoriously challenging yet profoundly important for understanding the response of Earth to climatic change. Within GRACE we have opened many new windows into the past atmosphere based on signals preserved within biota ranging from the enzyme character of modern day algae, and the footprints of the working of those enzymes on the isotopic record contained in siliceous and calcareous microfossils.
We have been able to peer into ancient atmospheres by looking at the evolution of algal Rubisco, the most abundant enzyme on Earth responsible for all marine photosynthetic carbon fixation, which underwent adaptation during ancient and distinct geological periods. We have defined positive selection within the large subunit of Rubisco to occur basal to the radiation of modern marine groups. This signal appears to be responding to changing intracellular CO2 triggered by physiological adaptation to declining atmospheric CO2 as it coincides with periods of falling Phanerozoic and Proterozoic CO2, and parallels the emergence of CCM components such as carbonic anhydrase and the pyrenoid.
Within GRACE we have found the exact converse to the prevailing view that Rubisco evolves to improve its ability to distinguish between the competitive substrates CO2/O2. Instead, from the largest dataset to date on the kinetics of Rubisco within diatoms and haptophyte algae, we have shown that improved CCMs have allowed Rubisco to relax to lower specificities, but faster turn over rates. The broad range of enzyme affinities for carbon from the diatoms mostly exceed those of C4 plant Rubisco, suggesting the carbon concentrating mechanism (CCM) efficiency in diatoms is more diverse, and more effective than previously predicted. Uniquely a negative relationship between carbon affinity and cellular Rubisco content has been found suggesting variation among diatom species in how they allocate their limiting cellular resources between Rubisco biogenesis and CCM efficiency. Furthermore, the unique variance found across the kinetic properties of these algal Rubiscos plays out in the size of the carbon isotopic fractionation associated with the Rubisco enzymes which revolutionises our understanding of why organic matter has become more isotopically heavy over the last 60 million years.
We have discovered that the adaptive history of Rubisco is informative on how carbon handling has evolved but does not provide significant resolution of the more recent geological record. So within GRACE we explored novel approaches to constraining the past based on biological signals within isotopes. We have used our emerging physiological knowledge and modelling to unpick the drivers of isotopic variance between different species of coccolithophores and diatoms and been able to obtain a barometer of CO2 over the more recent past, notably showing a distinct decline across the initiation of the Antarctic ice sheet some 33.5 million years ago. We have also developed novel tools for pulling ancient physiologically relevant molecules out of fossils and shown how the isotopes and biochemistry of these coccolith associated polysaccharides track adaptation of the cells and calcification to the changing carbon environment.