System-level effects of CO2 and RuBisCO concentration on carbon isotope fractionation
Carbon isotope biosignatures preserved in the Precambrian geologic record are primarily interpreted to reflect ancient cyanobacterial carbon fixation catalyzed by Form I RuBisCO enzymes.
The average range of isotopic biosignatures generally follows that produced by extant cyanobacteria. However, this observation is difficult to reconcile with several environmental (e.g., temperature, pH, and CO2 concentrations) and physiological factors that likely would have differed during the Precambrian and can produce fractionation variability in contemporary organisms that meets or exceeds that observed in the geologic record.
To test a range of genetic and environmental factors that may have impacted ancient carbon isotope biosignatures, we engineered a mutant strain of the model cyanobacterium Synechococcus elongatus PCC 7942 that overexpresses RuBisCO and characterized the resultant physiological and isotope fractionation effects. We specifically investigated how both increased atmospheric CO2 concentrations and RuBisCO regulation influence cell growth, oxygen evolution rate, and carbon isotope discrimination in cyanobacteria.
We found that >2% CO2 increases the growth rate of wild-type and mutant strains, and that the pool of active RuBisCO enzyme increases with increased expression. At elevated CO2, carbon isotope discrimination (ϵp) is increased by ~8 per mille, whereas RuBisCO overexpression does not significantly affect isotopic discrimination at all tested CO2 concentrations. Our results show that understanding the environmental factors that impact RuBisCO regulation, physiology, and evolution is crucial for reconciling microbially driven carbon isotope fractionation with the geologic record of organic and inorganic carbon isotope signatures.
Amanda K. Garcia, Mateusz Kedzior, Arnaud Taton, Meng Li, Jodi Young, Betul Kacar
doi: https://doi.org/10.1101/2021.04.20.440233
