Marine phytoplankton in a high CO2 world

  • Katharine Crawfurd

    Student thesis: Doctoral ThesisDoctor of Philosophy


    Marine phytoplankton is responsible for ~50% of global primary productivity, it supports the oceanic food web and affects biogeochemical cycles. I participated in a large mesocosm experiment that observed altered community structure and carbon drawdown in response to increased CO2. There was a 27% reduction in community primary production at the peak of an Emiliania huxleyi-dominated bloom in mesocosms initially at 760 ppm CO2 compared to present day pCO2. There were changes in community structure but not dominance; Synechococcus and large pico-eukaryote abundances were reduced by ~60%, E. huxleyi was reduced by ~50%. A number of E. huxleyi strains persisted throughout the experiment in both treatments and no malformation or significant change in lith size occurred at increased CO2. In a second field experiment in the oligotrophic ocean off the Canary Islands, 760 ppm pCO2 did not change community structure or cell division rates of Synechococcus, Prochlorococcus or pico-eukaryotes.In laboratory experiments, I maintained the diatom, Thalassiosira pseudonana CCMP1335 at 760 ppm and present day pCO2 for ~100 generations in gas equilibrated continuous cultures – one of the longest experiments that has been attempted to investigate the effect of increased CO2 on marine phytoplankton. No clear evidence of adaptation or acclimation to increased CO2 was found, neither were there consistent changes in transcription of RuBisCO or carbonic anhydrase genes. Non-calcified E. huxleyi CCMP1516 and calcified CCMP371 grown in gas equilibrated semi-continuous cultures for several weeks showed no change in cell division rate at 760 ppm CO2. An understanding of the underlying changes in communities is required for modelling responses to increasing CO2, molecular tools may prove useful for this task. The strong community response in the mesocosms shows that rising atmospheric CO2 can greatly affect phytoplankton productivity and biogeochemical cycling.
    Date of Award2010
    Original languageEnglish
    SponsorsNatural Environment Research Council
    SupervisorJohn Raven (Supervisor) & Ian R. Joint (Supervisor)


    • Phytoplankton
    • Ocean acidification
    • Global change

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