The Biological Effects of Ocean Acidification

2008-13 GC Seed Grant

The Morel group is conducting field and laboratory research to assess the impact of increasing atmospheric CO2 and the resulting ocean acidification on the marine biota. The focus is on marine phytoplankton, the photosynthetic organisms responsible for nearly half of primary production on Earth. The ongoing rise in atmospheric CO2 is increasing the dissolved CO2 concentration in the surface ocean, decreasing its pH, and consequently modifying the chemistry of many seawater constituents. These chemical changes in turn affect the ocean biota via a multitude of mechanisms. For example, the decrease in the degree of saturation of calcium carbonate makes it harder for calcifying organisms to precipitate their mineral structures. The decrease in pH changes the bioavailability of essential algal nutrients, including trace metals such as zinc and iron. Most directly, the increase in CO2 decreases the energy necessary for photosynthetic organisms to synthesize biomass and affects its elemental composition, particularly its C:N ratio. Such biological effects potentially lead to major perturbations in marine biogeochemical cycles including notably the biological export of CO2 to the abyss and may provide a key feedback, negative or positive, on the CO2 buildup in the atmosphere and surface ocean.

Electron microscope image of Emiliania huxleyi, the most abundant coccolithophores in the ocean. Phytoplankton, such as the one pictured here, live in the sunlit portion of the world’s oceans and are responsible for nearly half of primary production on Earth. Primary production is organic matter mostly produced via photosynthesis either in the marine or terrestrial biosphere. Almost all life on Earth is directly or indirectly reliant on primary production. (Photo: Yan Xu)

The project has demonstrated that decreasing seawater pH results in a lower bioavailability of essential trace metals such as zinc and iron to phytoplankton. This is an important result as phytoplankton are known to be limited by iron in large regions of the oceans and are likely also affected by the bioavailability of other metals, including zinc.  In addition a lower pH also decreases the efficiency of nitrogen fixation in the model marine cyanobacterium Trichodesmium, the dominant N2 fixer in the oceans. Because N2 fixation requires large quantities of iron, the effects on Fe availability and N2 fixation should amplify each other.To relate photosynthetic activity to the ambient CO2 concentration, researchers in the Morel group have also quantified the fluxes and concentrations of CO2 in sub-cellular compartments in marine diatoms. The results show that a doubling of ambient CO2 concentration decreases the energy expended on carbon fixation by about 5% with a corresponding increase in growth rate.  Such a beneficial effect partially mitigates the effects on Fe availability and N2 fixation mentioned above.

This research project is based on a series of mechanistic hypotheses on the biological effects of ocean acidification and comprises laboratory experiments as well as oceanographic field studies.

Educational Impacts

A central goal of the Grand Challenge program is to integrate environmental research into the undergraduate curriculum. We have endeavored to do so through introduction of the topic of ocean acidification in formal courses and involvement of undergraduate students in our research program. Three Formal courses have been directly impacted by this Grand Challenge project:

  • The Freshman Seminar FRS 122 “Global change and the impact of human activities on the biosphere: the Everglades today and tomorrow.” This seminar includes a three-week laboratory module on the effect of acidification on the dissolution of the calcite liths of coccolithophores. In addition to a formal introduction to the topic in lectures, this module includes hands-on activities spread over three weeks: 1) growing cultures of coccolithophores; 2) harvesting the cells and exposing them to seawater at various pH; and examining the effect of acidity on the the liths by scanning electron microscopy.
  • The junior course “Oil to Ozone: Chemistry of the Environment” (CHM 333/ENV 333) is being revamped and now includes one full lectures on the topic of ocean acidification, in addition to discussion of various connected topics in other lectures.
  • The senior class “Environmental Aqueous Geochemistry” (GEO 418/CHM 418) which deals in depth with the question of the chemistry of natural waters, including their acid-base balance, provides an ideal conduit to examine the question of ocean acidification. The topic was thus discussed in class on many occasions and the final examination dealt specifically with its principal quantitative aspects.

Other Outcomes

The project has resulted in the following publications to date:

D. Shi, Y. Xu, F. M. M. Morel. Effects of the pH/pCO2 control method on medium chemistry and phytoplankton growth. Biogeosciences 6: 1199-1207 (2009) doi:10.5194/bg-6-1199-2009

Shi D.,Y. Xu, B. M. Hopkinson and F. M. M. Morel. Effect of ocean acidification on iron availability to marine phytoplankton. Science 327: 676-679 (2010) doi:10.1126/science.1183517

Egleston, E.S., C. L. Sabine and F. M. M. Morel. Revelle Revisited: Buffer factors that quantify the response of ocean chemistry to changes in DIC and alkalinity.Global Biogeochemical Cycles 24, GB1002, (2010) doi: 10.1029/2008GB003407

Xu, Y., J. M. Boucher and F. M. M., Morel. Expression and diversity of alkaline phosphatase EHAP1 in Emiliania huxleyi (Prymnesiophyceae). Journal of Phycology 46: 85-92 (2010) doi: 10.1111/j.1529-8817.2009.00788.x

Hopkinson, B.M., Y. Xu, D. Shi, P.J. McGinn, and F.M.M. Morel. The effect of CO2on the photosynthetic physiology of phytoplankton in the Gulf of Alaska. Limnology and Oceanography 55: 2011-2024 (2010) doi: 10.4319/lo.2010.55.5.2011

Hopkinson, B.M., C.L. Dupont, A.E. Allen, and F.M.M. Morel. Efficiency of the CO2-concentrating mechanism of diatoms. PNAS 108: 3830-3837 (2011) doi:10.1073/pnas.1018062108

Xu, Y., D. Shi, L. Aristilde and F. M. M. Morel. The effect of pH on the uptake of zinc and cadmium in marine phytoplankton: Possible role of weak complexes. Limnology and Oceanography 57: 293-304 (2012)

Losh, J.L., F. M. M. Morel and B.M. Hopkinson, Modest Increase in the C:N Ratio of N-limited Phytoplankton in the California Current in Response to High CO2, Marine Ecology Progress Series, 468: 31-42   DOI: 10.3354/meps09981 (2012)

Shi, D., S. A. Kranz, J.-M. Kim and F. M. M. Morel, Ocean acidification slows nitrogen fixation and growth in the dominant diazotroph Trichodesmium under low-iron conditions, PNAS, 109 (45) 18255-18256; E3094-E3100   DOI: 10.1073/pnas.1216012109 (2012)

Lomas M., B.M. Hopkinson, J. Losh, D. Ryan, D. Shi, Y. Xu, and F.M.M. Morel. Effect of ocean acidification on cyanobacteria in the subtropical North Atlantic.Aquatic Microbial Ecology. 66: 211-222. (2012)

Aristilde, L., Y. Xu, and F.M.M.  Morel. Weak Organic Ligands Enhance Zinc Uptake in Marine Phytoplankton. Environmental Science & Technology. 46: 5438-5445(2012)

Losh, J. L ., J.N. Young and F.M.M. Morel. Rubisco is a small fraction of total protein in marine phytoplankton. New Phytologist. 198:52-58 (2013)

Morel F. M. M. The bioavailability of trace metals and its modification by microbes. Crystal Ball feature. Environ. Microbiology Reports 5(1) 10-11 (2013)

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Albert G. Blanke, Jr. Professor, Geosciences