The Biological Effects of Ocean Acidification
2008 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. Phytoplankton are responsible for nearly half of primary production on Earth and are known to be limited by iron in large regions of the oceans. 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 may change its composition e.g., its carbon to nitrogen (C:N) ratio as shown in preliminary experiments. 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, cadmium, and iron to phytoplankton. This surprising result is explained by the bioavailability of weak organic complexes of the metals whose concentration decreases at low pH. 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.
A focus of continuing work is the effect of ocean acidification on nitrogen fixation by the marine cyanobacterium Trichodesmium, the dominant N2 fixer in the oceans. Because N2 fixation requires large quantities of iron, its response to ocean acidification is made complicated by the dual (and likely opposite) effects of low pH on iron availability and high CO2 on photosynthesis.
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 Challenges program is to integrate environmental research into the undergraduate curriculum. Undergraduate students working in the Morel lab have investigated the response of specific phytoplankton groups to ocean acidification and effects on the nitrogen and carbon cycles in the open ocean. In addition, the topic of ocean acidification has been introduced in the following three courses:
- The Freshman Seminar FRS 122 “Global change and the impact of human activities on the biosphere: the Everglades today and tomorrow” 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; 3) examining the effect of acidity on 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 lecture 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.
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 CO2 on 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)
Participating Departments
- Chemical and Biological Engineering
- Chemistry
- Ecology and Evolutionary Biology
- Geosciences
- Princeton Environmental Institute
