(AciDification effects of AGgregation In the Ocean)

ADAGIO is a joint U.S.-German study of the effects of ocean acidification on particle aggregation. The project is funded by the National Science Foundation Chemical Oceanography Program. It is directed by Cindy Lee of the School of Marine and Atmospheric Science (SoMAS) at Stony Brook University in Stony Brook, New York, and Anja Engel of the Helmholtz-Zentrum für Ozeanforschung Kiel (GEOMAR), Germany, and SoMAS. Two experiments make up the bulk of ADAGIO work:

2010 Chemostat Experiment 2011 Mesocosm Experiment

Large-scale changes in surface ocean chemical equilibrium and elemental cycling are occurring due to ocean acidification. The ocean is the largest global sink for anthropogenic CO2, and as the CO2 inventory of the surface ocean increases due to enhanced uptake from the atmosphere, the pH of seawater decreases. This relative acidification of the surface ocean is expected to intensify in the future. By the year 2300 a further decrease in surface ocean pH of 0.7 units is expected, which will exceed anything experienced in the past 300,000 yr of earth history. CO2-related changes in the carbonate chemistry of the ocean affect biological processes like photosynthesis and calcification.

Cells of the coccolithophore Emiliania huxleyi. Fully calcified cells grown at present day CO2 (a), and cells showing reduced and absent calcification at 1400 ppm CO2 (b). Aggregates formed from calcifying E. huxleyi (c) differ in size and settling velocity from those of non-calcifying cells (d). Pictures: A. Engel; Engel et al. (2009a)

Calcifying phytoplankton species are thus likely to be particularly sensitive to changes in CO2 concentration. Biominerals (especially CaCO3) play an important role in exporting organic matter (OM) to the deep ocean.  Minerals in association with OM act as ballast, thereby increasing OM sinking rate and hence the depth to which OM is exported. Biominerals affect the rate of formation and properties of aggregates and can delay their microbial degradation. Aggregates play a pivotal role in organic matter cycling and export, as they mediate vertical mass fluxes and elemental cycling in the ocean.  Aggregation processes in the ocean range over a wide size spectrum, cascading from the nano scale up to the size of fast settling marine snow, primarily by the process of gel particle formation from organic polymers. In general, the reactivity and rigidity of a gel depends on its molecular composition and on the type of binding and interactions between the individual polymer chains. Gel particle formation is likely to become affected by ocean acidification due to changes in the reactivity and production rate of organic polymers. The ADAGIO project will investigate some of the mechanisms of aggregation of marine particles that specifically relate to organic matter-ballast mineral interactions and their sensitivity to ocean acidification. Our intention is to understand the mechanisms but also consider the properties of aggregation that might most result in alterations of sedimentation and export in the future ocean. Three hypotheses will be tested:

  1. Gel particles enhance aggregate formation. The production of gel particles is sensitive to ocean acidification.
  2. Biomineral ballast (particularly calcium carbonate) is an important factor in carbon export in the ocean. Lower pH will decrease ballast-particle interactions and consequently export.
  3. In addition to carbohydrates, gel particles also include proteins and lipids that will affect rate and mechanisms of organic matter aggregation in the ocean. Changes in the composition of gel particles in seawater are expected at different pH levels.

Schematic view of the potential effects of future seawater CO2 concentrations on the formation of TEP and subsequent enhancement of export fluxes by increased aggregate formation. Figure: Arrigo (2007).