Getting a sinking feeling yet?

There has been a recent spate of papers regarding the phytoplankton and carbon sequestration. I take a particular interest in this subject, as my undergraduate thesis looked at the effects of iron limitation on oxidative stress responses in diatoms. In this post, I'll attempt to synthesize the findings of these different papers to find the "take-home message", as they say.

First up is Le Quéré et al (1). They used data from atmospheric monitoring stations in the southern hemisphere tracking CO2 concentrations from 1981-2004 and an ocean general circulation model (OGCM) called PISCES-T to model the flux of carbon dioxide into/out of the ocean from/to the atmosphere. The details are complicated, but basically the message here is that the OGCM shows a flattening in the trend of the carbon dioxide flux from the atmosphere to the ocean (the blue line), which is different from the trend that you would predict based on atmospheric carbon dioxide concentrations alone (the red lines).

One of the little-known (at least by the lay public) facts about the oceans is that vast areas are virtual biological deserts due to the lack of nutrients. Unlike coastal areas, however, the limiting nutrient in most of the open oceans is actually iron. Hence, dust storms that carry sand from the Sahara actually end up fertilizing plankton growth in the Atlantic Ocean through iron deposition. Cassar et al (2) contend that a similar process of fertilization results in enhanced phytoplankton productivity in the Southern Ocean.


However, complicating the issue, Sunda 2010 (3) and Shi et al (4), discuss the effect of increasing ocean acidification on biological availability of iron.

In a nutshell, even though increasing ocean acidity (decreasing pH) due to increasing atmospheric carbon dioxide concentrations increases the solubility of iron, the biological availability of this iron to phytoplankton actually decreases. Keep in mind that this represents a double-whammy for calcifying phytoplankton, because increased CO2  concentrations also impair the ability of these organisms to grow and sequester CO2 (5).

As an aside, I'll note that one of the things I was looking at as an undergrad was whether iron limitation also inhibited phytoplankton's ability to handle oxidative stress (i.e., damage from ultraviolet radiation and/or high irradiance); iron is a crucial component of the ascorbate peroxidase enzyme, which reduces damaging hydrogen peroxide to harmless water. Thus, it is theoretically possible that the effects of increased UV radiation due to the Antarctic ozone hole could be exacerbated by iron limitation in phytoplankton by compromising their ability to manufacture heme-containing enzymes necessary to mitigate oxidative stress effects.

On the other hand, Peck et al (6) argue that melting ice sheets are exposing more open ocean and thus creating new carbon sinks - about 3.3 million tonnes' worth so far. They speculate that a further 50 million tonnes of carbon could be fixed annually if another 15% of the remaining Antarctic ice is lost.

So where does this leave us? First, that ocean acidification is the 500-pound gorilla in the room that few (if any) in the mainstream media are talking about. Second, that the effects of acidification may be even worse than we first thought, if it results in a reduction in biological availability of iron to phytoplankton. This is further complicated by uncertainty associated with the effects of climate change on desertification and wind patterns. And third, that the magnitude and - to a lesser extent - the direction of ocean-atmosphere carbon cycle feedbacks remains uncertain.

1. Le Quere, C., Rodenbeck, C., Buitenhuis, E., Conway, T., Langenfelds, R., Gomez, A., Labuschagne, C., Ramonet, M., Nakazawa, T., Metzl, N., Gillett, N., & Heimann, M. (2007). Saturation of the Southern Ocean CO2 Sink Due to Recent Climate Change Science, 316 (5832), 1735-1738 DOI: 10.1126/science.1136188

2. Cassar, N., Bender, M., Barnett, B., Fan, S., Moxim, W., Levy, H., & Tilbrook, B. (2007). The Southern Ocean Biological Response to Aeolian Iron Deposition Science, 317 (5841), 1067-1070 DOI: 10.1126/science.1144602

3. Sunda WG (2010). Iron and the carbon pump. Science (New York, N.Y.), 327 (5966), 654-5 PMID: 20133563


4. Dalin Shi, Yan Xu, Brian M. Hopkinson, François M. M. Morel (2010). Effect of Ocean Acidification on Iron Availability to Marine Phytoplankton Science, 327 (5966), 676-679 : 10.1126/science.1183517

5. Riebesell U, Zondervan I, Rost B, Tortell PD, Zeebe RE, & Morel FM (2000). Reduced calcification of marine plankton in response to increased atmospheric CO2. Nature, 407 (6802), 364-7 PMID: 11014189

6. Peck, L., Barnes, D., Cook, A., Fleming, A., & Clarke, A. (2009). Negative feedback in the cold: ice retreat produces new carbon sinks in Antarctica Global Change Biology DOI: 10.1111/j.1365-2486.2009.02071.x