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Free-drifting icebergs as proliferating dispersion sites of iron
enrichment, organic carbon production and export in
the Southern Ocean


Iron limitation

One of the central questions we’re asking in this project is: Are plankton Fe-limited, and if so, is this limitation relieved in iceberg-impacted waters?

To begin to answer this, here are some of the data we need to piece together from our combined disciplines:

In both environmental sampling and in controlled Seawater Enrichment Experiments (SEEx) we will look at the relationships between microbe and phytoplankton community composition (who's there), abundance (how many are there), community activity (who's translating RNA into protein), nutrients (what's available for food), and primary production (how much carbon the phytoplankton produce). We'll quantify the concentration of iron Fe(II) and Fe(III) in particulate (> 0.4-um), dissolved (<0.02-um), and colloidal (>0.02-um and <0.4-um) forms, as well as iron-binding ligands, We'll collect both near-field (<2-km from the iceberg) and far-field (> 4-km from the iceberg) samples, to compare the communities and activity within and outside the Iceberg Zone of Influence (IZI).

Current track plot

The seawater enrichments aim to specifically test iron limitation in a controlled environment. If Iron is limited, the community should respond to iron addition. It's a direct approach – add the "missing" ingredient. We are planning to run these experiments in waters that are both iron limited in the central Weddell Sea and those that should not be iron limited, in the western Weddell Sea (where we are starting off this expedition). We have a number of additions which will be carried out in both light and dark conditions, and plan to run the experiments for enough time to see an experimental effect as measured by growth of bacteria or phytoplankton (~6-12 days). We're interested in detecting the difference between control treatments with no additions to those with different additions of iron and other substrates. The sampling schedule looks like it'll be pretty intense, as we're planning to start the nearfield SEEx tomorrow, and the farfield SEEx the following day – then both experiments will be running in parallel (32 different 4-8 liter bottles) and will be sampled every other day following the schedule below.

sample schedule

How Might Iron Limitation be Alleviated by the Iceberg?

What we do know: The Southern Ocean is considered to be the largest body of iron-limited water on the planet (known as a high nutrient, low chlorophyll, HNLC). Though not all of the Southern Ocean is truly iron-limited – the majority of it is, especially the main current that circumnavigates the continent – the Antarctic circumpolar current. Here, a number of research cruises have conducted iron enrichment experiments, to test the hypothesis that if iron is added to the surface ocean, it will stimulate carbon dioxide draw down, resulting in export to the deep sea – which potentially can mediate the effects of carbon dioxide-induced climate change.

It turns out that iron, which is an essential micronutrient (toxic in high amounts, but essential in small amounts), is often a primary element which controls phytoplankton growth, and the phytoplankton often need more iron than is dissolved and bio-available in the ocean. Requirements for bacterioplankton are less well understood. We also know that glaciers, airborne dust, and river sediments contribute vast quantities of iron into the oceans in particulate or colloidal form, but these sources of iron (a) may not be in a form readily available for uptake by microbes, and (b) wind up primarily in deep ocean waters and sediment. One of the main findings of the first iceberg cruise was that biological activity was stimulated in the zone of influence surrounding the icebergs. As mentioned above, one of the goals of the current project is to test the hypothesis that one of the reasons for high productivity near the icebergs is that they are a natural source of iron fertilization.

Icebergs could provide another mobile "sink" of particulate and colloidal iron. Do iron-binding ligands produced by microbes in the water column help make colloidal and/or particulate iron from iceberg melting and ablation, available to bacterioplantkon and phytoplankton, and thus relieve iron limitation? Our microbial community data and iron-binding ligand analyses may shed some light on these questions.

Vivian cleaning bottle

Fe-binding Ligands: Another interesting source of Iron Relief for a Potentially Starved Community

As we've discussed, phytoplankton need iron, especially soluble Fe(III). Iron moves between these two species via a series of reactions called oxidation-reduction, or red-ox, in which the electron that differentiates Fe(II) from Fe(III) moves between the two, depending upon the conditions. For example, Fe(II) oxidizes to Fe(III) more slowly in lower pH waters, yet this oxidation occurs rapidly in highly oxygenated water, such as the shallow mixed region of the ocean. Bacteria are capable of producing iron scavengers that "seek" iron to fulfill the cell's needs. Some are bound to the cell's surface acting as an iron conduit between the seawater and the cell. Other bacteria produce siderophores, which are Fe-binding ligands that dissociate completely from the cell to retrieve iron. Based on DNA sequence analysis from bacterial genomes and Dr. Alison Murray's Antarctic metagenome sequence, we can identify microbes that possess the capability to produce Fe-binding ligands. The Iron Men will quantify the presence and type of Fe-binding ligands.

The principle investigators in this collaboration have assembled a diverse toolbox, but until the data are in, here are some ideas to ponder...

If we provide additional carbon to a thriving community of phytoplankton (which produce carbon), and a community of bacteria (who need carbon) not yet at their population peak, what would you expect to see? Do you think there would be more or less of one community versus another? Would there be any change at all?


Project Summary

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