Iron is one of the nutrients in seawater necessary for phytoplankton growth. Unlike other nutrients, iron is present at extremely low concentrations. This makes things difficult for both life in the ocean and the scientists trying to study it. Life in the ocean has developed strategies for acquiring iron just as trace metal oceanographers have for sampling it. Ironically, the difficulty for scientists is compounded by the fact that we sail on an iron-clad research vessel. This increases the likelihood of contaminating the samples. Trace-metal chemists have circumvented this issue by constructing a specially-designed clean room aboard the vessel, affectionately called the bubble, and collecting samples with metal-free bottles. With clean samples, we hope to better understand the iron cycling in a West Antarctic Peninsula fjord and how the abundant life in this region is sustained.
Iron is the fourth most abundant element in the Earth’s crust, but is only present in trace concentrations (parts per billion) in the ocean. The reason why there is scarce iron in seawater is due to its low solubility in oxygenated water. Iron readily forms minerals when combined with oxygen and these minerals sink to the ocean floor where they are unavailable for life at the surface. This paradigm limits primary production in vast regions of the global ocean. Without organisms at the base of the food chain, larger fauna cannot be supported. This is not the case here in Andvord Bay, where plentiful krill and whales aggregate during the productive season.
In Andvord Bay, there are multiple sources of iron that are typical of a polar coastal environment. These include seafloor sediments, glacial meltwater and sediment plumes originating at the interface between the ice and bedrock, and iron advected by incoming water masses. In an effort to understand how life is sustained by these sources, we need to understand their reactivity and how long they remain available for biological use. This includes both their chemical reactivity and processing by bacterial activity. Here in Antarctica, we collect seawater, snow and ice to measure a suite of complimentary variables, including iron concentrations and form, and surveys of bacterial abundance and metabolism.
In addition to measuring seawater samples directly, we also set up a series of experiments to simulate different environmental conditions. Over the course of these experiments, we monitor phytoplankton and bacterial growth to capture a key step in the biogeochemical cycle of iron, remineralization. Remineralized iron can contribute to the pool of nutrients sustaining life in this harsh environment.
In order to do all of this, we rely on a lot of plastic. A fabricated plastic bubble ensures a clean work space, free from metal contaminants. All of our bottles and sampling equipment were carefully acid-cleaned far in advance of this cruise. We will send our samples back to the Scripps Institution of Oceanography, where we make all our measurements using specialized instrumentation (voltammetry, chemiluminescence, mass spectrometry, epifluorescence microscopy, and next generation sequencing) in our clean analytical labs. Once we have our data, we can start putting together a picture of iron cycling in Andvord Bay. Even though it is a lot of work, we’re motivated by the unique environment and majestic coastline of this Antarctic fjord and our passion for understanding and preserving this climate sensitive seascape.
Written by Lauren Manck and Kiefer Forsch.