Trace Metals in the Ocean—The Link Between Rust and Life
Phytoplankton, tiny plants of the sea, are the life-sustaining force of our beautiful blue planet. They produce most of the oxygen we respire, devour massive amounts of CO2 like delicious candy, and feed ocean creatures, including those charismatic whales and furry marine mammals we love!
Just like any other living organism, phytoplankton need food to live, grow, and reproduce. Their diet consists mostly of carbon, nitrogen, and phosphorus. However, they also require metals like iron (Fe), zinc (Zn), and cadmium (Cd) to activate important cellular processes (such as photosynthesis). In seawater, these biologically-utilized metals are often present at extremely low concentrations, and photosynthesis is predicted to be limited by metal availability in approximately 40% of the ocean. So, in a way, the global carbon cycle, Earth’s climate, our precious oxygen, and that tuna sandwich you had at lunch depend on the availability of metals in seawater.
Hello, my name is Paulina Pinedo-Gonzalez, and I’m the trace metal chemist aboard the RV/IB Nathaniel B. Palmer. At this point, you may be intrigued by the relationship between life on Earth and trace metals in the ocean. Well, it is indeed a fascinating story that starts about 3.5 billion years ago in the primeval sea. There early evolution of primitive organisms selected the elements essential for life according to 2 basic principles: i) their abundance and ii) their suitability for a given task. According to these 2 principles, life evolved incorporating metals like Fe and Zn as the active centers in enzymes. Due to their high concentrations, they were abundant (or had high bioavailability) in the primeval sea.
The story gets more complicated and considerably more interesting when, about 2.3 billion years ago, cyanobacteria invented the art of making oxygen. Photosynthesis—by far life’s greatest invention—allowed the development of advanced life by creating the ozone layer, which shielded the earth from harmful UV radiation. It also allowed the advent of aerobic metabolism, which produces 16-18 times more energy per unit of sugar than anaerobic metabolism (metabolism without the presence of oxygen).
However, with time, photosynthesis became a double-edged sword. On the one hand, organisms invented a way to produce their own food out of water and thin air (isn’t it fantastic?!). But, on the other hand, the oxygen produced by photosynthesis drastically changed the biogeochemistry on Earth. For thousands of years after the initiation of photosynthesis, free oxygen was chemically captured by dissolved iron or organic matter through the process of oxidation. But after these sinks became saturated, the excess free oxygen started accumulating in the ocean and atmosphere. This produced major changes in the chemistry on Earth, with particular importance for a number of elements essential to life.
Oxygen caused the oxidation of elements such as sulfur, nickel, copper, zinc, molybdenum, and iron, impacting the availability of trace metals for biological processes. For example, soluble iron decreased several orders of magnitude (from about 100 parts per million to about 0.1 parts per billion) due to the precipitation of Fe oxides, making it difficult for phytoplankton to find and absorb this essential nutrient. These new environmental conditions—in particular, the changes in the bioavailability of essential elements in the ocean—put selective pressure on the evolution of life.
To this day, trace metals play a major role in controlling the growth of organisms, the production of oxygen, and the concentration of atmospheric CO2. Despite the enormous influence that trace metals have on processes that affect the whole earth as a system, our understanding of their sources, sinks, and internal cycling is very limited, in part because their study is analytically challenging.
In seawater, trace metals are typically present in concentrations of parts per billion (ppb) or below. One can begin to imagine a quantity so infinitesimally small by thinking of a chocolate milkshake made with a drop of chocolate syrup in one million liters (1,000,000 L) of milk. That one drop of syrup in an Olympic-sized swimming pool would be one part in a billion.
The main source of metals to the ocean is dust from the land. For example, in the Atlantic Ocean, the frequent sandstorms in Africa’s Sahara Desert are a source of trace metals. But the Southern Ocean is far from a landmass that delivers dust. Antarctica, while the most arid place on Earth, is covered in snow and ice. So, the concentration of trace metals in the Southern Ocean is particularly low.
Due to these extremely low metal concentrations in seawater relative to the ubiquitous presence of metals in other components of the environment—for example rust, dust, paint, and dirt—sampling and handling seawater samples is very challenging.
While on firm land, I work in a sophisticated metal-free lab that looks like Magneto’s plastic prison from the movie X-men. Onboard the Palmer, I work in a little “clean bubble” that looks like an improvised plastic fort built by children.
Paulina, queen of the clean bubble fort aboard the RV/IB N.B. Palmer
The roof of my clean “fort” is an air filter that constantly pumps clean air into the bubble, maintaining the space at positive pressure and keeping dirty air out. Inside, everything is covered in plastic, and I use a hairnet and clean plastic sleeves while handling my samples.
Paulina (at right) on deck with marine technicians, Joe and Garret, as her trace metal pump is retrieved after pumping.
With the help of a little air-operated pump and tens of meters of tubing, seawater is pumped from a depth of 30 meters directly into acid-washed containers sitting inside the bubble. This setup prevents seawater from being in contact with the dirty environment of this big metal ship, reducing the risk of metal contamination.
After sampling, all bottles are bagged and boxed to be analyzed back on land, where I use a range of analytical procedures to extract and concentrate trace metals. Using a technique called mass spectrometry, I measure the abundance of 25 different trace metals, as well as the ratios of various isotopes of Fe, Cd, and Zn.
The results from the samples collected during this research expedition will generate new evidence of the physical, biological, and chemical processes that influence the concentration and bioavailability of trace metals in the Southern Ocean, where trace metal distribution (in particular Fe) is considered to be the key factor controlling phytoplankton productivity. My results will also provide an important piece of the puzzle in understanding the sources and sinks for trace metals in the modern Southern Ocean.
We know the climate is changing, so we need to understand the processes that control trace metals in the Southern Ocean today in order to predict how the effects of climate change—such as the loss of sea ice and continental ice cover—might affect the concentration and bioavailability of trace metals in the future, thereby impacting photosynthesis and further impacting Earth’s climate.