The Diatom Wrangler
Hi there! My name is Colin Jones (aka The Diatom Wrangler), and I’m writing here today to tell you about what I’m trying to accomplish aboard the RV/IB Nathaniel B. Palmer. First, perhaps you’d like to hear a little bit about why I love my job? If so, dear reader, read on! I grew up in Washington State, whose landscape bears the indelible marks of glaciers charging past and retreating, carving islands in the sky (which I promptly hiked) and islands on the water (on whose shores I swam and played). One formative memory sticks out to me: while on one of those islands at night, I encountered bioluminescence. Think of millions of tiny creatures glowing as the moonlit waves crash to shore! This really impressed on me the wonder of the microscopic world. Years later, I’m in awe of the power of the microscopic world—to think that diatoms in all their multitudes have played a significant role in global climate for millions of years is incredible, and I get to study them. In this way, I’ve been able to merge two ideas that have captured my attention for most of my life: how the world changes and the microscopic creatures that do the heavy lifting.
Twenty thousand years ago, Earth was very different than it is today—glaciers covered a large portion of North America, sea level was more than 100m lower, and there was about 30% less CO2 in the air. Scientists call this period the last glacial maximum, or LGM. Our lab works with sediment (seafloor mud) from today to the LGM (and even older!) that is filled with diatoms. We measure the sediment and the diatoms to find out what the causes and mechanisms were for these large changes in Earth’s environment.
One familiar way that Earth’s temperature can be changed is through CO2. Marine organisms play a large role in regulating CO2 in the atmosphere: when they grow, phytoplankton take CO2 from the atmosphere and incorporate it into their growing cells. As they die and fall to the seafloor as marine snow, they are removing the carbon that they’ve taken up from the atmosphere, sometimes all the way to the seafloor! This is called the biological pump. Diatoms are among the most abundant phytoplankton (especially in the Southern Ocean!), with an estimate of about half of the ocean’s total productivity.
Diatomaceous mud from Station 3
In the Southern Ocean, there are lots of most nutrients that phytoplankton like, but the lack of some key nutrients means that they’re not drawing CO2 down to their full capacity—the biological pump isn’t running at its most efficient. We can measure the efficiency of the biological pump with nitrogen isotopes. Phytoplankton uptake nitrate (just like the plants in your garden), and because they prefer an isotopically lighter nitrate, we can calculate what fraction of the available nitrate has been consumed.
The measurement we make back in the lab looks at the relative abundance of light 14N and heavy 15N in our sample and comes up with δ15N to represent it. In a diatom, some portion of the nitrogen is completely encased in the silica frustule (you can think of this as nitrogen protected by glass!). Because we know that unprotected nitrogen in the sediment record can be altered chemically and biologically, we think that measuring diatom bound δ15N gives us a more accurate representation of what came down to the seafloor in the past. While we’re pretty confident that there is minimal alteration to the diatom bound nitrogen on the seafloor, there appears to be a difference in the isotope ratio (δ15N) between diatom bound nitrogen and the total nitrogen in diatoms. We’ve also found that individual species of diatom have different effects on the nitrogen isotope ratio, which are highly reproducible. That’s where I come in!
Culture results comparing diatom bound δ15N (triangles), total organic δ15N (squares), and dissolved nitrate δ15N (circles) in two Southern Ocean species. In each species, there is a different, internally consistent offset between the diatom bound δ15N and total organic δ15N. The solid lines show the theoretical Rayleigh approximations for each δ15N. The big takeaway from this figure is that there’s quite a large isotopic difference in F. cylindrus but only a small one in F. kerguelensis — different species having quite a different impact! [Figure modified from Horn et al, 2011, Paleoceanography]
In Diatom Alley, I am growing the diatoms that were present when we sampled the water—we call this the assemblage—to see if the assemblage behaves predictably in how the δ15N of the whole biomass is related to diatom bound δ15N. Here’s what I mean when I say predictably: if we know the offset of each species and how much of each species there is, we can predict the actual relationship with simple arithmetic. That’s where our big question comes in! Does the difference in the ratio of nitrogen isotopes (the offset) we see in Diatom Alley match the offset we predict for it?
To really answer this question, I’ll also need to know how each individual species in the assemblage changes the nitrogen isotope ratio. Back home, I’ll set up another Diatom Alley—and in this one we’ll be measuring the same process for individual species. Here on the ship, we’ve been isolating individual diatom cells from the seawater (a process involving a good microscope, glass sampling tips about 1 cell width across, and a lot of patience) to grow a culture of just a single species. That way, we’ll be able to see how each individual species in the assemblage acts on its own and as part of the group.
I’m excited to see what the answer is, and in the end, we’ll be much better informed about this complex system!