NSIC (Natural Silicon Isotopic Composition)—The Forensics of Diatoms

Hi everyone! I'm Ivia, and I’m your guest blogger today. I'm French, and I'm here to sample seawater, sediments, and what we called pore water to study the silicon cycle and silicon isotopes in this amazing Southern Ocean. I'm taking samples mainly from the rosette of Niskin bottles lowered with the CTD and the megacorer , and I’ll analyze everything in our lab in California.

Now that you know a lot about phytoplankton and diatoms, I can tell you a little more about how we can spy and learn a lot of things about these little organisms. My work is indeed like a big investigation where the main suspects measure less than 1mm, and I really feel like Sherlock Holmes when I try to reconstruct the story of phytoplankton in the ocean.

Using a chemical tool—the stable isotopes of silicon—I can track information about diatoms' life in the far distant past as well as what they did last week. But let me explain how that works...

Silicon (or Si) is the second most abundant element in the Earth's crust after oxygen. As with other atoms in the periodic table of elements, silicon is composed of an arrangement of 3 different particles: protons, neutrons, and electrons. In most cases, silicon is composed of 14 neutrons, but in some rare cases, it can have 15 or 16 neutrons. These little variations in the atomic composition of silicon are reflected in its mass number (which is literally the sum of both protons and neutrons in the atom).

Thus, we can find 3 variants (or isotopes) of silicon on Earth: 28Si (mass number = 28, with 14 neutrons), which is 92% of Si and the most abundant; 29Si (5%, mass number = 29 with 15 neutrons); and the least abundant 30Si (3%, mass number = 30 with 16 neutrons). Because these isotopes are a little different from each other, they will react differently during physical, chemical, or biological reactions (such as dissolution—when the frustule dissolves—or during the formation of their glass skeleton).

You know now that diatoms build an opaline structure, called a frustule. To do that, these little glassblowers absorb the silicon dissolved in seawater (also called DSi). Then they accumulate DSi in small vesicles (like small pockets in their cell) where the silicon can polymerize into biogenic silica (also called opal). Finally, they use this vesicle to build their frustule, decorated with spines, holes, antennae, etc.

In some parts of the ocean, diatoms can consume all the DSi in surface water, just as they do at the station where we’re currently sampling. Indeed, at our last stations, the DSi concentration in surface water was below 1µmol.L-1 (which is surprisingly low!).

Diatoms still think they can steal silicon from the seawater without leaving any trace of their crime?! Well, do not tell them, but we have found a way to track their fingerprints in the ocean and in the sediments...

Because the lightest isotope (28Si) reacts more quickly than the 2 other isotopes, diatoms preferentially use the 28Si when they silicify (i.e., transform the DSi into opaline structures that will constitute their frustules and which we called biogenic silica). By doing that, they "fractionate" (or discriminate) between the silicon isotopes.

Scanning electron microscope picture of the diatom Frangilariopsis kerguelensis showing a complete frustule (lateral view) and 2 separate valves (Credit: Ivia Closset). A diatom uses DSi to build its biogenic silica frustule (Production arrow). When it dies, the biogenic silica dissolves and releases DSi in theseawater (Dissolution arrow).

It sounds like a very difficult concept? It's not! Let me give you an example...

Diatoms are just like you and me: they have some preference for the nutrients they use to grow. Imagine that you have a bag full of dark and white chocolate chips in equal proportions. You like both, but you have a preference for the white chocolate. Because you're wise and you want to save some chocolate, you decide to take only 10 chocolate chips every day. The first day, you obviously grab more white chocolate compared to dark chocolate (well, it's your favorite, right?!)—let’s say 8 white chips and 2 dark chocolate chips.

The second day, you do the same thing, but you notice that, now, there are more dark than white chocolate chips in your bag. The third day, you have only 7 white chocolate chips and 3 dark chocolate chips in your hand (but you're still happy because it's still a lot of white chocolate!). The fourth day, you notice the color in your bag has changed. It's darker because you have less white chocolate compared to the first day; you have 6 white chocolate chips and 4 dark chocolate chips in your hand, so it's darker in your hand, too.

The last day, it's difficult to find white chocolate in your bag, and the chocolate in your hand looks much darker... By preferentially eating white chocolate, you have changed the composition of chocolate in your bag and in your hand. You have increased the proportion of dark chocolate (your less favorite chocolate) in both the bag and your hand.

Well, diatoms do the same thing with silicon isotopes in the ocean. They preferentially use the 28Si, and so, as they consume DSi, they increase the proportion of 30Si in the seawater and in the biogenic silica.

Schematic view of a diatom cell (from Martin-Jézéquel et al., 2000). DSi (little black dots) enters the cell through specific proteins or just by diffusion (uptake). Then it is transported to the vesicle (SDV) where it polymerizes into biogenic silica. The diatom uses this vesicle to build its frustule (grey line). Some DSi can return to the seawater, either by dissolution of the frustule or by the diatom itself removing extra DSi from its cell (efflux).

Actually, the fractionation of silicon isotopes by diatoms is a little bit more complex than when you eat chocolate chips, and there are many things we don't yet know about it. For example, we assume that diatoms always take up the isotopes in the same ratio. That means the proportion of 30Si and 28Si they take won't change, i.e., to use our example, that you always take the same proportion of white and dark chocolate.

Today, we are starting to think this ratio can change, but why? We don't know yet. It is very likely that different diatom communities and diatom species fractionate the isotopes differently. They probably change the ratio of 30Si and 28Si they take depending on environmental conditions such as the availability of nutrients, maybe light... These are the ideas I'm testing now in our lab at UCSB.

The inverse relationship between DSi concentration and silicon isotopic composition (d30Si) in seawater and sediment samples from the Southern Ocean (from Egan et al., 2012). The relative proportion of 30Si compared to 28Si (d30Si) increase as diatoms consume silicon and decrease the DSi concentration in the surface waters.

Regardless, their activity leaves an imprint in both seawater and in the diatoms themselves. When they die, diatoms sink through the deep ocean and are buried in the sediments, saving this fingerprint—or "silicaprint"—for us. By measuring the proportion of the 3 silicon isotopes, and especially the ratio between the 28Si and the 30Si, I can investigate their activity in the surface water.

For example, when I measure the silicon isotopes directly in the seawater, I can calculate the amount of silicon they have used in the last week or last month. If there are a lot of 30Si, diatoms were very active and consumed a lot of DSi.

When I measure these isotopes in the particles that sink in the deep ocean, I can identify whether the diatoms have produced their frustules in an icy environment or if the ocean was free of ice. (Diatoms growing in icy environments fractionate silicon isotopes differently.)

And finally, when I measure them in the sediments, I can investigate part of the lifecycle of diatoms that lived in the ocean thousands of years before. Were they abundant? Very active? Did they consume a lot of DSi, and a so a lot of CO2?

This forensic science seems to be easy and work very well. Actually, it is, but we still have a lot of questions regarding the fractionation of isotopes by diatoms. Do all species discriminate between isotopes in the same way? If you change the conditions of their environment (for example the temperature, the quantity of nutrients, etc.), do they change the way they fractionate isotopes? Are they doing the same thing everywhere?

These are important questions because we use silicon isotopes to reconstruct how diatoms influence ocean chemistry and how they have interacted with the climate in the past. We cannot reconstruct accurately what happened in the past if we're not able to understand how the tools we're using for that work in the present. Moreover, we also use silicon isotopes to understand how diatoms respond to the current climate change and to make predictions about how these changes will affect the ocean chemistry and biological activity in the future.