Dr Susan Little talks about the effects that pinhead-sized algae have on the chemistry of the world's oceans when they 'binge' feed in summer.

Dr Little is an isotope geochemist from Imperial’s Department of Earth Science and Engineering. Her work focuses on analysing the chemistry of the world’s oceans to understand more about its past, present and future.

In her latest scientific collaboration, Dr Little and scientists from ETH Zürich and the Universities of Southampton, Otago and Oxford have been investigating the impact of diatoms on ocean chemistry. Colin Smith talks to Dr Little about diatoms and the effect they are having on oceans around the world.

What is a diatom and what is its purpose?

Diatom blooms are so big that they can be seen by satellites (Copyright Michael Köbberich ETHZ)

A diatom is approximately the size of a pinhead. They are disc-shaped and have a hard shell made of silica.

Silica is the same material used to make glass, so they are often referred to as cells living in glass houses.

Diatoms are phytoplankton. They capture solar energy during photosynthesis and produce a quarter of our planet’s oxygen.

They also capture carbon dioxide from the atmosphere, using the carbon to make their cells. When they die they sink from the surface of the ocean and decompose. The carbon dioxide stored in their cells can then be stored in the deep parts of oceans for hundreds or thousands of years.

Despite their tough shell, they are also a vital part of the food chain, being an abundant source of food for many creatures.

Where do they live?

They thrive in the Southern Ocean, which skirts the continent of Antarctica.

Why are diatoms important?

In short, they play a uniquely important role in our oceans.

Diatoms need nutrients to grow, just like plants in our garden. They need the same nutrients that you would find in fertiliser, such as phosphate and nitrate. They also need silicon to make their shell and a range of what we call ‘micro-nutrients’ including iron and zinc.

All other aquatic plants and algae also need these ingredients to survive. So, if diatoms absorb more of these ingredients, or use them in different proportions, then this may have a knock-on effect on life in other parts of the ocean, or higher up the food chain.

Aquatic life are important for regulating atmospheric carbon dioxide levels, so anything that affects them can have a knock-on effect in our atmosphere.

What do we already know about diatoms?

We know that diatoms undergo a ‘bloom and bust’ life cycle in the Southern Ocean.

Diatoms absorb nutrients like zinc and silicon in the sunlit upper level of the Southern Ocean (Copyright Peter Andreas Rüegg ETHZ)

In spring-summer, increased light and plentiful nutrients cause the diatoms to undergo a feeding frenzy. For reasons we don’t fully understand, Southern Ocean diatoms have a particularly strong preference for silicon and zinc compared to other nutrients. They literally absorb much of the zinc and silica in the upper layer of the Southern Ocean, stripping it almost bare and altering the chemistry of the upper layer of water. The frenzy leads to huge algal blooms, which are so big that they can be captured via satellite images in space.

When they run out of nutrients, they enter the ‘bust’ phase of their life cycle, dying and sinking towards the bottom of the ocean. On their way down to the seafloor, they start to decompose at around 200 to 400 metres below the sea level, releasing the zinc and silicon back into the seawater. This, in turn, enriches the lower layer of water in zinc and silicon.

Why do they absorb zinc and silicon?

Diatoms absorb zinc from seawater for use in their physiological processes. They absorb silica to form their hard casing.

What have you discovered?

Dr Susan Little

In our new study, we’ve learnt that currents in the Southern Ocean play a unique role in magnifying the impact that these algal blooms have on the zinc and silicon supply in oceans around the world.

Try and picture the world’s oceans as if they were a massive layered sponge cake. Our oceans, like sponge cakes, have many layers with different currents and unique chemical fingerprints.

The currents in the Southern Ocean transport the upper nutrient depleted layer all the way to the equator. The warmer top layer of water in this equatorial region acts like a lid and traps the nutrient depleted layer of water just below it.

Meanwhile, similar currents are also transporting the zinc and silicon enriched deep layer of water to the equatorial regions of our oceans, where it remains trapped deep below the surface of the ocean.

How does this information improve our understanding of the world’s oceans?

Behind iron, zinc is the second most important micronutrient in the world’s ocean, used by life to carry out many physiological processes at the cellular level.

The feeding frenzy of diatoms in the Southern Ocean, coupled with the ocean currents, means that these nutrients (zinc and silicon) are in short supply in oceans elsewhere in the world.

The vast majority of life lives in these upper sunlit layers, so we think this process is probably impacting on the types of communities of organisms that are able to grow at these upper levels.

In the case of diatoms, they are much less common outside of the Southern Ocean. It may be that their own binge feeding in the Southern Ocean prevents them from growing elsewhere because they’ve depleted the zinc and silica supply to the rest of the world oceans. 

What are you planning to do next with this work?

The big question is have diatoms around Antarctica always behaved like this? Or is this a new phenomenon? We want to investigate why they are absorbing so much zinc and silicon. Maybe the chemistry of the Southern Ocean has changed over tens of thousands of years and their nutrient preferences have changed too.

It is fascinating to see how the growth of one particular organism, in one particular region, plays such a vital role in global ocean chemistry and life elsewhere in the oceans. We want to know more, in order to get a better understanding of our oceans and how they may evolve in the future.