Marine algae: Defying climate change on a rollercoaster

10 September 2018, by Prof. Elisa Schaum

Photo: UHH/Wohlfahrt

Elisa Schaum in the laboratory

Half of the oxygen we breathe comes from the ocean. Minute algae, known as diatoms, produce this oxygen. At the same time, they are also the basis of the entire food chain in the ocean. Climate change is altering their habitat, but what effect is it having on these tiny powerhouses?

If we imagine the world in the year 2100, climate change may well have altered quite a few things in the sea. That’s why, at the Center for Earth System Research and Sustainability (CEN), I’m investigating how algae will react in the long term to new and extreme conditions. A ten-degree rise in water temperature? Significantly more CO₂ dissolved in the sea – making the water more acidic? Many of today’s algae wouldn’t be able to tolerate this, and computer models predict that the population could decrease by a fifth in the long term – with far-reaching repercussions for the oceans and the atmosphere around the globe.

But is the alga of today the same as the alga of tomorrow? Algae reproduce rapidly and their populations are enormous. A generation is replaced within one or two days – an advantage when it comes to mutation, which can lead to beneficial genetic adaptations.

We put a type of diatom known as Thalassiosira pseudonana in hundreds of mini aquariums at different temperatures. Their “comfort zone“ is 22 degrees Celsius, but we also kept them in water that was four degrees warmer, at 26 degrees. According to the IPCC climate report, the temperature will have risen by this much on average by 2100. And this is just a mean value; peak temperatures – like those in a heat wave – will be much higher, which is why we wanted to provoke the algae still further. In another 100 containers, we are exposing them to continuous temperatures of 32 degrees.

Once an experiment has been set up, it means that my team has to monitor, clean, and feed the algae several times a week. For one to two years! Including at Christmas and during the summer vacations. Evolution may be beautiful, but it takes time! Usually, we only finish the experiment after 300 generations. And sometimes, in the end it turns out that the algae were unable to permanently adapt to the test factor and struggled to thrive. An important insight, but rarely one that makes us do a victory dance.

Couldn’t we get the algae to adapt in a shorter time? I wanted to try out a new method out for the first time: Instead of introducing the Thalassiosira to the changing conditions gradually, I put them on a rollercoaster. In a further series of experiments, I immersed them in alternating baths – warmer, cooler, warmer, cooler. Every four days I turned the thermostat up from 22 to 32 degrees, then after another four days turned it back down again. Could they tolerate this?

The surprising answer: Yes, very well! The results were impressive, demonstrating that while the algae in constant 32-degree modus were unable to cope with the heat and developed poorly for about a year, they were able to tolerate the same temperature in fluctuating modus without any problem. The population came to life – like at 26 degrees – and grew quickly. Faster even than at the usual 22 degrees. Our analyses also show that the genetic changes in Thalassiosira are greatest in fluctuating conditions.

An unstable environment promotes genetic adaptation in algae – a groundbreaking discovery. This means that they are better equipped to adapt to future extreme situations. A positive side effect: Our experiments can be shorter.

For the moment we don’t have to worry about the flexible diatoms. Instead we need to keep an eye on their predators, which may not be able to adapt as quickly. With their versatility, the algae appear to always be one step ahead.

This content was first published as a guest article in the newspaper Hamburger Abendblatt in September 2018.

Elisa Schaum is an evolution expert at Universität Hamburg’s Center for Earth System Research and Sustainability (CEN)

Scientific article: Schaum EC, Buckling A, Smirnoff N, Studholme DJ, Yvon-Durocher G (2018): Environmental fluctuations accelerate molecular evolution of thermal tolerance in a marine diatom, Nature Communications 9, 1719

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