Algae & the asteroid: how single-celled plankton succeeded where the dinosaurs failed

New research has revealed how photosynthetic algae managed to survive the asteroid that killed the dinosaurs, by quickly adapting to hunt other microbes in the absence of sunlight. Not only did this help them thrive when so many species were wiped out, but it also provided a much needed lifeline to our early oceans.

An artists interpretation of the asteroid that ended the cretaceous period with an extinction event 66 million years ago

Around 66 million years ago the cretaceous period came to an abrupt end when a massive asteroid collided with Earth in an event that wiped out over 3/4 of life on the planet. Whilst the initial impact would have caused a shockwave around the world, unlike anything we could possibly imagine, the real damage was actually done in the following months and years. Vast amounts of debris, soot and aerosols were shot into the atmosphere, plunging the planet into darkness, cooling the climate, and acidifying the oceans. This meant extinctions in every major group and the complete loss of others, such as the dinosaurs. This was no different in the oceans where almost all photosynthetic algae were wiped out, causing a near collapse of the food chain and the disappearance of large ocean predators. Luckily a rare type of plankton were able to not only survive but thrive in this post-apocalyptic ocean, which meant a much needed lifeline for future marine life.  

Secrets of success

For years it has remained a mystery as to how ocean life, in particular its single-celled community, was able to survive and bounce back after the cretaceous-ending asteroid. Especially considering most photosynthetic algae were wiped out due to a lack of available sunlight. To researchers like Andrew Ridgwell, a geologist at University of California Riverside, it was a fascinating dilemma. “If you remove algae, which form the base of the food chain, everything else should die”, he explained in a recent press release, “we wanted to know how Earth’s oceans avoided that fate, and how our modern marine ecosystem re-evolved after such a catastrophe”. That was the aim of a group of researchers including Ridgwell and others from the University of Southampton, University College London and the University of Gibraltar, among others.

Diatoms are modern-day photosynthetic algae that would have been wiped out by the asteroid, however their silica shells (like those of the planktonic survivors) are easily preserved

However to do this, they would first need to find some of these ancient algal survivors to study. That is easier said than done when you are looking for intact single-celled specimens that are over 66 million years old, but luckily they were eventually able to find some samples in special layers of fast accumulating and high-clay-content sediments. Like today’s diatoms, the surviving plankton had a calcium carbonate shell, which meant they were fossilized in these in these environments. These shell fossils (or coccospheres) were then scanned by a computer to simulate what they would have looked like when they were alive and more importantly how they evolved.  

Mixing it up

The researcher’s computer scans revealed that these plankton had small holes in their calcium carbonate shells which are consistent with the presence of flagella (small tail-like structures that allow tiny organisms to swim). This meant they could control their own directional movement to some degree, which suggested that in addition to photosynthesising, these plankton were also hunting prey. So whilst other photosynthetic algae were wiped out due to a lack of sunlight, these survivors were able to switch things up and start hunting for their food instead. Being able to switch between strategies like this is known as mixotrophy and the research team suggest that this is what allowed these plankton to survive and ultimately thrive in a post-asteroid ocean. They recently shared their findings in a new paper released in the journal Science Advances.

These computer scans taken of the ancient coccospheres shows a clear opening where flagella would have extended from

An ocean lifeline

The idea of mixotrophy is not a new concept and a lot of single-celled algae today can gain nutrients through both photosynthesis and hunting other plankton. However, although these plankton were unlikely to be the first to do this, it is believed to be the first time mixotrophy became evolutionarily advantageous and the knock-on effects of it were vital to the entire ocean ecosystem. After the asteroid hit and the Earth was plunged into darkness, these plankton survived by hunting prey. But when the skies cleared they could resume photosynthesising, and because other algae had died off they had no real competition or predators. This meant they could quickly became the dominant plankton in the depleted oceans and provide the rest of the surviving marine life with a much needed food source. This didn’t happen overnight and it took around 2 million years for the oceans to recover to the levels of diversity and biomass they contained before the extinction event, but without these innovative plankton it may have never happened at all.

Importance of algae

This discovery not only solves an evolutionary mystery surrounding our oceans, but it also highlights the importance of single-celled algae. Without these planktonic survivors being able to rapidly adapt to changing conditions and evolving to become the base of the food web in our oceans, things could have turned out very differently for marine life (and the rest of us). The same is still true today, without plankton and other ocean microbes propping up ocean ecosystems, life on Earth would quickly break down. Unfortunately this is not just a worrying thought, but also an uncomfortably more realistic scenario. Today’s plankton face a wide range of problems, including rising temperatures, ocean acidification, pollution and more recently microplastics, which are all effecting their ability to play their crucial roles in the wider ocean ecosystem. Luckily we now know that they have the capacity to quickly adapt to these kinds of problems. However the question remains – why are we forcing them to adapt at all? And what happens if this time we aren’t so lucky?

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