Plankton turned hunters to survive dinosaur-killing asteroid impact

After the last global mass extinction, 66 million years ago, most of the plankton that survived were those able to capture food to eat, according to a study led by UCL and University of Southampton researchers.

The findings, published in Science Advances, support the theory that darkness drove the global extinctions, after an asteroid impact, as plankton and plants would not have been able to use photosynthesis to get their energy from the sun.

Co-lead author Professor Paul Bown (UCL Earth Sciences) said: "We found that before the dinosaurs went extinct, plankton were similar to modern-day plankton as they got their energy directly from the sun - but that all changed very suddenly."

"After a massive asteroid strike, most of the plankton that persisted used a different survival strategy, by hunting and ingesting food.

"Our findings add to a growing body of evidence that the last global mass extinction was driven by a global darkness, and was probably the only truly geologically instantaneous mass extinction event."

The research team used an exceptional record of plankton fossils and eco-evolutionary modelling techniques to examine how organisms behaved before and after this event, which killed three quarters of life on earth - and why some survived and some didn’t.

The team found that prior to the asteroid impact, species of nannoplankton - microscopic algae - were exclusively reliant on harnessing energy from sunlight ( photoautotrophs) , but those living afterwards were capable of capturing food and eating it in addition to using photosynthesis to feed ( mixotrophs) . This suggests the blocking of light from the sun played an important role in killing off some species and over time, encouraging others to evolve and adapt.

The research team’s breakthrough came when they found that many of the nannoplankton skeletons (coccospheres) post mass-extinction included a large hole, indicating the position of flagella - tiny tail like structures used by the algae for movement and feeding. This indicates these microscopic organisms, which survived the asteroid strike, were capable of hunting and ingesting food.

Co-lead author Dr Samantha Gibbs (University of Southampton) said: "Those species that were lost at the mass extinction show no evidence of a mixotrophic lifestyle and were likely to be completely reliant on sunlight and photosynthesis. Fossils following the Cretaceous-Paleogene (K-Pg) extinction show that mixotrophy dominated and our model indicates this is because of the exceptional abundance of small prey cells - most likely surviving bacteria - and reduced numbers of larger ’grazers’ in the post-extinction oceans."

Opposing evolutionary forces led to the emergence of more diverse feeding strategies and eventually a return to greater reliance on photosynthesis in open ocean nannoplankton. Most nannoplankton today only photosynthesise.

The K/Pg event was triggered by an asteroid impact that formed the Chicxulub crater in Mexico, and is well known for the extinction of dinosaurs, plesiosaurs, ammonites and many other groups.

Professor Bown said: "This huge impact flung vast amounts of debris, aerosols and soot into the atmosphere, causing darkness, cooling and acidification over days and years.

"The significant bias found in the nannoplankton extinctions - removal of open-ocean photoautotrophs but survival of mixotrophs that could hunt and feed - can only be fully explained by the darkness caused by the asteroid impact acting as a kill mechanism."

Dr Gibbs added: "This ’blackout’ or shutdown of primary productivity would have been felt across all of Earth’s ecosystems and reveals that the K/Pg event is distinct from all other mass extinctions that have shaped the history of life, both in its rapidity, related to an instantaneous impact event, and its darkness kill mechanism, which shook the foundations of the food chains."

The study was conducted by researchers at UCL and University of Southampton alongside colleagues in Paris, California, Bristol and Edinburgh.


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