Night of the living algae
UNIVERSITY OF CALIFORNIA – RIVERSIDE
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IMAGE: K / PG or CRETACEOUS-PALEOGENE EXTINCTION EVENT refers to the aftermath of the ASTEROID HITTING EARTH 66 million years ago. Show more CREDIT: ODYSSEUS ARCHONTIKIS / UNIVERSITY OF OXFORD
Tiny, seemingly harmless marine plants survived the darkness of the asteroid strike that killed the dinosaurs by learning a creepy behavior – to eat other living things.
Huge amounts of debris, soot, and aerosols shot into the atmosphere when an asteroid hit Earth 66 million years ago, plunging the planet into darkness, cooling the climate and acidifying the oceans. Along with the dinosaurs on the land and the giant reptiles in the ocean, the dominant species of seaweed – except for one rare type – were instantly wiped out.
A team of scientists, including researchers from UC Riverside, wanted to understand how these algae managed to thrive while mass extinction hit the rest of the global food chain.
"This event was the closest thing to wiping out all multicellular life on this planet, at least in the ocean," said UCR geologist and study co-author Andrew Ridgwell. “If you remove algae, which are the basis of the food chain, everything else should die. We wanted to know how the earth's oceans avoided this fate and how our modern marine ecosystem has developed again after such a catastrophe. "
To answer their questions, the team examined well-preserved fossils of the surviving algae and created detailed computer models to simulate the likely evolution of the algae eating habits over time. Their results are now being published in Science Advances.
According to Ridgwell, scientists were somewhat lucky to find the nano-sized fossils in the first place. They were located in rapidly accumulating, high clay sediments which helped to preserve them in the same way that the La Brea tar pits provide a special environment for mammoth conservation.
Most fossils had calcium carbonate shields and holes in their shields. The holes indicate flagella – thin, tail-like structures that tiny organisms can swim with.
"The only reason you have to move is to get your loot," explained Ridgwell.
Modern relatives of the ancient algae also have chloroplasts, which enable them to use sunlight to make food from carbon dioxide and water. This ability to survive through both feeding on other organisms and through photosynthesis is known as mixotrophy. Examples of the few land plants with this ability are Venus flytraps and sundew.
The researchers found that after dark after the asteroid, these mixotrophic algae spread from the coastal shelf areas into the open ocean, where they became a dominant form of life for the next million years and contributed to the rapid rebuilding of the food chain. It also helped that larger creatures that would normally feed on these algae were initially absent from the oceans after extinction.
"The results show both the extreme adaptability of ocean plankton and its ability to evolve quickly, but also for plants with a generation time of just one day that they are always only a year of darkness away from extinction," said Ridgwell.
It wasn't until much later that the algae evolved, lost the ability to eat other creatures, and re-established themselves to become one of the dominant species of algae in today's ocean.
"Mixotrophy was both the means of initial survival and a benefit after the darkness after the asteroid subsided due to the abundance of tiny pretty cells, likely surviving cyanobacteria," said Ridgwell. "It's the ultimate Halloween story – when the lights go out, everyone starts eating each other."