One of the most extraordinary things about the impact of an asteroid that hit Earth 66 million years ago is not what died, but what survived. A new study found that one order of marine algae made it through by changing its source of energy, feeding on other life forms instead of harvesting sunlight. In the process, they greatly sped up the recovery of oceanic ecosystems.

In the Cretaceous, as today, photosynthetic algae formed the base of the open ocean's food webs. When the asteroid struck, causing the Cretaceous-Paleogene Extinction Event, or K/PG, the world became shrouded in darkness from soot cutting off access to sunlight.

"If you remove algae, which form the base of the food chain, everything else should die. We wanted to know how Earth's oceans avoided that fate, and how our modern marine ecosystem re-evolved after such a catastrophe." Professor Andrew Ridgwell of the University of California, Riverside said in a statement. 

Coccolithophores are photosynthesizing single-celled algae with calcium carbonate scales. Studying those buried on the seafloor at the start of the Paleogene era, Ridgwell found the fossilized shells have openings that indicate they once had flagella to move around, although these have not been preserved. Modern coccolithophores with flagella consume bacteria and smaller algae, in addition to the energy they capture from sunlight, using the flagella to move in on their prey.

Moreover, Ridgwell and co-authors report in Science Advances, long after the asteroid struck, many of the dominant coccolithophores were those that during the Cretaceous had been restricted to continental shelves. Today species living on these shelves are much more likely to supplement their energy intake by feeding on other life than their open ocean counterparts.

The authors draw the conclusion that during the Cretaceous, coccolithophores were much like today – most relying on sunlight alone, but some, particularly on continental shelves, snacking on even tinier creatures as supplements. In a period of global darkness those with only one source of energy died, but the so-called “mixotrophic species” survived, recycling nutrients from the before-times.

Once sunlight returned, mixotrophic species remained dominant for 1.5-2 million years until exclusively photosynthesizing species evolved again to reclaim the open oceans.

The authors were puzzled as to why the mixotrophic coccolithophores remained dominant for so long. They created a computer model providing a simplified version of conditions after sunlight returned, and found it closely replicated what they had seen, with mixotrophy remaining widespread for long periods of time once established.

"Mixotrophy was both the means of initial survival and then an advantage after the post-asteroid darkness lifted because of the abundant small pretty cells, likely survivor cyanobacteria," Ridgwell said. "It is the ultimate Halloween story – when the lights go out, everyone starts eating each other."

The findings strengthen the case it was the extended darkness, rather than drastic temperature changes or ocean acidification, that most affected the base of the marine food web post-impact.