Plankton revived oceans after dinosaur-era extinction

ScienceFair: Plankton revived oceans after dinosaur-era extinction
The revival of the world's oceans after the extinction event that felled the mighty dinosaurs, suggests a study, may rest on the ability of humble plankton to survive a century of seafloor burial. Plankton image from University of Hawaii Institute of Marine Biology
CAPTIONPhoto Collection of Dr. James P. McVey, NOAA Sea Grant Program
Poor old Tyrannosaurus Rex enjoying his apex position at the top of the food chain when ... WHAMMO ... along comes a space rock that blasts a hole in the Gulf of Mexico some 65.5 million years ago.

The impact, "ejected large amounts of material into the atmosphere, blocking sunlight to levels below photosynthesis compensation, and contributing to a global collapse of terrestrial and marine food webs," begins the Nature Communications study led by Sofia Ribeiro of Denmark's University of Copenhagen. Massive earthquakes, tsunamis, ozone depletion, wildfires and a cloudy blackout of the sky lasting perhaps nine months also resulted from the blast, with global cooling that lasted decades as well.

But not every critter fared too badly, the study notes. Deep ocean microbes fared badly for the next three million years. But much less catastrophic extinction rates appear to have afflicted their coastal critter cousins. "In fact, life resurgence in coastal areas appears to have been relatively rapid, possibly occurring in less than a century," says the study.

Why did that happen?

Plankton, says the team, based on an analysis of 450 plankton cysts found in coastal seafloor sediment cores. The team dated the mud in the cores and then attempted to grow the plankton:

We isolated phytoplankton resting stages from sediment cores retrieved from a low-oxygen sill fjord, and found these to be viable for up to 87 ± 12 years ... The growth performance of the tested strains was not affected by nearly a century of dormancy. As this species belongs to a lineage well-established by the Cretaceous, and presumably little affected by the (65.5 million-year-ago) crisis, we argue that resting stages could effectively have contributed to the survival of phytoplankton groups possessing this life-history trait. Our findings indicate that resting stages in coastal sediments may play a significant role for the recovery of primary production after events of global photosynthesis disruption.

Plankton includes bacteria and other microbes, even tiny jellyfish, any type of drifting sea surface critter, basically. But the coastal ones in the study were of the "cyanobacteria" variety, blue-green algae that obtain their energy from photosynthesis. Other types, such as single-celled algae and a variiety seen today in toxic algae blooms, also survive long dormancy.

Some of these same sort of bugs likely first filled the early Earth's skies with oxygen, billions of years ago. And they saved our hash one more time after the impact they wiped out many other creatures some 65.5 million years ago, the researchers conclude. "Germination after long-term dormancy appears to be common."

We, thus, propose that phytoplankton resting stages accumulated in coastal sediments have aided survival across the (extinction event) and played a crucial role for the resurgence of primary production and ecosystem recovery. Phytoplankton resting stage 'banks' in coastal sediments should be incorporated in scenarios aimed at reconstructing or predicting recovery from catastrophes. It should be noted, however, that photosynthesis and biodiversity recovery do not occur at the same rates. Considering that only a fraction of modern phytoplankton groups are able to form resting stages (for example, less than 20% of modern dinoflagellate species are known to produce cysts), removing non-cyst formers at the (extinction event) likely resulted in a larger diversity loss than that inferred from the fossil record (because non-cyst formers do not fossilize). Although the recovery of primary productivity after the (extinction event) was relatively rapid, the recuperation in biodiversity is expected to have been delayed, due to intrinsic limits to how quickly global biodiversity can recover after extinction events.