The famous asteroid impact 66 million years ago shrouded the earth in dark clouds of ash – how could the normally light-dependent marine algae survive the darkness? Apparently this enabled predatory abilities, according to an investigation of fossil algae deposits. After the impact, the oceans were shaped by species of single-cell organisms that not only fed through photosynthesis, but were also able to exterminate bacteria. These predatory algae may have played an important role in the recovery of marine ecosystems. Later they were largely replaced by the “peaceful” algae species, the results show.
The Cretaceous Period ended with a bang: Even today, traces of the asteroid can still be seen in the area of the Mexican peninsula Yucatan, which, according to dating, crashed into the earth about 66 million years ago. It is believed that the cosmic bomb, in addition to fire storms and gigantic tsunamis, also caused thick clouds of ash in the atmosphere. The result was probably a strong darkening, which probably only cleared up over the course of years. The paleontological finds from the time of the impact clearly show that the inferno was associated with a devastating mass extinction: the dinosaurs were the most famous victims, but with them a large part of all other living things disappeared – on land and in the sea.
Important tiny things in sight
However, it is also clear that there was no complete destruction. Somehow some living beings managed to survive the inferno. From them today’s animal and plant species developed. However, the current study now focuses on the survival of tiny animals. The researchers led by Samantha Gibbs from the University of Southampton investigated the question of how the unicellular algae (nannoplankton) of the oceans coped with the consequences of the impact. As they emphasize, these tiny creatures were as important then as they are today, because marine algae form an important basis for the food chain.
As part of their study, the researchers dealt with a group of these creatures whose cell body is enclosed in a ball made of calcium carbonate platelets. In some cases, these tiny “skeletons” (coccospheres) have survived for millions of years. As part of their study, the researchers have now examined the microfossils of these algae from different locations and from different times and compared them with representatives living today. Most of today’s species of these tiny creatures are photoautotrophic. This means that they only feed on the production of energy-rich substances from the use of light. However, there are also representatives who, in addition to photosynthesis, capture bacteria in order to secure another source of food – they are known as mixotrophs.
The researchers have now been able to show that before the asteroid impact, the algae in the oceans had characteristics associated with a purely photoautotrophic way of life. In the nannoplankton skeletons of the representatives of the algae from the time after the disaster, however, the researchers then found striking holes in the spherical structures. As they explain, this is typical of the predatory species of unicellular algae. Because in the holes there are flagella, which are used for locomotion – but also for food intake: With them, the single-celled cells catch bacteria in their environment and absorb them.
Only the robbers did not starve to death
The results thus show that after the impact, the mixotrophic species dominated the sea, which, in addition to photosynthesis, were also able to provide for their nutrition through hunting. “The species that were lost in mass extinction, on the other hand, did not show any characteristics that indicate a mixotrophic way of life – they were probably completely dependent on sunlight,” says Gibbs. In addition to these findings, the researchers also carried out computer simulations to model how the impact of the impact might have affected the development of the algae. As they report, the results match the fossil findings. The models also reflect that, after the impact, the predatory algae could also have benefited from a presumably increased bacterial density in the water.
The fact that some plankton algae were able to survive due to their ability to eat alternatively may have played an important role in rebuilding the destroyed food chains in the ocean, the researchers say. The findings indicate that the mixotrophic algae dominated the oceans for at least a million years. Later, however, a revolutionary return trend set in towards a renewed stronger dependence of nannoplankton on photosynthesis. Today most representatives live again only from the energy supply from sunlight.
As the scientists finally explain, the results now also make it clear that the darkening after the asteroid impact was apparently a central aspect of the mass extinction. “This massive impact hurled huge amounts of debris, aerosols and soot into the atmosphere, causing darkness, cooling and acidification,” says co-author Paul Bown of University College London. “The significant distortion that we found in the nannoplankton extinction can only be fully explained by the darkness caused by the asteroid impact,” said the researcher. Gibbs finally adds: “The event probably represents the only really geologically instantaneous mass extinction in the history of the earth.”
Source: University of Southampton, Article: Science Advances, doi: 10.1126 / sciadv.abc9123