Where will life survive when Earth enters a deep freeze?MITScientists believe that the meltwater pools scattered across the Earth's ice may be one of the refuges where life can survive.In Nature CommunicationsIn a published study, researchers suggest that some early complex life forms may have survived in shallow pools of melt between 635 million and 720 million years ago - the "Snowball Earth" period when much of the Earth was covered in ice.
The team found that eukaryotes — the complex cells that eventually gave rise to all multicellular life — could survive in small bodies of water that form on the surface of shallow ice sheets near the equator. In these areas, dust and debris can darken the ice surface, increase heat absorption, and cause localized melting. At temperatures close to 0 degrees Celsius, this process may have created a habitable meltwater environment for early life.
To support their hypothesis, the researchers turned to similar conditions in Antarctica today. They studied conditions similar to the small melt pools that may have existed during Snowball Earth along the edge of the ice sheet.
The research team collected samples from multiple ponds on the McMurdo Ice Shelf. Members of Robert Falcon Scott's 1903 expedition described the McMurdo Ice Shelf as "dirty ice." MIT scientists found evidence of eukaryotes in every pond they sampled. The composition of these life forms varies from pond to pond, displaying an astonishing biodiversity. They also observed that salinity had a significant effect on the types of organisms present: Ponds with higher salinity had more similar communities, while ponds with fresher water had distinct populations.
Researchers Ian Hawes from the University of Waikato and Mark Shallenberger from the University of Otago measured the physicochemical conditions of the meltwater ponds. Photo credit: Roger Sammons
"We have shown that melt pools are candidate sites where early eukaryotes might have taken shelter during these global glacial events," said the study's lead author Fatima Husain, a graduate student in MIT's Department of Earth, Atmospheric and Planetary Sciences (EAPS). "It shows that diversity exists and is possible in these types of environments. It's really a story about the resilience of life."
MIT co-authors of the study include Schlumberger Professor of Geobiology Roger Summons and former postdoctoral researcher Thomas Evans. Other collaborators include Jasmin Millar of Cardiff University, Anne Jungblut of the Natural History Museum in London, and Ian Hawes of the University of Waikato in New Zealand.
"Snowball Earth" is the common name for the icy period in Earth's history. It usually refers to two consecutive glacial periods that occurred during the Cryogenian period and lasted for millions of years, which geologists call the period between 635 million and 720 million years ago. Whether the Earth was more like a hard snowball or a softer "ball of mud" at that time is still debated. But scientists are sure of one thing: much of the planet is mired in a deep freeze, with average global temperatures dropping to minus 50 degrees Celsius. The question is: how and where did life survive?
"We are interested in understanding the basis of complex life on Earth. We find evidence of the presence of eukaryotes around the Proterozoic in the fossil record, but we largely lack direct evidence of where they might have lived," Hussain said. "The point of this mystery is that we know life has survived. We're just trying to understand how and where they survived."
A marine sponge is transported from the ocean floor to the surface near melt pools on the McMurdo Ice Sheet. Photo credit: Roger Summons
There are many ideas as to where organisms might have taken refuge during Snowball Earth, including certain areas of open ocean (if such environments existed), in and around deep-sea hydrothermal vents, and under ice sheets. When considering meltwater pools, Husain and her colleagues hypothesized that meltwater from surface ice at that time might also have been able to support the presence of early eukaryotes.
"There are many hypotheses about where life might have survived and sheltered during the Proterozoic Era, but we don't have perfect analogies for all of them," Hussein noted. "The presence and accessibility of supraplacial melt pools on Earth now gives us the opportunity to really focus on the eukaryotic organisms that live in these environments."
In the new study, researchers analyzed samples collected from meltwater ponds in Antarctica. In 2018, Sammons and colleagues from New Zealand traveled to an area of East Antarctica's McMurdo Ice Shelf known for hosting small melt pools, each just a few feet deep and meters wide. There, the water freezes all the way to the ocean floor, trapping dark sediment and marine life in the process. Wind-driven ice loss from the ocean surface creates a kind of conveyor belt, and over time, these trapped fragments are carried to the surface, where they absorb the sun's heat, causing the ice to melt, while ice without surrounding fragments reflects incoming sunlight, creating shallow melt pools.
The bottom of each pond is covered with a microbial mat that accumulates over the years to create layers of sticky cell colonies.
“These mats can be several centimeters thick, colorful and layered,” Hussain said.
These microbial mats are composed of cyanobacteria, which are prokaryotic, single-cell photosynthetic organisms that lack a nucleus or other organelles. While these ancient microorganisms are known to survive some of the harshest environments on Earth, including meltwater pools, researchers wondered whether eukaryotes—complex organisms that evolved cell nuclei and other membrane-bound organelles—could withstand similar rigors. Answering this question will require more than a microscope, as the characteristics of the tiny eukaryotes present in the microbial mats are too subtle to be distinguished by the naked eye.
Fragments of cyanobacterial mat collected from the edge of a meltwater pond. Photo credit: Roger Summons
To identify the eukaryotes, the team analyzed the microbial mats for specific lipids they produce, called sterols, as well as a genetic component called ribosomal ribonucleic acid (rRNA), both of which can be used to identify organisms with varying degrees of specificity. These two independent sets of analyzes provide complementary fingerprints for certain eukaryotic groups. As part of their study of lipids, they discovered a number of sterol and rRNA genes in the microbial mat that are closely related to specific types of algae, protists and microbes. The researchers were able to assess the type and relative abundance of lipid and rRNA genes in different ponds and found that the diversity of eukaryotes in these ponds was stunning.
“No two ponds are the same,” Hussein said. "They had some repeating characteristics, but varying abundances. We found diverse communities of eukaryotes from all major taxa in all the ponds studied. These eukaryotes are descendants of eukaryotes that survived Snowball Earth. This really highlights that meltwater ponds during Snowball Earth may have acted as supraglacial oases, nourishing eukaryotes and thus facilitating the diversification and proliferation of later complex life, including us humans."
Compiled from /scitechdaily