Ancient microbes may help us find alien lifeforms


Artist’s rendering of the process by which microbes capture sunlight to produce energy with rhodopsin proteins. Credit: Sohail Wasif/UCR

Light-catching proteins illuminate Earth billions of years ago.

Scientists have reconstructed the lives of some of Earth’s earliest organisms using light-capturing proteins in living microbes. These efforts could help us recognize signs of extraterrestrial life on other planets, whose atmospheres may be more like our pre-oxygen planet.

The first living things on Earth, which included bacteria and single-celled organisms called archaea, inhabited a mostly oceanic planet with no ozone layer to protect them from solar radiation. These microbes evolved rhodopsins, proteins capable of turning sunlight into energy, and used them to fuel cellular processes.

“On early Earth, energy may have been very scarce. Bacteria and archaea figured out how to use the sun’s abundant energy without the complex biomolecules needed to photosynthesis“said Edward Schwieterman, a Riverside astrobiologist at the University of California and co-author of a study describing the research.

Rhodopsins are related to the rods and cones in human eyes that allow us to distinguish between light and dark and to see colors. They are also widely distributed among modern organisms and environments such as salt ponds, which exhibit a rainbow of vibrant colors.

Using a type of artificial intelligence called machine learning, the team of scientists analyzed rhodopsin protein sequences from around the world and tracked how they changed over time. Next, they created a type of family tree that allowed them to piece together rhodopsins from 2.5 to 4 billion years ago, and the conditions they likely faced.

Their findings are detailed in an article recently published in the journal Molecular biology and evolution.

“Life as we know it is as much an expression of the conditions of our planet as it is of life itself. We resurrected the old DNA sequences of a molecule, and it allowed us to make the connection to biology and the environment of the past,” said Betul Kacar, a University of Wisconsin-Madison astrobiologist and lead on the study.

“It’s like taking the DNA of many grandchildren to replicate the DNA of their grandparents. Only, they are not grandparents, but tiny things that lived billions of years ago, all over the world,” Schwieterman said.

Modern rhodopsins absorb blue, green, yellow, and orange light and may appear pink, purple, or red due to light they do not absorb or additional pigments. However, according to the team’s reconstructions, ancient rhodopsins were tuned to primarily absorb blue and green light.

Since ancient Earth did not yet have an ozone layer, the research team hypothesizes that microbes billions of years old lived several meters deep in the water column to protect yourself from intense UVB rays on the surface.

Blue and green light penetrate water best, so it is likely that early rhodopsins primarily absorbed these colors. “That might be the best combination of being shielded and still being able to absorb light for energy,” Schwieterman said.

After the Great Oxidation Event over 2 billion years ago, the Earth’s atmosphere began to experience an increase in the amount of oxygen. With additional oxygen and ozone in the atmosphere, rhodopsins evolved to absorb additional colors of light.

Rhodopsins today are able to absorb colors from light that plant chlorophyll pigments cannot. Although they represent totally independent and independent light-harvesting mechanisms, they absorb complementary regions of the spectrum.

“This suggests co-evolution, in that one group of organisms exploit light not absorbed by the other,” Schwieterman said. “It could be because the rhodopsins grew first and filtered out the green light, so the chlorophylls grew later to absorb the rest. Or it could have happened the other way around.

In the future, the team hopes to resuscitate model rhodopsins in a lab using synthetic biology techniques.

“We engineer ancient DNA inside modern genomes and reprogram bugs to behave as we thought millions of years ago. Rhodopsin is an excellent candidate for travel studies in the lab time,” Kacar said.

In the end, the team is delighted with the research possibilities opened up by the techniques used for this study. Since other signs of life from the deep geological past must be physically preserved and only certain molecules can be preserved long term, many aspects of life history have not been accessible to researchers until now. .

“Our study demonstrates for the first time that the behavioral histories of enzymes lend themselves to evolutionary reconstruction in a way that conventional molecular biosignatures do not,” Kacar said.

The team also hopes to take what they learned about the behavior of early terrestrial organisms and use it to search the skies for signs of life on other planets.

“Primitive Earth is an alien environment compared to our world today. Understanding how organisms here have changed over time and in different environments will teach us crucial things about how to search for and recognize life elsewhere. said Schwieterman.

Reference: “Earliest Photic Zone Niches Probed by Ancestral Microbial Rhodopsins” by Cathryn D. Sephus, Evrim Fer, Amanda K. Garcia, Zachary R. Adam, Edward W. Schwieterman and Betul Kacar, May 7, 2022, Molecular biology and evolution.
DOI: 10.1093/molbev/msac100


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