Oxygen And Multicellularity, A Complicated Relationship (Astronomy)

A study published in Nature Communications challenges the prevailing theory on the development of multicellular life forms on Earth, according to which the concentration of gas in the atmosphere played a crucial role in the evolution of large and complex organisms. The new findings instead highlight that oxygen would have behaved like a double-edged sword: providing significant metabolic benefits when abundant, but suppressing the evolution of large multicellular organisms in conditions of scarcity.

Scientists have long thought that at the basis of the transition from single-celled organisms to the first multicellular life forms was the increase in oxygen in the Earth’s atmosphere, which began 2.5 billion years ago with the so-called Great Oxygenation Event . This theory, called by the experts “ hypothesis of oxygen control”, Suggests that the transition from unicellular to multicellular life, in which single cells are able to cooperate with the typical mechanisms of more complex life forms, has strictly depended on the amount of oxygen available. Furthermore, the hypothesis predicts that with the increase in the concentration of oxygen in the atmosphere, the size of the multicellular organisms that populated the Earth also increased. According to a new study published this month in Nature Communications,  conducted by a team of researchers from the Georgia Institute of Technology in Atlanta (USA), this is not exactly the case.

Through laboratory experiments that used the unicellular yeast Saccharomyces cerevisiae as an animal model , and thanks to sophisticated evolutionary models, the researchers obtained important new information about the relationship between oxygenation of the early Earth and the emergence of large multicellular organisms. The results of the study suggest that the effect of oxygen on the evolution of multicellularity would not always have been positive, on the contrary: the initial oxygenation of the Earth’s atmosphere would have even severely limited the development of multicellular individuals, rather than selecting larger organisms and complex.

“The positive effect of oxygen on the evolution of multicellularity is dose-dependent: the first oxygenation of our planet would have strongly limited, and not promoted, the development of multicellular life forms”, says Ozan Gonensin Bozdag , researcher at the Georgia Institute of Technology and lead author of the study. “The positive effect of oxygen on the size of multicellular organisms was realized only when it reached high levels.”

Left, photograph obtained by confocal microscopy showing several multicellular clusters of yeast. On the right, the enlargement of a single cluster with the typical shape of a snowflake. Credits: Shane Jacobeen, Will Ratcliff, and Peter Yunker, Georgia Institute of Technology

As anticipated, the researchers used as a model the single-celled yeast Saccharomyces cerevisiae , a eukaryotic microorganism capable of obtaining energy both in the presence of oxygen through respiration and in its absence through fermentation – the chemical process that we have been using for centuries to produce bread. wine and beer. But the one used in the study is not the wild strain – or wild type , as they say in the jargon – but a mutant in the ability to divide and reproduce, and for this reason able to form a multicellular “individual” whose shape resembles the flakes of snow, hence the name snowflake yeastby which these cell clusters are called. After selecting around 800 generations of multicellular forms of this microorganism, the researchers examined their ability to evolve into larger multicellular aggregates by subjecting them to different concentrations of oxygen.

“Large sizes evolved easily when our yeasts lacked or had abundant oxygen, but not when oxygen was present at low levels,” explains Will Ratcliff , also a researcher at the Georgia Institute of Technology and co-founder. author of the study.

These results can be explained by a divergent oxygen-mediated selection mechanism that acts on the size of the organism. An outcome, also confirmed by mathematical models, of almost universal evolutionary and biophysical compromises.

“We have worked hard to show that this is actually a fairly predictable and understandable consequence of the fact that oxygen, when limited, acts as a resource if the cells that can use it get a huge metabolic benefit from it,” he adds. Ratcliff. “When oxygen is in short supply, it can’t spread much, so there’s an evolutionary incentive that leads multicellular organisms to be small in size, which allows most of their constituent cells to access oxygen. This limitation does not exist when oxygen is simply not present, or when there is enough to diffuse much deeper into the tissues [ in the internal cells of large multicellular clusters, ed. ] ».

This study, the researchers continue, not only challenges the oxygen control hypothesis, but helps us understand why the world of multicellular organisms evolved so little in the billion years after the Great Oxygenation Event. In this period – which geologists call the ” Boring Billion ” ( Boring Billion , in English), or the Middle Ages of the Earth – oxygen in the atmosphere was present, but its low levels, rather than selecting larger and more complex organisms , have exerted evolutionary pressure that has pushed multicellular organisms to remain relatively small and simple.

“In previous works, the relationship between oxygen and size of multicellular organisms has been studied mainly through the physical principles of gas diffusion,” emphasizes Bozdag. “This reasoning is fundamental, but when we study the origin of the complex multicellular life forms on our planet it is necessary to include the principles of Darwinian evolution as well,” concludes the researcher. Being able to grow microorganisms through numerous generations has made it possible to achieve this goal.

Featured image: Artistic illustration of the primeval Earth. Credits: Nasa

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