Genes captured by chlamydiae were allowed to live a complex life without oxygen

An international team of researchers has discovered a new group of Chlamydiae – Anoxychlamydiales – that live without oxygen under the ocean floor. These Chlamydiae have genes that allow them to survive without oxygen in the production of hydrogen gas. The researchers found that our single-parent ancestors ‘captured’ these hydrogen-producing genes from ancient Chlamydiae ‘up to two billion years ago’ – an event that was critical to the evolution of all modern life. The results are published in Science Advances.

Life on earth can be divided into two main categories: eukaryotes (e.g. plants, animals, fungi, amoeba) and prokaryotes (e.g. bacteria and archaea). Compared to relatively simple prokaryotic cells, eukaryotic cells have complex cellular organization. How such cellular complexity has evolved has amazed scientists for decades. The prevailing hypothesis for the evolution of eukaryotes includes the fusion, as a symbiosis, of two prokaryotes – an argeon and a bacterium – almost two billion years ago, in environments with little oxygen. Scientists believe that these microbes work together to survive without oxygen by exchanging nutrients. While we do not know what this nutrient was, many scientists think that hydrogen may be the answer.

To find an answer to this two billion year old mystery, scientists have taken genomes from modern prokaryotes and eukaryotes to find genes for living without oxygen and food metabolism with hydrogen. Many like fossils have taken clues to the evolutionary history of their ancestors. In our cells we have a specialized factory called the mitochondrion – as a power pump of the cell – which helps us to generate energy with the oxygen we breathe and the sugar we eat. However, some mitochondria can produce energy without producing oxygen through hydrogen gas. Since hydrogen has been proposed to be an important nutrient for the origin of eukaryotes, scientists believe that hydrogen production was present in one of the two billion-year-old partners: the argeon or the bacterium. However, there is no evidence for this with current data.

In an article published in Science Advances, a team of international researchers has discovered an unexpected source of these genes at the bottom of the ocean of the Anoxychlamydiales, a newly discovered group Chlamydiae. Anoxychlamydiales live without oxygen, and have genes for producing hydrogen – a property never before identified in Chlamydiae. The researchers were surprised to find that the chlamydial genes for hydrogen production closely resembled those found in eukaryotes. This strongly suggests that ancient chlamydiae contributed these genes to the evolution of eukaryotes.

“In our study, we identified the first evidence of how eukaryotes got the genes to make hydrogen and it was from a completely unexpected source!” says co-lead author Courtney Stairs, postdoctoral researcher at Uppsala University in Sweden. Co-lead author Jennah Dharamshi, Ph.D. student of Uppsala University, adds: “We found new evidence that the eukaryotic genome has a mosaic evolutionary history, and came not only from Archaea and the mitochondrion, but also from Chlamydiae.”

“Understanding where hydrogen metabolism comes from in eukaryotes is important for gaining insight into how our two billion year old ancestors evolved,” says senior author Thijs Ettema, professor at Wageningen University and Research in the Netherlands, and coordinator of the international team of researchers. “For years I thought that if we ever figured out where eukaryotic hydrogen metabolism came from, we would have a clearer picture of how eukaryotes evolved – yet finding out that these genes may have come from Chlamydiae has raised even more questions,” Courtney Trappen adds. .

How did the eukaryotes get their hands on these genes?

“We know that microorganisms routinely share genes together in a process called ‘gene transfer.’ We can find these transmission events by building pedigrees of each gene and looking for patterns in their evolution, ”explains Courtney Stairs. Today, the closest relatives of the archaeon are those who participated in the first symbiosis Asgard archaea. These archaea are also found at the bottom of the ocean where Anoxychlamydiales resides. “Asgard archaea and Anoxychlamydiales are both found under the ocean floor where there is no oxygen,” explains Thijs Ettema, “their cohabitation could have transferred genes between the ancestors of these microbes.”

Finding chlamydiae that can live without oxygen has important implications in itself. These bacteria are typically known as pathogens in humans and other animals, although they can also infect e-cellular eukaryotes such as amoebae. All hitherto known chlamydiae live in eukaryotic cells.

“Finding chlamydiae that can live without oxygen, produce hydrogen and live outside a eukaryote challenges our previously held beliefs,” says Jennah Dharamshi. “Our findings suggest that chlamydiae may be important members of the oceanic ecosystem and that perhaps all chlamydiae are not so bad after all.”