Tentacled microbe could be missing link between simple cells & complex life

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https://www.sciencemag.org/news/2019/08/...mplex-life

EXCERPT: . . . The tree of life has three major branches—bacteria and archaea make up two, both of which are microbes that lack nuclei and mitochondria, distinct membrane-bound compartments to store DNA or generate energy, respectively. Those components, or organelles, characterize cells of the third branch, the eukaryotes. The prevailing thinking is that roughly 2 billion years ago, a microbe belonging to a group called the Asgard archaea absorbed a bacterium called an alphaproteobacterium, which settled inside and became mitochondria, producing power for its host by consuming oxygen as fuel. But isolating and growing Asgard archaea has proved a challenge, as they tend to live in inhospitable environments such as deep-sea mud. They also grow very slowly, so they are hard to detect. Most evidence of their existence so far has been fragments of DNA with distinctive sequences.

But Hiroyuki Imachi and Ken Takai, microbiologists from the Japan Agency for Marine-Earth Science and Technology in Yokosuka, and their colleagues have persisted in trying to grow such a microbe from a seabed core that a submersible brought up from a dive in 2006. [...] A year later, they detected microbes in one of the tubes, which also contained four antibiotics to kill any contaminating bacteria. DNA analyses of samples from the tube indicated it included an Asgard archaeon, the microbe they were hoping to grow. [...] Patience proved the key ingredient to what researchers are saying may be an important discovery about how complex life evolved. After 12 years of trying, a team in Japan has grown an organism from mud on the seabed that they say could explain how simple microbes evolved into more sophisticated eukaryotes.

Eukaryotes are the group that includes humans, other animals, plants, and many single-celled organisms. The microbe can produce branched appendages, which may have helped it corral and envelop bacteria that helped it—and, eventually, all eukaryotes—thrive in a world full of oxygen. “This is the work that many people in the field have been waiting for,” says Thijs Ettema, an evolutionary microbiologist at Wageningen University in the Netherlands. The finding has not yet been published in a peer-reviewed journal, but on Twitter, other scientists reviewing a preprint on it have already hailed it as the “paper of the year” and the “moon landing for microbial ecology.”

Having grown the microbe, the researchers used an electron microscope to image it, revealing multiple branched appendages. The team hypothesizes that, eons ago, an archaeon encircled the protomitochondrion and put it to work. The researchers propose that as the concentration of oxygen increased on early Earth, archaea like Prometheoarchaeum took in oxygen-using partners and did better than other microbes. [...] Previously, most researchers had assumed that the mitochondria were pulled into their archaeal hosts—the “outside in” theory, with the nucleus and the cell’s internal membranes evolving from engulfed components. The newly cultured microbe’s appendages suggest otherwise. These appendages surrounded the protomitochondrion and their membranes gave rise to the internal ones.

Ettema cautions that the archaeal ancestor to eukaryotic cells that lived 2 billion years ago may not have looked and acted just like Prometheoarchaeum. Moreover, DNA studies indicate that other archaea are more closely related to eukaryotes than this one. He expects, however, that the 12 years the Japanese team devoted to culturing this microbe will help him and others isolate and grow related archaea in the lab: “I’m sure it will not take 12 years to get the next Asgard into culture.” (MORE - details)
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