September 29, 2023

Bacterial relatives of mitochondria have been identified

More than 1.6 billion years ago, the ancestors of our cells entered into symbiosis with bacteria that have played an indispensable role in cell metabolism ever since: mitochondria appeared. A study has now identified living sea bacteria that may be descendants of ancient bacteria. The new discoveries thus complete an important piece of the puzzle in the decades-old debate about mitochondrial origins.

Mitochondria are the powerhouses of our cells. During cellular respiration, they gain energy from sugar and oxygen, making them available to our cells. Scientists hypothesize that mitochondrial precursors, the so-called protomitochondria, were absorbed into other cells by endosymbiosis about 1.6 to 1.8 billion years ago. One indication of this is that, until now, mitochondria have their own DNA and are surrounded by a double membrane. Only through coexistence with the original bacterial powerhouses could eukaryotes, that is, cells with a cell nucleus, evolve to their present form and thus form the basis for the development of all complex organisms, from algae to humans.

List of metabolic features

“The exact bacterial origin of the mitochondria and thus also the origin of the aerobic metabolism of our cells remains controversial, despite the comprehensive genomic information available today,” explains a team led by Otto Geiger of the Autonomous University of Mexico in Cuernavaca. . “We combined several methods to identify the most likely living relatives of primitive bacteria from which mitochondria originated.” Previous research had already shown that the ancestors of mitochondria might have been called alphaproteobacteria. However, the genus of this large group of bacteria to which they belong has so far been controversial.

For their study, Geiger and his team identified specific characteristics that potential candidates must have. This includes not only the ability to metabolize oxygen, which is essential to the aerobic metabolism of our cells, but also the ability to produce certain fatty substances, called lipids. “One of these traits affects enzymes involved in the metabolism of cardiolipin, a typical prokaryotic lipid found only in the mitochondrial membranes of eukaryotic cells,” Geiger and his team explain. “The guiding principle of our strategy is that in the emergence of the first eukaryotic cell, there was a genomic transfer of metabolic traits from a bacterium that could have descendants alive today.”

Similar ecological niche as in the primeval ocean

Using genetics data, the researchers analyzed which of the proteobacteria met the criteria he set. A group of parasitic bacteria previously proposed as potential relatives of mitochondria, the parasite called Rickettsiales, was excluded because they lacked relevant metabolic properties. “Our new approach indicates that the proto-mitochondrial ancestor was most likely related to marine alpha-bacteria, which had not previously been considered for the evolution of mitochondria,” the research team wrote.

This resulted in the highest match with a bacterium of the genus Iodidimonadales, which was first described in 2016. Capable of both aerobic and anaerobic metabolism, this marine bacterium is found primarily in environmental niches where the oxygen concentration in the water varies widely. “This environment could resemble the conditions that prevailed in the oceans at the time that proto-mitochondrion evolved,” Geiger and his team explain. “Thus, our findings fit the emerging picture that bacteria that could regulate metabolism with or without oxygen, depending on environmental conditions, were most likely proto-mitochondrial progenitors.”

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Source: Otto Geiger (Center for Genome Sciences, UNAM Campus de Morelos, Cuernavaca, Mexico) et al., Science Advances, Available here. doi: 10.1126/sciadv.adh0066