More than 1.6 billion years ago, the precursors of our cells entered into a symbiosis with bacteria that have played an indispensable role in cell metabolism ever since: the mitochondria emerged. A study has now identified living sea bacteria that are probably descendants of the ancient bacteria. The new findings thus complete an important piece of the puzzle in the debate about the origins of mitochondria that has been going on for decades.
The mitochondria are considered the powerhouses of our cells. During cell respiration, they gain energy from sugar and oxygen, which they make available to our cells. Scientists assume that the precursors of the mitochondria, the so-called protomitochondria, were absorbed into other cells via endosymbiosis around 1.6 to 1.8 billion years ago. One indication of this is that to date, mitochondria have their own DNA and are surrounded by a double membrane. Only through the symbiosis with the original bacterial power plants could the eukaryotes, i.e. the cells with a cell nucleus, develop into their present form and thus form the basis for the development of all complex organisms, from algae to humans.
List of metabolic traits
"The exact bacterial origin of the mitochondria and thus also the origin of the aerobic metabolism of our cells is still controversial, despite the extensive genomic information that is available today," explains a team led by Otto Geiger from the Autonomous University of Mexico in Cuernavaca. "We combined several approaches to determine the most likely living relatives of the primordial bacteria from which the mitochondria developed." Previous research had already shown that the ancestors of the mitochondria were probably so-called alphaproteobacteria. To which lineage of this large group of bacteria they go back, however, has so far been controversial.
For their study, Geiger and his team defined specific characteristics that potential candidates should have. This not only includes the ability to metabolize oxygen, which is fundamental to the aerobic metabolism of our cells, but also the ability to produce certain fatty substances, so-called lipids. "One of these features affects the enzymes involved in the metabolism of cardiolipin, a typical prokaryotic lipid that is only found in the mitochondrial membranes of eukaryotic cells," explain Geiger and his team. "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 surviving today."
Similar ecological niche as in the primordial ocean
Using phylogenetic data, the researchers analyzed which alphaproteobacteria meet the criteria they set. A group of parasitic bacteria previously suggested as possible relatives of mitochondria, the parasitic so-called Rickettsiales, were excluded because they lack the relevant metabolic characteristics. "Our new approach indicates that the protomitochondrial ancestor was likely related to marine alphaproteobacteria, which have not previously been considered for mitochondrial evolution," write the research team.
This resulted in the highest match with bacteria of the genus Iodidimonadales, which was first described in 2016. Capable of both aerobic and anaerobic metabolism, these marine bacteria are found primarily in ecological niches where the oxygen concentration of the water varies greatly. "This environment could resemble the conditions that prevailed in the oceans at the time when the protomitochondria developed," explain Geiger and his team. "Thus, our findings fit the emerging picture that a bacterium that could organize its metabolism with or without oxygen, depending on environmental conditions, was the most likely ancestor of the protomitochondria."
Source: Otto Geiger (Center for Genomic Sciences, UNAM Campus de Morelos, Cuernavaca, Mexico) et al., Science Advances, doi: 10.1126/sciadv.adh0066