ATP -- Nick Lane --The Vital Question, Post #5 -- June 12, 2019

See this link, dated April 23, 2019.]

Archaea and bacteria, April, 2018.

A 2018 paper suggest alphaproteobacteria were not the ancestors. If accurate, as of 2018, we still have not identified the bacterium that became the mitochondrion. Having said that, researchers who "believe" the alphaproteobacteria story are not ready to accept this new study. But it looks like it will take much more research to up-end the alphaproteobacteria argument.

Mitochondria in the same family as Rickettsia.

Alphaproteobacteria, January 12, 2017. When I read this paper, I get the feeling that there could have been many cases of endosymbiosis, but the "ingestion" of alphaproteobacteria was the most successful. Any other cases/examples of bacterial endosymbiosis/mitochondrial generation is therefore lost.

Alphaproteobacteria, wiki.

The Alphaproteobacteria is a diverse taxon and comprises several phototrophic genera, several genera metabolising C1-compounds (e.g., Methylobacterium spp.), symbionts of plants (e.g., Rhizobium spp.), endosymbionts of arthropods (Wolbachia) and intracellular pathogens (e.g. Rickettsia). 

Moreover, the class includes (as an extinct member) the protomitochondrion, the bacterium that was engulfed by the eukaryotic ancestor and gave rise to the mitochondria, which are organelles in eukaryotic cells (See endosymbiotic theory). 

A species of technological interest is Rhizobium radiobacter (formerly Agrobacterium tumefaciens): scientists often use this species to transfer foreign DNA into plant genomes.

Aerobic anoxygenic phototrophic bacteria, such as Pelagibacter ubique, are alphaproteobacteria that are a widely distributed and may constitute over 10% of the open ocean microbial community.

Alphaproteobacteria: purple nonsulfur bacteria (today’s photosynthetic alphaproteobacteria).

See this link, dated 1999.