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Mitochondria Are Related to Ocean Bacteria, But Not to the Ones We Thought

The views expressed are those of the author and are not necessarily those of Scientific American.


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Your floor-model animal mitochondrion. Public domain. Click for link.

Two billion years ago, around the time atmospheric oxygen levels were rising, one cell engulfed another, and instead of becoming lunch, the ingestee became an Earth-changer and, eventually, a vital part of you: mitochondria.

These microscopic cell inhabitants/engines allowed their host cell to suddenly begin to burn oxygen when digesting their food, an energy source that vastly expanded the amount of energy they could harvest from a given morsel of food. The magic born of this union helped enable nearly all multicellular life on Earth to evolve and get big, complicated, and, in our case, hairy and prone to back problems.

Most multicellular organisms would agree it was a good move. However, there have long been fights among biologists as to whether this aborted lunch was actually the source of mitochondria, and then, when that was all settled, what the identity of the engulfer and engulfee actually were.

Many clues lead us to the former conclusion, not least of which is the fact that mitochondria contain their own DNA separate from your cells’ own nucleic DNA, and that mitochondrial DNA looks suspiciously like bacterial DNA. As for the latter, the identity of the engulfer is still hotly contested. But scientists have gotten closer to the identity of that first mitochondrion-to-be.

Rickettsia ricketsii, the cause of Rocky Mountain Spotted Fever, hanging out in some host cells. CDC; public domain. Click image for link.

For years, they have known it was likely an Alphaproteobacterium, and that the mitochondria are closely allied with the Rickettsiales, a group of largely parasitic intracellular bacteria that include organisms that cause typhus and Rocky Mountain Spotted Fever. That would make sense: both mitochondria and intracellular parasites obviously have lots of adaptations that make them good at living inside other cells.

But what is the identity of the extant free-living bacterium that is most closely related to the mitochondria/Rickettsiales? Recently, biologists identified it as some member of the SAR11 group of bacteria including, most notably, Pelagibacter ubique (which still has not been properly named according to the taxonomic code, so my use of italics is not quite right). As its name implies, it may be the most common bacterium on the planet. Under the right conditions, it can make up of half of the living cells in seawater, and likes to hang out in freshwater too. But a recent paper by a team of Swedish scientists in PLoS ONE has found that not only does it seem that mitochondria are *not* most closely related to this group, their closest free-living relatives might be quite obscure indeed.

To gauge the relationships between these bacteria and attempt to identify better the closest free-living mitochondrial relative, scientists looked at the sequences of the respiration-related mitochondrial genes. Scientists have done this before, but they tended to consider groups of bacteria with completely sequenced genomes, which are filled with medically and agriculturally-signficant  bacteria, because those are the ones that tend to interest us most financially and get their genomes sequenced. The authors of the study noted that this doesn’t cover very well the diversity of bacteria living in sunlit, oxygenated surface ocean water, the kind of environment in which mitochondria were likely to evolve.

So they turned to the Global Ocean Survey, a marine metagenomic sequencing initiative, to see what else was out there. Perhaps to their surprise, they found that the most closely related group in their samples was an obscure little group of bacteria that made up less than 1% of the cell population of ocean surface waters in their Global Ocean Survey samples. They named it Ocean Mitochondrial Affiliate Clade, or OMAC.

A relatedness (phylogenetic) tree of the Alphaproteobacteria based on the genes COX1 and COX2. OG is the outgroup. Couretesy PLoS ONE, Creative Commons License. Click image for link.

To explain the earlier results implicating SAR11 bacteria as mitochondrial relatives, they speculated that members of the SAR11 clade — including P. ubique — and members of the mitochondrial Rickettsiales clade are rich in the nucleotide bases adenosine and tyrosine in their DNA (as opposed to the bases guanine and cytosine). Because most other alphaproteobacteria are GC-rich,  this may be a convergent feature that lead them to falsely conclude they were related. When they looked at genes that were more essential, more conserved, less AT-rich, and likely more similar to the common ancestors of these bacteria, they found that the mitochondria/Rickettsiales were *not* closely related to SAR11/P. ubique.

The authors note that since we’ve always thought it was likely that aerobic respiration — the neat trick that mitochondria (and many other bacteria) do to harvest vastly more energy from their food using oxygen — evolved in ocean surface waters where oxygen was most likely to filter in first once the atmosphere started filling with oxygen (how that happened is a different story), it makes sense that mitochondria’s closest extant free-living relative lives there still. They also noted that after billions of years of climate change and mass extinctions, conditions are likely to have changed for the descendants of the last free-living ancestor of our mitochondria, and thus it’s not surprising they may not represent a huge proportion of the oceanic bacterial party.

Jennifer Frazer About the Author: Jennifer Frazer is a AAAS Science Journalism Award-winning science writer. She has degrees in biology, plant pathology/mycology, and science writing, and has spent many happy hours studying life in situ.
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Follow on Twitter @JenniferFrazer.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. Christopher Himes 3:14 pm 04/16/2012

    [...] из ленты +Bora Zivkovic: Mitiochondria Are Related to Ocean Bacteria, But Not to the Ones We Thought | The Artful Amoeba, Sci… Two billion years ago, around the time atmospheric oxygen levels were rising, one cell engulfed [...]

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  2. 2. Madame Etta 4:13 pm 04/16/2012

    I cannot adjudicate the accuracy, nor can I contest it. It was so interesting (I am a microbiology buff) and informative that all I did was read it. However, my compliments to the author, Jennifer Frazer, for making what could be considered a rather dull, scientific research article into a readable, enjoyable, and thoroughly well-written piece. I read it twice. Hats off to such excellent, thoughtful writing!

    Link to this
  3. 3. jtdwyer 7:10 pm 04/16/2012

    In retrospect, it seems quite reasonable that it would not be the most successful, ubiquitous bacterium that would be coopted by multicellular organisms but some marginal bacterium that could most greatly benefit from some support provided by its new host cells…

    Link to this
  4. 4. Jennifer Frazer in reply to Jennifer Frazer 10:45 pm 04/16/2012

    Thanks guys! So glad to hear you enjoyed it.

    Link to this
  5. 5. kdimoff 12:18 am 04/17/2012

    ok, so is this not the craziest amazingest thing ever?? how is this even possible? side question: i understand how embryos get their regular DNA, but how does an embryo get it’s own individual mitochondrial DNA? my mind is blown.

    Link to this
  6. 6. tiurlumphd 3:10 am 04/17/2012

    so who is the one in your mind? speaking about mitocondrial always remain me about phosporilation oxidative, energy transport between membran and desease which caused by DNA mitocondrial mutation on man. I didn’t find it in this paper.

    Link to this
  7. 7. David Marjanović 9:20 am 04/17/2012

    Perhaps to their surprise, they found that the most closely related group in their samples was an obscure little group of bacteria that made up less than 1% of the cell population of ocean surface waters in their Global Ocean Survey samples. They named it Ocean Mitochondrial Affiliate Clade, or OMAC.

    Then why does their tree show OMAC and Rickettsiales together as the closest relative of mitochondria? It shows OMAC and Rickettsiales as more closely related to each other than to mitochondria.

    how does an embryo get it’s own individual mitochondrial DNA?

    It doesn’t; it inherits the mother’s. The oocyte contains lots and lots and lots of mitochondria.

    (Sperm cells contain so few mitochondria that it usually doesn’t matter.)

    I didn’t find it in this paper.

    That’s because the paper isn’t titled “Everything about mitochondria”.

    Link to this
  8. 8. jgrosay 2:31 pm 04/18/2012

    If I’m right, although Mitochondria do have their own DNA, that is arranged in a circular shape, like for example the DNA in E coli, at least in humans a lot of DNA containing genes that regulate both the building of structural and functional proteins in the Mitochondria is already incorporated into the human cell’s nuclear DNA. This would make an unsurmontable wall for modern Frankensteins wanting to produce interspecies hybrids and that kind of stuff. The main value of mtDNA today, besides the rare diseases that have a mitochondrial basis and heredity, would be ascertaining the ethnic group your female parent and grandparent and so on belonged to, and the geographical places they lived in before you arriving to where you currently live. As you know, nationalgeographic.com/genographic is conducting a worldwide research about this.

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  9. 9. kdimoff 1:37 am 04/22/2012

    after i commented, i did find out that mitochondrial DNA is passed down from the mother (which is also incredibly interesting). so (jennifer, since you are so much more socially adept at answering a laywoman-who-just-likes-fun-science’s questions), does tracing mitochondrial DNA (instead of regular DNA) make it easier to do things like trace lineage when looking into things like ancient human/pre-human migrations or “first mother ever”, etc?

    Link to this
  10. 10. Another Week of GW News, April 22, 2012 [A Few Things Ill Considered] 12:03 pm 04/23/2012

    [...] 2012/04/16: SciAm:TAA: Mitiochondria Are Related to Ocean Bacteria, But Not to the Ones We Thought [...]

    Link to this
  11. 11. Jennifer Frazer in reply to Jennifer Frazer 3:40 am 04/26/2012

    Kati — Yes it does! In fact, there is a whole line of research and thought around this. They call the concept “Mitochondrial Eve”, and there’s been a lot of ink spilled over it. See here for more info. Because of higher mutation rates in mt DNA than in nuclear DNA it has also been used to identify people, especially through female lines of inheritance. The remains of Jesse James and Tsarina Alexandra and her children where identified using mitochondrial DNA in this way.

    David — I left out an important phrase in that sentence. It should read “most closely related free-living group”. The Rickettsiales aren’t free-living. I had hoped this was implied by the start of the previous paragraph “To gauge the relationships between these bacteria and attempt to identify better the closest free-living mitochondrial relative . . . ” but it never hurts to be as explicit as possible.

    Sorry for the delay in responding — you’ll see why in my next post!

    Link to this
  12. 12. Earthling Bulletin #4 | Earthling Nature 8:42 am 04/30/2012

    [...] Mitochondria are related to ocean bacteria, but not the ones we tought, by Jennifer Frazer, about the recently discovered relationships between mitochondria and the bacteria they came from, published at PLoS One. [...]

    Link to this
  13. 13. kdimoff 11:55 pm 05/11/2012

    so cool!

    Link to this
  14. 14. “The past isn’t dead. It isn’t even past.” « Rturpin's Blog 10:35 am 05/19/2012

    [...] scientists looking at data from Ventner’s Global Ocean Sampling expedition have identified a clad of ocean bacteria that is more closely related to eukaryote ribosomes than those previously so [...]

    Link to this
  15. 15. Steven 9:55 pm 08/7/2013

    It must have been a leap to get the nuclear DNA in the cell to control mitochondrial development.

    Link to this
  16. 16. cuitlahuacrivasg 3:02 am 03/4/2014

    [...] These microscopic cell inhabitants/engines allowed their host cell to suddenly begin to burn oxygen when digesting their food, an energy source that vastly expanded the amount of energy they could harvest from a given morsel of food. The magic born of this union helped enable nearly all multicellular life on Earth to evolve and get big, complicated, and, in our case, hairy and prone to back problems.” http://blogs.scientificamerican.com/artful-amoeba/2012/04/16/mitiochondria-are-related-to-ocean-bact... [...]

    Link to this

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