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Study identifies relatives of microbe that became part of us

July 25, 2011
Courtesy of University of Hawaii - SOEST
and World Science staff

Bil­lions of years ago, a strange and fate­ful step in ev­o­lu­tion took place, bi­ol­o­gists say. Some bac­te­ria started liv­ing in­side oth­er cells, and over genera­t­ions lost the abil­ity to live any­where else. Their hosts al­so lost the abil­ity to do with­out them.

Now, they’re simply part of us. Those bac­te­ri­a’s de­scen­dants are “mi­to­chon­dria”: es­sen­tial, energy-generating sub­com­part­ments of nor­mal an­i­mal and plant cells.

A strain of SAR11 bacteria under an electron micro­scope. (Cred­it: Mich­ael Rappe, SOEST/UHM)


While most bi­ol­o­gists have ac­cept­ed this sto­ry for dec­ades—based on si­m­i­lar­i­ties be­tween bac­te­ria and mi­to­chon­dria (which have their own DNA and even look like bac­te­ri­a)—it has been a mys­tery just who these in­ter­lop­ers were, or where they came from.

Now, it may be less of a mys­tery.

A new study pro­poses mi­to­chon­dria are rel­a­tives of a line­age of ma­rine bac­te­ria known as SAR11, per­haps Earth’s most abun­dant group of mi­croor­gan­isms. It ac­counts for as many as one third of the cells found in the up­per oceans, and has pro­duced sim­ple, ef­fi­cient or­gan­isms well-a­dapt­ed to low-nutrient con­di­tions, ac­cord­ing to Or­e­gon State Uni­vers­ity sci­en­tists.

“This is a very ex­cit­ing disco­very,” said Mi­chael Rap­pé of the Uni­vers­ity of Hawaii-Manoa, one of the re­search­ers in the new stu­dy, pub­lished on­line June 14 in the jour­nal Na­ture Scien­tific Re­ports.

The pro­pos­al makes sense “in a lot of ways,” he added. “the phys­i­ol­o­gy of SAR11 makes them more apt to be de­pend­ent on oth­er or­gan­isms, and based on the con­tem­po­rary abun­dance of SAR11 in the glob­al ocean, the an­ces­tral line­age may have al­so been abun­dant in the an­cient ocean.” This abun­dance, he spec­u­lat­ed, could have led to fre­quent en­coun­ters be­tween SAR11 mi­crobes and “the host of the orig­i­nal sym­bi­o­sis event.” Sym­bi­o­sis is a rela­t­ion­ship of mu­tu­al de­pend­ence be­tween two spe­cies.

SAR11 bac­teria are part of plank­ton, float­ing mass­es of micro­scopic organ­isms of­ten eat­en by big­ger sea crea­tures.

Col­leagues of Rap­pé at Or­e­gon State Uni­vers­ity used sev­er­al in­ter­con­nected com­put­er pro­grams to com­pare the genomes of mi­to­chon­dria from widely rang­ing spe­cies, with those of var­i­ous SAR11 strains. In this way they con­clud­ed that the bac­te­ria and the mi­to­chon­dria must have had a com­mon an­ces­tor. The find­ings al­so led the re­search­ers to pro­pose the ex­ist­ence of a new family of bac­te­ria, Pelag­ibac­ter­aceae

The SAR11 lineage “con­tains a sig­nif­i­cant amount of ge­net­ic di­vers­ity, which po­ten­tially in­di­cates sig­nif­i­cant di­vers­ity in metabolis­m,” said Rap­pé.

Mi­to­chon­dria are a part of all “eukary­otic” cells, a com­plex, com­part­men­talized type of cell that char­ac­ter­izes an­i­mals, plants and fun­gi and their close single-celled an­ces­tors.


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Billions of years ago, a strange and fateful step in evolution took place, biologists say. Some bacteria started living inside other cells, and over generations lost the ability to live anywhere else. Their hosts also lost the ability to do without them. Now, they’re simply part of us. Those bacteria’s descendants are “mitochondria”: essential, energy-generating subcompartments of all animal and plant cells. While most biologists have accepted this story for decades—based on similarities between bacteria and mitochondria (which have their own DNA and even look like bacteria)—it has been a mystery who these interlopers were, or where they came from. Until now, apparently. A new study proposes mitochondria are relatives of a lineage of marine bacteria known as SAR11, perhaps the most abundant group of microorganisms. The group accounts for as many as one third of the cells found in the upper oceans, and has produced simple, efficient organisms well-adapted to low-nutrient conditions, according to Oregon State University scientists. “This is a very exciting discovery,” said Michael Rappe of the University of Hawaii-Manoa, one of the researchers in the new study. The proposal makes sense “in a lot of ways,” he added. “the physiology of SAR11 makes them more apt to be dependent on other organisms, and based on the contemporary abundance of SAR11 in the global ocean, the ancestral lineage may have also been abundant in the ancient ocean,” he went on. This abundance, he speculated, could have led to frequent encounters between SAR11 microbes and “the host of the original symbiosis event.” Symbiosis is a relationship of mutual dependence between two species. Colleagues of Rappe at Oregon State University used several interconnected computer programs to compare the genomes of mitochondria from widely ranging species, with those of various SAR11 strains. In this way they concluded that the bacteria and the mitochondria must have had a common ancestor. The findings also led the researchers to propose the existence of a new family of bacteria, Pelagibacteraceae. “The lineage of highly abundant marine bacteria known as SAR11 contains a significant amount of genetic diversity, which potentially indicates significant diversity in metabolism,” said Rappe. Mitochondria are a part of all “eukaryotic” cells, a complex, compartmentalized type of cell that characterizes animals, plants and fungi and their close single-celled ancestors.