Page path:

20.09.2012 Un­usual sym­bi­osis dis­covered among mar­ine mi­croor­gan­isms

Tiny single-celled al­gae and ni­tro­gen-fix­ing bac­teria ex­change car­bon and ni­tro­gen in a mu­tu­ally be­ne­fi­cial re­la­tion­ship that helps fer­til­ize the oceans.
 
Unusual symbiosis discovered among marine microorganisms

Tiny single-celled algae and nitrogen-fixing bacteria exchange carbon and nitrogen in a mutually beneficial relationship that helps fertilize the oceans

An in­ter­na­tional team of sci­ent­ists from France, Ger­many and the USA have dis­covered an un­usual sym­bi­osis between tiny single-celled al­gae and highly spe­cial­ised bac­teria, the first sym­bi­otic re­la­tion­ship known between these types of or­gan­isms. Their part­ner­ship plays an im­port­ant role in mar­ine eco­sys­tems, fer­til­iz­ing the oceans by tak­ing ni­tro­gen from the at­mo­sphere and "fix­ing" it into a form that other or­gan­isms can use.
De­tails of the sym­bi­osis emerged from the in­vest­ig­a­tion of a mys­ter­i­ous ni­tro­gen-fix­ing mi­crobe with a drastic­ally re­duced gen­ome. First de­tec­ted in 1998 by Jonathan Zehr, a pro­fessor of ocean sci­ences at the Uni­versity of Cali­for­nia, Santa Cruz, it now ap­pears to be the most wide­spread ni­tro­gen-fix­ing or­gan­ism in the oceans. The mi­crobe be­longs to a group of pho­to­syn­thetic bac­teria known as cy­anobac­teria, but it lacks the genes needed to carry out pho­to­syn­thesis and other es­sen­tial meta­bolic path­ways. Ap­par­ently, its as­so­ci­ation with a pho­to­syn­thetic host cell makes those genes un­ne­ces­sary.

"The cy­anobac­terium is a ni­tro­gen-fixer, so it provides ni­tro­gen to the host cell, and the host cell provides car­bon to the cy­anobac­terium, which is lack­ing the meta­bolic ma­chinery to get its own car­bon," says Anne Thompson, co-first au­thor of the pa­per and a postdoc­toral re­searcher in Zehr's lab at UC Santa Cruz. Rachel Foster of the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy is the other lead au­thor and con­trib­uted equally to this work.

In or­der to un­ravel the mys­tery, the re­search­ers used tech­niques such as cell sort­ing and gene se­quen­cing. The host cell is a type of single-celled al­gae in a class known as "prym­ne­siophytes," which are found throughout the world's oceans. In sea­wa­ter samples sor­ted by flow cyto­metry, which sep­ar­ates cells by size and col­our, the host cells are sor­ted into the "pho­to­syn­thetic pi­c­oeuk­a­ryote" pop­u­la­tion, mean­ing tiny single-celled al­gae in the 1 to 3 mi­cron size range. The cy­anobac­teria are mostly seen in an in­dent­a­tion at one end of the host cell.
Confocal microscope images of a UCYN-A symbioses. The blue stains the DNA and the green results from a highly specific probe to the UCYN-A cell. Images are of cells collected in the North Pacific Ocean at Station Aloha. Arrows designate the UCYN-A cell found attached to a larger cell. Scale bar is 1.0 μm. (images are courtesy of co-author Dr. Niculina Musat and MPI colleague, Dr. Cristina Moraru and were taken with a Zeiss LSM510 Cofocal Microscope).
UCYN-A cells were detected and sorted by flow cytometry (left panel) that detects cells by their size (forward scatter, X-axis) and pigments (red fluorescence, Y-axis). Nitrogen fixation and carbon fixation were measured by incubating seawater with stable isotopes, sorting the cells by flow cytometry, and then quantifying stable isotopes of individual cells using secondary ion mass spectrometry (nanoSIMS)(Right panel). The green in the image shows were nitrogen gas was fixed into organic matter by UCYN-A and transferred to the host picoplankton. Scale bar 3.0 μm (Image courtesy of co-first author Rachel A Foster, MPI Bremen, Germany).
The nanoscale secondary ion mass spectrometry instrument (nanoSIMS 50L) housed at the Max Planck Institute for Marine Microbiology in Bremen, Germany, where the single cell stable isotope measurements of nitrogen and carbon fixation in the UCYN-A symbioses were performed. (Image courtesy of Manfred Schlösser, MPI Bremen, Germany).
Left: Photograph of seawater sampling during the BIOSOPE (Biogeochemistry and Optics South Pacific Experiment) cruise to the Southeast Pacific. (Image courtesy of co-author Daniel Vaulot of the Station Biologique, Roscoff, France).Right: Map of the stations in the North and South Pacific where UCYN-A symbioses were found.
"Aside from the im­port­ance of ni­tro­gen fix­a­tion in mar­ine eco­sys­tems, this is such an in­ter­est­ing sym­bi­osis from an evol­u­tion­ary per­spect­ive, be­cause it can be seen as ana­log­ous to an early stage in the en­dosym­bi­osis that led to chloro­plasts," ex­plains Zehr.
Chloro­plasts, which carry out pho­to­syn­thesis in all plants, evolved from sym­bi­otic cy­anobac­teria that even­tu­ally be­came in­cor­por­ated into their host cells in a pro­cess known as en­dosym­bi­osis. The newly dis­covered ni­tro­gen-fix­ing part­ner­ship is re­min­is­cent of the in­ter­ac­tions thought to have led to the evol­u­tion of chloro­plasts.. "At this point, it's un­clear ex­actly how the cy­anobac­teria are as­so­ci­ated with the host cells. It looks like there may be a little groove in the host cell where the cy­anobac­teria fits," says Thompson. "The as­so­ci­ation is ro­bust enough to go through the cell sorter and other pre­par­a­tions, but del­ic­ate enough that they sep­ar­ate if they're filtered or frozen and thawed."

In pre­vi­ous work Zehr's team had stud­ied the cy­anobac­teria, which they called UCYN-A, in samples pro­cessed at sea and brought back to the lab for cell sort­ing and ge­netic ana­lysis. Des­pite be­ing un­able to grow it in the lab, they were able to se­quence the mi­crobe's com­plete gen­ome and dis­cover that it was miss­ing the genes for sev­eral key meta­bolic path­ways, sug­gest­ing that it might live in as­so­ci­ation with an­other or­gan­ism. Thompson said re­search­ers were only able to see the sym­bi­otic part­ners to­gether when they sor­ted fresh sea­wa­ter samples on board the ship.

The ex­change of car­bon and ni­tro­gen between the two part­ners was demon­strated us­ing power­ful ana­lytic tech­niques de­veloped and car­ried out by col­lab­or­at­ors from the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy in Bre­men, Ger­many. Co-first au­thor Rachel Foster, sci­ent­ist at the In­sti­tute, de­scribes how they found out how the part­ner­ship between the two works. "We used la­belled sub­strate and could track down the in­ter­cel­lu­lar traffic. The strategy was to in­cub­ate the sea­wa­ter samples with stable iso­topes of car­bon (13C) and ni­tro­gen (15N), and then sort the 1-3.0 µm dia­meter cells by flow cyto­metry.”
A highly spe­cific ge­netic probe de­veloped by Niculina Musat, a Max Planck re­searcher, was used to identify the UCYN-A cells among the "pho­to­syn­thetic pi­c­oeuk­a­ryotes" sep­ar­ated out by the cell sort­ing equip­ment. Mar­cel Kuypers, dir­ector at the Max Planck In­sti­tute, adds: "This probe is la­belled with flu­or­ine (F), an ele­ment with a mass of 19, which like the car­bon and ni­tro­gen iso­topes can be de­tec­ted by mass in the high res­ol­u­tion nano­meter scale sec­ond­ary ion mass spec­tro­meter (nanoSIMS) in­stru­ment.” The nanoSIMS data proved the iden­tity of the UCYN-A and their as­so­ci­ation with an­other cell by the 19F sig­nal, and also quan­ti­fied and im­aged the car­bon and ni­tro­gen iso­topes within in­di­vidual cells.
"It's a great tool for mi­cro­bi­o­logy, coup­ling phylo­gen­etic iden­ti­fic­a­tion with meta­bolic ana­lysis," Thompson said. "We could see that the cy­anobac­teria were fix­ing the la­belled ni­tro­gen and trans­fer­ring it to the host cells."
Ge­netic ana­lysis of the host cell in­dic­ates its closest re­l­at­ive is the spe­cies Braarudosphaera bi­gelowii. In many spe­cies of prym­ne­siophytes, in­clud­ing B. bi­gelowii, the cells form ex­ternal cal­ci­fied plates, sug­gest­ing that the host cell in the sym­bi­osis may have plates that are eas­ily dis­lodged dur­ing pro­cessing of sea­wa­ter samples. "That would be im­port­ant, be­cause cells with plates sink faster than other or­gan­isms, so the car­bon they fix could end up be­ing trans­por­ted to the deep sea or the sea­floor," ex­plained Zehr..
Zehr noted that it is very dif­fi­cult to es­tim­ate the con­tri­bu­tion of this sym­bi­osis to global car­bon and ni­tro­gen cycles. Other al­gae are more abund­ant and prob­ably much more im­port­ant in terms of oceanic car­bon fix­a­tion than the algal host in this sym­bi­osis. But the cy­anobac­terial part­ner prob­ably makes a sig­ni­fic­ant con­tri­bu­tion to global ni­tro­gen fix­a­tion in the oceans, he said.
"Plank­tonic sym­bi­oses are very un­der­stud­ied and dif­fi­cult to study, as the as­so­ci­ations are of­ten fra­gile and dif­fi­cult to keep in­tact," said Foster. "Here we used mul­tiple tools and kept the re­la­tion­ship in­teg­rity, and also iden­ti­fied one of the first ex­amples of a seem­ingly mu­tu­al­istic part­ner­ship present in the plank­ton."
Zehr has named the cy­anobac­terium Candidatus Atelocyanobacterium thalassa. ("Candidatus" in­dic­ates a can­did­ate or pro­vi­sional name, since the rules of bac­teri­olo­gical no­men­clature re­quire that a mi­crobe be grown in cul­ture be­fore the name be­comes of­fi­cial.)
In ad­di­tion to Thompson and Zehr, the coau­thors of the pa­per in­clude co-first au­thor, Rachel Foster, An­dreas Krupke, Niculina Musat, and Mar­cel Kuypers of the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy; Brandon Carter of UC Santa Cruz; and Daniel Vaulot of the Pierre and Marie Curie Uni­versity in Paris. This re­search was fun­ded by the Gor­don and Betty Moore Found­a­tion and the Max Planck So­ci­ety.


For more information please contact


Dr. Rachel Foster, +49 421 2028 655, rfoster@mpi-bre­men.de

Prof. Dr. Mar­cel Kuypers, +49 421 2028 602, mkuypers@mpi-bre­men.de

or con­tact the press of­ficers of the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy

Dr. Man­fred Schloesser, +49 421 2028704, mschloes@mpi-bre­men.de
Dr. Rita Dunker, +49 421 2028856, rdunker@mpi-bre­men.de

Original article
Novel uni­cel­lu­lar cy­anobac­terium is sym­bi­otic with a single-celled eu­k­a­ryotic alga. Anne W. Thompson, Rachel A. Foster, An­dreas Krupke, Brandon J. Carter, Niculina Musat, Daniel Vaulot, Mar­cel MM Kuypers, & Jonathan P. Zehr
Sci­ence 21 Septem­ber, 2012, doi/​10.1126/​sci­ence.1222700
 
Back to Top