South Pacific Gyre, Largest Ocean Desert, Driving Global Cycles

The middle of the South Pa­cific is as far away from land as you can pos­sibly get. Solar ir­ra­di­ance is dan­ger­ously high, reach­ing a UV-in­dex that is la­belled ‘ex­treme’. There are no dust particles or in­flows from the land and as a res­ult these wa­ters have ex­tremely low nu­tri­ent con­cen­tra­tions, and thus are termed ‘ul­trao­l­i­go­troph­ic’. Chloro­phyll-con­tain­ing phyto­plank­ton (minute al­gae) are found only at depths greater than a hun­dred meters, mak­ing sur­face South Pa­cific wa­ters the clearest in the world. Due to its re­mote­ness and enorm­ous size – the South Pa­cific Gyre cov­ers 37 mil­lion km2 (for com­par­ison, the US cover less than 10 mil­lion km2) –, it is also one of the least stud­ied re­gions on our planet.

There are five major ocean-wide gyres—the North Atlantic, South Atlantic, North Pacific, Indian, and South Pacific Ocean Gyre.

 Credit: © NOAA

Des­pite its re­mote­ness, both satel­lite and in situ meas­ure­ments in­dic­ate that the mi­croor­gan­isms liv­ing in the wa­ters of the South Pa­cific Gyre (SPG) con­trib­ute sig­ni­fic­antly to global biogeo­chem­ical cycles. Thus, the sci­ent­ists from Bre­men were in­ter­ested in dis­cov­er­ing which mi­crobes are liv­ing and act­ive in this ocean desert. Dur­ing a six-week re­search cruise on the Ger­man re­search ves­sel FS Sonne, or­gan­ized and led by the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy, Greta Re­intjes, Bernhard Fuchs and Tim Fer­del­man col­lec­ted hun­dreds of samples along a 7000 kilo­metre track through the South Pa­cific Gyre from Chile to New Zea­l­and. The sci­ent­ists sampled the mi­cro­bial com­munity at 15 Sta­tions in wa­ter depths from 20 to more than 5000 metres, that is, from the sur­face all the way down to the sea­floor.

Looking at our planet from the right side, there is a lot of water and little earth. RV Sonne crossed the SPG from Chile to New Zealand. The picture also shows chlorophyll concentrations derived from NASA imagery. Dark areas show the gyre middle or “desert”
. Credit: © modified from Google Earth / NASA

 

Low cell numbers and unexpected distributions

“To our sur­prise, we found about a third less cells in South Pa­cific sur­face wa­ters com­pared to ocean gyres in the At­lantic”, Bernhard Fuchs re­ports. “It was prob­ably the low­est cell num­bers ever meas­ured in oceanic sur­face wa­ters.” The spe­cies of mi­crobes were mostly fa­mil­iar: ”We found sim­ilar mi­cro­bial groups in the SPG as in other nu­tri­ent-poor ocean re­gions, such as Prochlorococcus, SAR11, SAR86 and SAR116”, Fuchs con­tin­ues. But there was also a sur­prise guest amongst the dom­in­ant groups in the well-lit sur­face wa­ters: AE­GEAN-169, an or­gan­ism that was pre­vi­ously only re­por­ted in deeper wa­ters.

 

At New Year’s Eve, the scientists were visited by this puffer fish. “The water in the South Pacific Gyre is incredibly blue”, says cruise leader Tim Ferdelman. “You can see very deep into the water, it is really beautiful.”
 Credit: © Tim Ferdelman / Max Planck Institute for Marine Microbiology

 

Re­intjes and her col­leagues dis­covered a pro­nounced ver­tical dis­tri­bu­tion pat­tern of mi­croor­gan­isms in the SPG. “The com­munity com­pos­i­tion changed strongly with depth, which was dir­ectly linked to the avail­ab­il­ity of light”, Re­intjes re­ports. Sur­pris­ingly, the dom­in­ant pho­to­syn­thetic or­gan­ism, Prochlorococcus, was present in rather low num­bers in the up­per­most wa­ters and more fre­quent at 100 to 150 meters wa­ter depth. The new player in the game however, AE­GEAN-169, was par­tic­u­larly nu­mer­ous in the sur­face wa­ters of the cent­ral gyre. 

 

Bernhard Fuchs busy sampling during the expedition

.Credit: © Tim Ferdelman / Max Planck Institute for Marine Microbiology

“This in­dic­ates an in­ter­est­ing po­ten­tial ad­apt­a­tion to ul­trao­l­i­go­trophic wa­ters and high solar ir­ra­di­ance”, Re­intjes points out. “It is def­in­itely something we will in­vest­ig­ate fur­ther.” AE­GEAN-169 has so far only been re­por­ted in wa­ter depths around 500 metres. “It is likely that there are mul­tiple eco­lo­gical spe­cies within this group and we will carry out fur­ther meta­ge­n­omic stud­ies to ex­am­ine their im­port­ance in the most oli­go­trophic wa­ters of the SPG.”

Methodological milestone

The cur­rent re­search was only pos­sible thanks to a newly de­veloped method that en­abled the sci­ent­ists to ana­lyse samples right after col­lec­tion. “We de­veloped a novel on-board ana­lysis pipeline”, Re­intjes ex­plains, “which de­liv­ers in­form­a­tion on bac­terial iden­tity only 35 hours after sampling.” Usu­ally, these ana­lyses take many months, col­lect­ing the samples, bring­ing them home to the lab and ana­lys­ing them there. This pipeline com­bines next-gen­er­a­tion se­quen­cing with fluor­es­cence in situ hy­brid­isa­tion and auto­mated cell enu­mer­a­tion. “The out­come of our method de­vel­op­ments is a read­ily ap­plic­able sys­tem for an ef­fi­cient, cost-ef­fect­ive, field-based, com­pre­hens­ive mi­cro­bial com­munity ana­lysis”, Re­intjes points out. “It al­lows mi­cro­bial eco­lo­gists to per­form more tar­geted sampling, thereby fur­ther­ing our un­der­stand­ing of the di­versity and meta­bolic cap­ab­il­it­ies of key mi­croor­gan­isms.”

Contacts and sources:
Max Planck Institute for Marine Microbiology

Citation: . On-Site Analysis of Bacterial Communities of the Ultraoligotrophic South Pacific Gyre.
Greta Reintjes, Halina E. Tegetmeyer, Miriam Bürgisser, Sandi Orlić, Ivo Tews, Mikhail Zubkov, Daniela Voß, Oliver Zielinski, Christian Quast, Frank Oliver Glöckner, Rudolf Amann, Timothy G. Ferdelman, Bernhard M. Fuchs Applied and Environmental Microbiology, 2019; 85 (14) DOI: 10.1128/AEM.00184-19