Mys­tery of mar­ine re­cyc­ling squad solved

One is miss­ing – in short, this sum­mar­izes the mys­tery. It is about ni­tri­fic­a­tion, the ox­id­a­tion of am­mo­nia via ni­trite to ni­trate, a key pro­cess in mar­ine ni­tro­gen cyc­ling. In the sea, both steps of this pro­cess are bal­anced and most avail­able ni­tro­gen ex­ists in the form of ni­trate, the fi­nal product of ni­tri­fic­a­tion. The or­gan­isms which are largely re­spons­ible for the first step of ni­tri­fic­a­tion in the ocean – the am­mo­nia ox­id­iz­ing ar­chaea - were dis­covered around a dec­ade ago, and it turns out that they are amongst the most abund­ant mi­croor­gan­isms on the planet.

The second part of ni­tri­fic­a­tion, the trans­form­a­tion of ni­trite to ni­trate, is car­ried out by ni­trite ox­id­iz­ing bac­teria, which mainly be­long to the Ni­tro­spinae phylum. Yet, Ni­tro­spinae are ten time less abund­ant than the am­mo­nia-ox­id­izers, rais­ing the ques­tion: is there an equally abund­ant, still un­dis­covered ni­trite ox­id­izer in the ocean?

“On the con­trary. Our res­ults in­dic­ate that Ni­tro­spinae are much more act­ive and grow much faster than the am­mo­nia-ox­id­iz­ing Ar­chaea. Thus, Ni­tro­spinae are clearly more ef­fi­cient than the Ar­chaea,” ex­plains Kath­ar­ina Kitzinger and adds: “As such, one would ex­pect the Ni­tro­spinae to be sig­ni­fic­antly more abund­ant. As this is not the case we as­sume that Ni­tro­spinae have such a low abund­ance be­cause they have a high mor­tal­ity rate. This ex­plains the bal­anced mar­ine ni­tri­fic­a­tion pro­cess in the ocean and makes the ex­ist­ence of fur­ther un­known, abund­ant ni­trite ox­id­izers un­likely.”

At the same time, the re­search­ers in­vest­ig­ated which ni­tro­gen com­pounds am­mo­nia-ox­id­iz­ing Ar­chaea and Ni­tro­spinae use for their cell growth. “While the Ar­chaea al­most ex­clus­ively grow us­ing am­monium, the Ni­tro­spinae seem to mainly use or­ganic ni­tro­gen, namely urea and cy­anate in­stead,” says Kath­ar­ina Kitzinger. “The util­iz­a­tion of or­ganic ni­tro­gen is likely key to the eco­lo­gical suc­cess of Ni­tro­spinae, as it al­lows them to avoid com­pet­i­tion with their friends, the Ar­chaea, on whom they de­pend on for ni­trite.” In this way the two mi­croor­gan­isms help each other: The Ar­chaea pro­duce ni­trite which serves the Ni­tro­spinae, while the Ni­tro­spinae pre­sum­ably re­lease some am­monium after they take up or­ganic ni­tro­gen. So, in turn, they provide the en­ergy source for the Ar­chaea – a sym­bi­otic win-win situ­ation.

The sci­ent­ists ac­quired their samples in the Gulf of Mex­ico, where the pro­cess of ni­tri­fic­a­tion is very im­port­ant due to the high nu­tri­ent in­put from rivers like the Mis­sis­sippi. “The mi­croor­gan­isms in­volved in ni­tri­fic­a­tion and their re­l­at­ive abund­ances are sim­ilar world­wide,” says Kath­ar­ina Kitzinger. “There­fore, it is very likely that our res­ults are also valid for the rest of the ocean.”

Kath­ar­ina Kitzinger, Han­nah K. Marchant, Laura A. Bris­tow, Craig W. Herbold, Cory C. Pa­dilla, Abiel T. Kid­ane, Sten Littmann, Hol­ger Daims, Petra Pje­vac, Frank J. Stew­art, Mi­chael Wag­ner, Mar­cel M. M. Kuypers: Single cell analyses reveal contrasting life strategies of the two main nitrifiers in the ocean. Nature Com­mu­nic­a­tions, Feb­ru­ary 2020

 DOI: 10.1038/s41467-020-14542-3

• Max Planck Institute for Marine Microbiology, Bremen, Germany
• Centre for Microbiology and Environmental Systems Science, University of Vienna, Austria
• Department of Biology, University of Southern Denmark, Odense, Denmark
• School of Biological Sciences, Georgia Institute of Technology, Atlanta, USA