Small bug, large im­pact: A new key player in the mar­ine ni­tro­gen cycle

Ni­tro­gen is es­sen­tial to all forms of life: It is part of pro­teins, nuc­leic acids and other cel­lu­lar struc­tures. However, many oceanic re­gions are lim­ited by the avail­ab­il­ity of ni­tro­gen, such as the trop­ical North At­lantic. In this re­gion, ni­tro­gen sup­ply to the open ocean comes mainly from ni­tro­gen fix­a­tion and to a lesser ex­tent from at­mo­spheric de­pos­ition. Ni­tro­gen fix­a­tion is the trans­form­a­tion of the in­ert ni­tro­gen gas from the at­mo­sphere into other forms of ni­tro­gen, which can then be taken up by other mi­croor­gan­isms such as primary pro­du­cers. The or­gan­isms per­form­ing ni­tro­gen fix­a­tion are called ni­tro­gen fix­ers or diazo­trophs. They can be con­sidered the fer­til­izers of the large areas of the nu­tri­ent-lim­ited oceans. Thus, it is cru­cial to de­term­ine the factors and key play­ers of ni­tro­gen fix­a­tion in or­der to un­der­stand how global changes might im­pact ni­tro­gen fix­a­tion in the fu­ture and the pos­sible res­ult­ing im­pacts on the ocean’s pro­ductiv­ity.

Clara Martínez-Pérez and co-au­thors from the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy in Bre­men (MPI Bre­men), the Uni­versity of Kiel, and the GEO­MAR in Kiel have now as­sessed the con­tri­bu­tion of one of the most abund­ant diazo­trophs (UCYN-A) in the ocean to total ni­tro­gen fix­a­tion in the trop­ical North At­lantic. Des­pite its re­l­at­ively high abund­ance com­pared to other diazo­trophs, the activ­ity and con­tri­bu­tion of this small diazo­troph, liv­ing in sym­bi­osis with a small uni­cel­lu­lar alga, has never been de­term­ined be­fore. Their sur­pris­ing res­ults show that pre­vi­ous know­ledge about the key play­ers in ni­tro­gen fix­a­tion might have to be re­vised.

The trop­ical North At­lantic, har­bor­ing about one quarter of global ni­tro­gen fix­a­tion, has pre­vi­ously been be­lieved to be dom­in­ated by Trichodesmium, a fil­a­ment­ous cy­anobac­terium bloom­ing in such large num­bers that it can be seen with the na­ked eye and by satel­lites. “However, there are many other diazo­trophs in the ocean whose im­port­ance had not yet been quan­ti­fied”, Martínez-Pérez ex­plains. To quantify the im­port­ance of UCYN-A, the sci­ent­ists used sev­eral meth­ods in­clud­ing a NanoSIMS, which al­lows for the de­tec­tion and quan­ti­fic­a­tion of the activ­ity of in­di­vidual cells. “By this, we can quantify the eco­lo­gical role of UCYN-A in the mar­ine ni­tro­gen cycle, which is es­sen­tial in­form­a­tion for global mod­els of nu­tri­ent cyc­ling”, says co-au­thor Wiebke Mohr from the MPI Bre­men.

Their res­ults were up for a sur­prise: Martínez-Pérez and her col­leagues show that UCYN-A is as im­port­ant for ni­tro­gen fix­a­tion in the trop­ical North At­lantic as is Trichodesmium. “Al­though Trichodesmium cells were very nu­mer­ous, they were not fix­ing much ni­tro­gen gas", says Martínez-Pérez. In con­trast, the much smal­ler UCYN-A was very act­ive. Liv­ing in as­so­ci­ation with a small alga also means that UCYN-A not only fixes ni­tro­gen for it­self but also the host alga. As a res­ult, it con­trib­uted as much as Trichodesmium to the total ni­tro­gen fix­a­tion across the trop­ical North At­lantic.

Oceanic cosmopolitans

The sci­ent­ists fur­ther ex­plored the global dis­tri­bu­tion of UCYN-A. They are found all over the oceans from the Arc­tic to the Ant­arc­tic circles (which is not the case for Trichodesmium which is usu­ally in wa­ters above 20 °C). “Thus, UCYN-A has the po­ten­tial to be one of the main con­trib­ut­ors to ni­tro­gen fix­a­tion not just in the trop­ics but around the globe”, says Martínez-Pérez. It is in­ter­est­ing to note that, des­pite their activ­ity and eco­lo­gical rel­ev­ance, the abund­ance of these sym­bi­oses is very low com­pared to other mi­croor­gan­isms in the oceans. “One of the ma­jor chal­lenges when quan­ti­fy­ing these or­gan­isms was to ac­tu­ally find them un­der the mi­cro­scope, since they are so rare”, ex­plains Mohr. The low abund­ance of these or­gan­isms im­plies that they are rap­idly con­sumed by grazers or oth­er­wise re­moved from the sur­face wa­ters. This would res­ult in a very ef­fi­cient trans­fer of the fixed ni­tro­gen to the oceanic food web, and sug­gests that the con­tri­bu­tion of UCYN-A to ni­tro­gen fix­a­tion is even higher than quan­ti­fied here. “Next, we would like to look into other re­gions of the ocean and quantify the abund­ance and activ­ity of UCYN-A there. This will al­low for deeper in­sights into their global role”, con­cludes Martínez-Pérez.