Diazotrophs: The ocean’s natural fertilizers

Similar to the human body that contains trillions of microbial cells that help us with our metabolic functions and health, marine microorganisms sustain major biogeochemical cycles and the function of all life in the ocean. Among those microorganisms, there are very special bugs that are able to fix di-nitrogen, called diazotrophs. Nitrogen is a key element for all organisms to grow as it occurs, for example, as a building block for DNA or proteins. Therefore, diazotrophs play a key role in the food web and global nitrogen cycle.

Breaking a triple bond

Di-nitrogen makes up ~ 78% of the earth’s atmosphere, but due to its strong triple bound (enthalpy = +945.5 kJ, which is roughly the energy it would take to bring 500 L of water to boil!) it is inaccessible to use as a nitrogen source for most organisms. Diazotrophs found a way to break this strong bond and turn the di-nitrogen into ammonia and organic nitrogen, which can then be used by other organisms. You might have heard about that there is also an artificial way, for which the two chemists Haber and Bosch won a Nobel prize. Their finding involved turning atmospheric nitrogen into ammonia which is used as fertilizers to increase productivity of agricultural land (the Green Revolution) but also disrupting natural habitats.

Diazotrophs have been fixing nitrogen for already hundreds of millions of years, and they contribute the largest fraction of natural bioavailable nitrogen to ecosystems. Yet, their contribution can be seen as much more resourceful than their artificial counterpart, because diazotrophs particularly occur where they are most needed. On land, they can be found as small nodules in the roots of trees, directly providing nitrogen to trees or in the ocean where other sources of nitrogen are limited and restrict phytoplankton growth.

Eddies as a hotspot for diazotrophs

Anticyclonic eddies, such as the one we are currently studying, can potentially be a hotspot for diazotrophs as they trap water – and with it the microbial community living within – for a longer amount of time. During the eddy’s lifetime, nitrogen becomes more and more depleted and the only way to have new, bioavailable nitrogen is through diazotrophically mediated di-nitrogen fixation.

During the cruise we are sampling these diazotrophs from different depths and across the eddy, by analyzing the DNA of all cells in the different water samples. Due to the structure of the eddy its center is more nutrient depleted potentially fueling nitrogen fixation more than at the edges of the eddy. Using multiple incubation bottles spiked with 15N (a stable istope that can be used as a tracer), we are also measuring their activity to see whether the diazotrophs are actively fixing di-nitrogen (notably, the presence of diazotrophs doesn’t necessarily imply that they are actively fixing nitrogen). We are curious to see how the communities and their activities will change between the center of the eddy and across the edges, but we will only find out after some more analyses in the lab back on shore!

Preparing all the incubations while filtering for DNA at the same time