 We study the global biogeochemical nitrogen cycle in the oceans. Nitrogen is especially important for all living organisms because it's a really abundant component of our bodies or cells. Therefore it's really important to understand how the cycling and interconversion of nitrogen compounds works in the environment. In my research I focus especially on one process of the marine nitrogen cycle which is exclusively carried out by microorganisms. This is the process of nitrification. Even though it has one name it actually consists of two distinct processes. The first one ammonia oxidation to nitrite and then the second one where nitrite is further oxidized to nitrate. This process in the marine environment appears to be really balanced. So both the first step ammonia oxidation and the second step nitrite oxidation appear to be really efficiently coupled because if we look at the inventories of the compounds relevant for nitrification in the oceans we see that both ammonium and nitrite are hardly detectable whereas the largest majority of available nitrogen compounds occurs in the form of nitrate. Keeping these two balanced processes in mind when we then look at the abundances of the microorganisms that catalyze these two processes these abundances are really far from balanced. The first step of nitrification, ammonia oxidation is catalyzed by extremely abundant microbes belonging to specific archaea the ammonia oxidizing archaea and they can make up up to 40% of the microbial community whereas the second step of nitrification, nitrite oxidation to nitrate is carried out by microbes that only make up about 1% of the microbial community the nitrite oxidizing bacteria. In our research we tried to understand the mechanisms that govern these two very different abundances of the key microbes catalyzing very balanced processes. We had the hypothesis that the ammonia oxidizing archaea which are so abundant in the marine environment may actually have the ability to use additional substrates in addition to ammonium namely organic nitrogen compounds such as urea and cyanate which can be really abundant in the marine water color. On the other hand we try to understand why the nitrite oxidizers do not match the abundance of the ammonia oxidizers and what the underlying mechanisms for this discrepancy may be. To answer these questions we had to find a way to both assess abundances of our target microorganisms. We had to see how fast their activity is so how fast nitrification occurs in the environment and we had to see how fast our microbes actually grow in situ so within their natural environment. To achieve this we made use of stable isotopes of both carbon and nitrogen which differ from the normal elements simply by having one neutron more in the atom nucleus. So we used 15n labeled compounds in our case this was urea and cyanate so the two organic nitrogen compounds that we investigated to track both nitrification activities starting from these compounds by tracking the 15n into the nitrite pool and we also looked at single cell growth rates by tracking isotope incorporations or heavy isotope incorporation into single AOA cells and also into single nitrite oxidizer cells and by this we could actually compare their environmental growth rates and activity within the same samples doing field incubations. In addition to these activity based measurements that we did in the field we also looked at the genomic capacity of our target microorganisms so which processes they actually encode the genes for and whether this makes sense with our field based experimental observations. What we found was actually regarding this high abundance and potential metabolic versatility of AOA in the oceans was that in addition to ammonia our highly abundant AOA were actually able to utilize both urea and cyanate the supplied alternative substrates as additional energy sources. So that means that the activity of AOA in the marine environment may be actually uncockled from the availability of ammonium and this ability to use alternative substrates might explain their high abundance in the world oceans. What we found especially intriguing was that AOAs could use cyanate despite lacking the canonical genes that are required for cyanate breakdown. However, we could also show that even cultured representatives are able to use cyanate without having these normal cyanate degrading genes which really opens up quite a large potential for marine AOA to use this substrate. What we found in relation to what explains these two very different abundances of AOA and nitrite oxidizers in the marine environment we were actually really surprised because previously this difference in abundance has been mainly attributed to the difference in theoretical energy gain when comparing ammonia oxidation and nitrite oxidation because based on thermodynamics ammonia oxidation should actually give a lot more energy than nitrite oxidation which might explain why one group of organisms is more abundant than the other. However, what we found in our field experiments where we could really compare both AOA and nitrite oxidizers in the same samples was that in contrast to what was previously thought we found that nitrite oxidizers despite gaining so little energy from the process that they catalyzed that they were extremely efficient in translating the energy that they gain from nitrite oxidation to biomass growth. This actually indicates that nitrospinase or nitrite oxidizers in general should be much more abundant in the marine environment than they actually are and based on our measurements the only factor that can actually explain and maintain the very different abundances between AOA and nitrospina or nitrite oxidizers is a very high mortality rate of the nitrite oxidizers that keeps this population size extremely low. Currently quite many of our ocean models on nitrification actually rely on the availability of ammonium to predict nitrification occurring and our findings actually show that ammonia oxidizers can evade kind of ammonium availability by simply switching their metabolism to use alternative substrates like urea and cyanate. Regarding the different abundances of AOA and nitrite oxidizers in the marine environment our findings are really important on one hand because they show that only because an organism doesn't occur at very high abundances they can still contribute a lot to an ecosystem process especially in the case of nitrite oxidizers to such a vital one within the nitrogen cycle and it also is relevant because both AOA and nitrite oxidizers are actually autotrophs meaning they fix their own carbon from CO2 into biomass to grow because nitrite oxidizers experience this very high mortality rate this actually could transport quite a large proportion of fresh organic carbon and also nitrogen to the dark microbial food web kind of providing them with fresh yummy substrates when nitrite oxidizers lies. For now we have mainly studied this phenomenon in one area which is a relatively shallow sea the Gulf of Mexico. The next steps or the next logical steps would be to investigate metabolic versatility of AOA in the large oligotrophic water bodies of the open ocean where ammonium is typically much more limiting than in areas closer to the coast it is really intriguing because in these large oligotrophic water bodies the availability of urea and cyanate and other organic nitrogen compounds is typically much higher than that of ammonium so it would make a lot of sense for AOA in these regions to actually tackle or use these organic substrates in addition to ammonium and from preliminary work that we have done in a more oligotrophic oceanic region we actually see that the relative importance of organic nitrogen use is much higher compared to more coastal regions that we have studied before taking a step further metabolic versatility is probably not restricted to this one group of microbes that we have studied so far the nitrify is ammonia oxidizes and nitrite oxidizes but it probably characterizes almost all microbial groups in the ocean and actually everywhere we are still in infant shoes kind of trying to understand how important this metabolic versatility might be and how it also affects nutrient cycling in general