Becoming and passing away in unexpected places
In the balance sheets of the oceans, huge amounts of nitrogen from the atmosphere are constantly being booked from debit to credit and back again. How long these account shifts take is important for climate researchers - and has apparently been misjudged so far. Life is the never-ending hunt for mates and procreation, for scarce resources like food, power, money and hopefully a few more things. But there shouldn't be a lack of at least one thing on earth: nitrogen. After all, our indispensable basic component from protein to genetic material molecules fills a proud eighty percent of the earth's atmosphere: so breathe in, and the coveted building material flows massively into your body.
And, unfortunately, when exhaling, out again just as quickly, without even a small N-atom being able to be stopped. The gaseous form of nitrogen, N2, is extremely stable, very inert and biochemically simply not tangible for the human body. It's the same with other animals and plants - they need help to get to the incredibly inert mass product N2.
This globally vital support is provided by biochemical specialists who, in the course of evolution, have learned the reaction mechanism of "nitrogen fixation" - they convert the useless N2 form into a mean one, too Field-forest-and-meadow metabolism, like that of human cells, usable form, such as nitrate, NO32- The occupational field is dominated of microbes that can be native to all sorts of places. Initially, the nitrogen fixers appealed to curious researchers because they colonize such exotic habitats as the guts of termites or the root nodules of pea relatives, where they maintain symbiotic relationships with the nitrogen-dependent host organisms. Much more often, however, the nitrogen fixers simply thrive in the soil - whereby archaea probably participate even more frequently than the "real" bacteria.
The proportion of fixed nitrogen in the sea can hardly be overestimated. In the ocean, business is a little different, especially apparently in other time dimensions and nicely geographically separated: In the warm, light-flooded surface water near the equator, masses of photosynthetic cyanobacteria bob up and down, which come from the nitrogen reservoirs N2 dissolved in the seawater Remove and fix. This only works until the other raw materials required for fixation run out, such as the rare resource iron. According to the conventional wisdom, this is less likely to happen in the Atlantic, because the sea here is repeatedly fertilized with iron by desert winds from the Sahara. The quantitatively decisive nitrogen fixers should therefore be the less resource-constrained microbes of the tropical Atlantic.
The disappearance of N2 in seawater is then compensated for by the opposite process: denitrification, in which nitrate is converted back into gaseous nitrogen. This happens in the oceans in the sediments, but especially in the low-oxygen layer between 200 and 700 meters water depth. So-called anammox bacteria, for example, play a role that has long been underestimated. Be that as it may: Denitrification takes place above all in the eastern tropical Pacific and in the Arabian Sea – so that, according to prevailing opinion, N2 is probably mostly fixed elsewhere (in the Atlantic) than it is created (in the Pacific and Arabian Seas). Until now, researchers thought that ocean currents would only compensate for this in a time frame of around a thousand years - which means that the system could only react very slowly and in the long term to sudden local interventions that are conceivable as a result of climate change.
Not quite right, say Curtis Deutsch and his colleagues. With the help of a computer, the researcher from the University of Washington has again taken on mountains of data that have accumulated during the analysis of water samples from all oceans. Their main focus was on the relationship between nitrate (NO32-) and phosphate (PO4), which had not previously been fully considered 3-). Even in the presence of feasting bacteria, this ratio normally remains constant in seawater, since average microbes eat both molecules equally.
However, a special case occurs in regions with many nitrogen fixers: These consume phosphate intensively, but ignore nitrate because they cover their nitrogen requirements with N2 – that PO 43-/NO32- ratio falls dramatically below the otherwise constant value.
Where this crash occurs and how strong it is should tell the researchers around Deutsch something about the local activity of nitrogen fixers. So far, a clear sagging had been detected, especially in the Atlantic - and that remained roughly the case in the studies by Deutsch and Co. Nevertheless, the evaluation of their analysis surprised the researchers: A few limited regions in the Pacific surface water were particularly noticeable, in which there was no less, even significantly more phosphate in relation to nitrate than the generally accepted mean.
This is clearly the work of particularly active anti-nitrogen fixers at depth, concludes Deutsch: Water rises to the surface from their habitat, from which the denitrifying bacteria have previously extracted much nitrate. The strangely shifted ratio of the relatively low-nitrate and phosphate-rich waters that have risen then very quickly return to the constant normal value on the surface. And that, so the inevitable conclusion, because the nitrogen fixers in the Pacific are also doing their job very thoroughly, using up phosphate and letting nitrate be nitrate.
Although the ratio rarely drops to the low-phosphate ratio that can be seen in the Atlantic and has always been interpreted as evidence of the high activity of the nitrogen fixers there - overall, however, the computer model showed that almost twice as much N 2 is fixed in the waters of the Pacific and Indian Oceans as in the Atlantic. The consequences on the PO43-/NO32- However, theratio is different in the two seas because denitrification in the Pacific is obviously much stronger than initially thought.
Overall, the results are likely to revolutionize the prevailing models of oceanic nitrogen cycles, say Douglas Capone and Angela Knapp of the University of Southern California. For example, it now seems clear that the cycles that bring about the balancing between the different forms of nitrogen are local rather than global - because N2 and NO3 2- doesn't just always arise in one ocean and perish in the other.
