ANT-XXVI/4, Weekly Report No. 4

27 April - 3 May 2010

The doldrums lie behind us and we welcome the fresh breeze that the northeast trade wind is providing. The fainter blue of the sky and its slightly yellowish colour above the horizon are clear signs of the presence of atmospheric dust. For several days now the model predictions have been showing a marked aerosol band at 10-15°N reaching from West Africa all the way to the Caribbean. And indeed the filters from our aerosol collectors show a yellow-brown colour after having been so immaculately white for weeks. Anyway, the dust theme has been addressed in quite detail in my 3rd weekly report.

I will dedicate my 4th weekly report to the presentations of two more research topics of our current cruise ANT-XXVI/4, which at first sight appear entirely unconnected by at closer look turn out to be very much linked. One research topic deals with the biogeography of marine nitrogen fixing organisms, which is carried by a biological working group from the Leibniz Institute for Marine Sciences in Kiel (Wiebke Mohr, Scarlett Sett). The other topic encompasses chemical measurements of hydrogen, which are carried out by Sylvia Walter from the Institute for Marine and Atmospheric Research of Utrecht University, Netherlands (IMAU).

In many ocean regions, macro nutrients such as nitrogen or phosphorus are limiting factors of marine primary production. Most organisms require nitrogen in combined form, i.e. as nitrate or ammonia. However – in contrast to molecular nitrogen (N₂) – their concentrations are typically very low in surface waters. Only nitrogen fixing organisms, also known as diazotrops, are capable of breaking the triple bond of molecular nitrogen to use it as their source of nitrogen – very much like Rhizobium bacteria that live in symbiosis with certain land plants. For this they require the special and evolutionary very old enzym complex nitrogenase. Nitrogen fixers themselves can be limited, however, by the availability of nutrients such as phosphorus and iron. It is known that these elements are released to seawater from atmospheric dust that is deposited on the ocean (see 3rd weekly report). Therefore dust plays a very important role for marine nitrogen fixation. The tropical north Atlantic with its strong Saharan dust imprint is a particularly interesting region for this kind of research.
The work program of this cruise is primarily focused at the biogeography of nitrogen fixing organisms. Using molecular tools, the abundance and transcription of the nifH gene, which encodes a major part of the nitrogenase enzyme complex, are determined. By taking water samples from various depths insight can be gained not only into the regional but also into the vertical distribution of nitrogen fixers. Additional incubation experiments with isotopically labelled nitrogen (¹⁵N₂) allow the determination of nitrogen fixation rates under ambient temperature and light conditions.

Hydrogen is one of the promising energy sources of the future. But a hydrogen-based energy would certainly come with the risk of significant anthropogenic hydrogen emissions. In contrast to other trace gases rather little is known about the atmospheric cycle of hydrogen and its major sources and sinks. In order to be able to estimate the potential influence of hydrogen emissions on the chemistry of the atmosphere, which would be prerequisite of large-scale hydrogen-based energy production, a better understanding of the hydrogen cycle is clearly needed. The ocean serves as a natural source for atmospheric hydrogen which is produced by photochemical degradation of organic matter as well as by biological processes by organisms such as – and this is the exiting link to the work on nitrogen fixers – by marine diazotrophs such as Trichodesmium. During this cruise hydrogen (and carbon monoxide) is continuously measured in surface seawater and air. In addition, discrete surface water samples are taken for shore-based analysis of hydrogen isotopes at IMAU, which will provide information about the hydrogen sources. The main focus of this cruise lies on the distribution patterns of hydrogen between the hemispheres and in the different ocean regions.  
This example of scientifically and methodologically rather different approaches, which look at tightly connected processes from different angles and can provide valuable additional information to each other is rather typical for the ocean as a natural system and also for marine sciences in general.

I am closing this report with the warmest regards from all scientists on board the R/V Polarstern,

Arne Körtzinger