Nitrogen processes and contributors in the sediments of the Gulf of Finland, Baltic Sea

Nitrogen availability controls phytoplankton production in oceans. Research on nitrogen cycling is challenging due to the complicated, mainly microbially mediated pathways of fixation, metabolism and final reduction back to atmospheric N2. An average 30 % of the annual nitrogen loading has been calculated to be removed by denitrification (sequential reduction of nitrate to nitrogen gas via nitrite, nitrous oxide and dinitrogen oxide) in the Gulf of Finland, but recently another nitrogen removing process, anammox, has been found in the area, too, challenging the validity of the earlier nitrogen removal estimates. So far there is no data available from the Baltic Sea of this process, in which ammonium is oxidized with nitrite to form nitrogen gas. Nitrification, a process producing nitrite and nitrate from ammonium, has so far only been studied in the water column of the Baltic Sea. Since nitrification is the key process producing substrates for both denitrification and anammox, the gap in knowledge directly affects our understading of the other two processes as well. Recently it has also been suggested that in the sediments of the Gulf of Finland, a considerable amount of fixed nitrogen enter neither denitrification, nor anammox pathway, but a process called dissimilatory nitrate reduction to ammonium, DNRA. Unlike denitrification and anammox, this process does not remove nitrogen from the water ecosystem, but stores it in the water, to be used by algae and bacteria again. Clearly, there is a need to re-evaluate the current conception of nitrogen cycling in the Baltic Sea.
Environmental bacteria have an essential role in mineralization processes in ecosystem, and in the recovery from internal or external loading of nutrients and pollutants. The microbial biodiversity of the Baltic Sea is virtually unexplored, and hardly any data exist about the bacteria participating in nitrogen cycling processes. The sediment microbes of the brackish Baltic Sea are even less known. No data exists about the diversity or genetic potential on nitrogen cycling bacteria in the sediments.
The proposed project aims at clarifying the relative importance and controlling factors of the different pathways in nitrogen cycling. It will quantify the nitrogen removal in the Gulf of Finland, Baltic Sea. The project covers all the main nitrogen cycling processes at the sediment-water interface, that is, nitrification, denitrification, anammox and DNRA. The microbial community behind the processes will be studied at DNA level to reveal genetic potential for nitrogen transforming processes and the active fraction of nitrogen transforming microbes and changes in the fraction seasonally and due to changes in environmental conditions will be studied using RNA level analyses.

The project combines the description of genetic potential for microbial nitrogen transforming processes, revealed by DNA, and expression of activity, revealed by RNA, to the measurements of actual process rates.