The role of Archaea in the nitrogen cycle of coastal lagoon sediments

Coastal aquatic transition ecosystems are hot-spots of organic matter and nutrient fluxes between terrestrial and marine aquatic ecosystems. However, the microbial and biogeochemical controls of many of these processes are still poorly understood. In particular, microbial activities in transitional area are important for aquatic nitrogen cycling. We have analysed the abundance and community structure of prokaryotes in sediments from three distinct sites of Sacca di Goro of the Po estuary (Italy) with specific chemical, physical and biological characteristics. This research significantly advances our understanding of the role of Bacteria and Archaea in coastal transition systems subject to high inputs of nitrogen and under strong anthropogenic pressure. First results suggest that Archaea are an important component of microbial community (20%) overlooked until now. They are also constant along the layers investigated, while Bacteria tend to decrease in the subsurface. In conclusion the abiotic parameters (temperature, dissolved oxygen, pH and salinity) explain the variations in the composition of prokaryotic communities. The niche partitioning between aerobic ammonia oxidizing Bacteria (AOB) and Archaea (AOA) is currently an important subject of study, but the factors that regulate and control the distribution and activity of ammonium oxidizing organisms (AOO) is still not clear. Recently it has been described that the availability of ammonium, the trophic status as well as the pH can influence the niche of AOO. In the second part of my thesis I have analysed the abundance and community structure of prokaryotes and nitrifiers, and nitrification in estuarine sediments from three distinct sites, Giralda, Gorino and Mare with specific chemical, physical and biological characteristics. The sampling area was the Sacca di Goro Lagoon of the Po estuary (Italy). Microcosm stable isotope probing (SIP) experiments with 13CO2 were carried out to identify dominating populations of ammonia oxidisers, and to clarify whether the contribution of AOA and AOB to gross nitrification is a function of their abundance and of pH in sediments. This study suggests that in Sacca di Goro lagoon at least three groups of ammonia oxidizers were actively involved in nitrification: members of Bacteria domain (potentially Nitrosospira sp.) and two taxa belonging to the Archaea (related to Nitrosopumilus and Nitrososphaera). Indeed, AOB seemed to dominate the process in Giralda sediments under high rates of nitrification and low salinity, whereas Archaea dominated in Gorino sediments, under low nitrification rates and high salinity. This advances our understanding of the role of Archaea in the nitrogen cycle in coastal transition systems subject to high inputs of nitrogen and under strong anthropogenic pressure.