Lipids as indicators of nitrogen cycling in present and past anoxic oceans

Nitrogen (N) cycling influences primary production in the ocean and, hence, the global climate. It is performed by a variety of microorganisms, including eukaryotes, bacteria and archaea in oxic, suboxic, and anoxic waters. Our knowledge of the reactions involved in marine N cycling and its associated microorganisms has greatly increased in the last decade due to the development of multiple culture-independent methods. Among them are gene and lipid biomarkers, which hold taxonomic potential and can be successfully applied in modern day and paleoenvironmental studies. However, many aspects of N cycling and their long-term implications for the marine environment and the global climate still require more study, especially in suboxic and anoxic waters, including the oxygen-deficient zones (ODZs), which are expanding in the modern oceans. Oxygen-depleted waters are not an exclu-sive feature of the modern ocean as oceanic anoxic events (OAE) have been occurring in the geological past and severely affected N cycling. This thesis investigates aspects of the marine N cycle in the modern ocean by applying molecular biomarkers. Particular focus was put on the modern oxygen-de-pleted water systems and on how these systems can be helpful to understand the past OAEs and how in turn past anoxic events can be predictive of the future of the ocean. The work described in this thesis provides new insights into N cycling in oxygen-depleted basins and ODZs. Additionally, it expands the current knowledge on the archaeal diversity in such settings. Finally, it has improved the understanding of past variations in N2 fixation. Future investigations using biomarkers may help to further increase our understanding of present and past N cycling in the ocean