The role of unicellular cyanobacteria in nitrogen fixation and assimilation in subtropical marine waters

Biological N2 fixation constitutes the major source of nitrogen in open ocean systems, regulating the marine nitrogen inventory and primary productivity. Symbiotic relationships between phytoplankton and N2 fixing microorganisms (diazotrophs) have been suggested to play a significant role in the ecology and biogeochemistry in these oceanic regions. The widely distributed, uncultured N2 fixing cyanobacterium UCYN A was suggested to live in symbiosis since it has unprecedented genome reduction, including the lack of genes encoding for oxygen evolving photosystem II and the tricarboxylic acid cycle. This thesis aims to study carbon and nitrogen metabolism on field populations of UCYN A using molecular biology, as well as mass spectrometry tools to visualize metabolic activity on a single cell scale. The development of a 16S rRNA oligonucleotide probe specifically targeting UCYN A cells and its successful application on environmental samples (Manuscript I and II) revealed a symbiotic partnership with a unicellular prymnesiophyte. We demonstrated a nutrient transfer in carbon and nitrogen compounds between these two partner cells, providing an explanation how these diazotrophs thrive in open ocean systems. Further, UCYN A can also associate with globally abundant calcifying prymnesiophyte members, e.g. Braarudosphaera bigelowii, indicating that this symbiosis might impact the efficiency of the biological carbon pump. In manuscript III, we provided quantitative information on the cellular abundance and distribution of UCYN A cells in the North Atlantic Ocean and identified the eukaryotic partner cell as Haptophyta (including prymnesiophyte) via double Catalyzed Reporter Deposition Fluorescence In Situ Hybridization (CARD FISH). The UCYN A Haptophyta association was the dominant form (87.0±6.1%) over free living UCYN A cells. Interestingly, we also detected UCYN A cells living in association with unknown eukaryotes and non calcifying Haptophyta cells, raising questions about the host specificity. During a follow up study (Manuscript IV), we conducted various nutrient amendment experiments (including iron, phosphorus, ammonium nitrate and Saharan Dust) in order to examine physiological interactions between individual UCYN A and Haptophyta cells. Single cell measurements using nanometer scale secondary ion mass spectrometry (nanoSIMS) revealed a tight physiological coupling in the transfer of carbon (R2 = 0.6232; n = 44) and nitrogen (R2 = 0.9659; n = 44) between host and symbiont. N2 fixation was mainly stimulated when iron rich Saharan Dust was added, emphasizing on aeolian dust deposition in seawater as a major parameter in constraining N2 fixation of UCYN A. Moreover, when fixed nitrogen species (ammonium and nitrate) were added, a third unknown microbial partner cell was observed within individual UCYN A Haptophyta associations, but their menaing is unclear. Based on this thesis work we revealed how UCYN A cells thrive in the environment and established a culture independent technique to assess the in situ activity in respect to CO2 and N2 fixation of this ecological relevant group of microorganisms. Furthermore, this unusual partnership between a cyanobacterium and a unicellular alga is a model for symbiosis and is analogous to plastid and organismal evolution, and if calcifying, may have important implications for past and present oceanic N2 fixation