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

• Main Author: Krupke, A.
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: University of Bremen 2013
• Subjects: North Atlantic
• Online Access: http://hdl.handle.net/21.11116/0000-0001-C69D-F; http://hdl.handle.net/21.11116/0000-0007-0899-4

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 ...