Beyond Symbiosis: Reframing Of The UCYN-A Marine N2-fixing Partnership

Primary endosymbiosis is the biological process which led to the domestication of mitochondria and consequently the evolution of complex life on Earth. It is also responsible for the origin of the two photosynthetic organelles in eukaryotes which gave rise to land plants and many algal lineages. Research into marine microbes has revealed the fourth known occurrence of primary endosymbiosis which resulted in a novel N2-fixing organelle called the nitroplast. Candidatus Atelocyanobacterium thalassa, or UCYN-A, is a metabolically streamlined N2-fixing cyanobacterium previously reported to be an endosymbiont of a marine unicellular alga. Here we show that UCYN-A has been tightly integrated into algal cell architecture and organellar division and that it imports proteins encoded by the algal genome that expand its metabolic capabilities. For instance, UCYN-A biosynthetic pathways previously reported to be incomplete are restored by participation of nuclear-encoded proteins. The eukaryotic cell supports the process of N2-fixation with cytochrome P450 monooxygenases and flavodoxin electron-transfer proteins, and regulates UCYN-A circadian rhythm with cryptochromes. Furthermore, nuclear and nitroplast genomes together encode a vitamin B12 (adenosylcobalamin) biosynthetic pathway which may enable B. bigelowii to avoid B12 deficiency in the marine environment. This new perspective on a key player in the marine nitrogen cycle provides insight into the mechanisms of eukaryotic nitrogen fixation and the transition from symbiont to organelle.

 

Dr. Tyler Coale

Postdoctoral Researcher, UCSC

Dr. Tyler Coale is a postdoctoral researcher affiliated with UCSC and has previously studied the physiological response of phytoplankton in low iron conditions. Dr. Coale completed a B.S. in Plant Sciences at UCSC and went on to work as a technician in the field of chemical oceanography with Ken Bruland at UCSC and later with Kristen Buck at the Bermuda Institute of Ocean Sciences.