About this event
It was believed that a symbiosis occurred between the unicellular nitrogen-fixing cyanobacterium UCYN-A and a unicellular alga (Braarudosphaera bigelowii). We discovered that UCYN-A has properties of an intracellular organelle and termed it a “nitroplast.” UCYN-A is metabolically limited, missing photosystem II, Rubisco, and the entire TCA cycle. The UCYN-A is importing proteins analogous to the known protein import process in chloroplasts and mitochondria. Intriguingly the complete set of photosystem I genes have been retained in UCYN-A, raising the prospect of cyclic photophosphorylation for nitrogen fixation, which may be a key to the environmental success of the nitroplast-containing B. bigelowii. The existence of an intracellular body that fixes N2 aerobically in the presence of oxygenic photosynthesis pertains to the question of engineering N2 fixation in agricultural plants, a long-term goal of the N2 fixation research.