The case
UCYN-A is a nitrogen-fixing cyanobacterium with a streamlined metabolism, previously reported as an endosymbiont of a unicellular marine alga. Coale et al. (LIT-1562) show that it is tightly integrated into the alga’s cell architecture and organellar division, and that it imports proteins encoded by the algal genome. They call these characteristics of organelles, and conclude that UCYN-A “has evolved beyond endosymbiosis” as an early-stage nitrogen-fixing organelle, the “nitroplast”. Nitrogen fixation had been known only in prokaryotes. LIT-1560 places it among the recently evolved organelles with protein import.
What it can show
- A boundary drawn by criteria. Coale et al. name two criteria for calling it an organelle: division coupled to the host’s, which is cohesion (R), and imported host-encoded proteins, which is control (C). That makes the case a test of whether those dimensions mark the transition QUESTION-277 asks about.
- Recency. Like Paulinella (CASE-174), it lets organellogenesis be studied without relying only on the ancient mitochondrion and plastid.
Documented in a work the record has not read. What it shows rests on its abstract until it is read.
Note of 2026-10-27 (batch 2): the review read
Sørensen et al. (LIT-1560, read in NOTE-1241) confirm the case from Coale et al.’s results: one UCYN-A per host cell, coordinated division, vertical transmission, and more than 350 imported nucleus-encoded proteins. They add three points:
- Import without transfer. No gene transfer from UCYN-A to the nucleus has been found. Import alone gives genetic integration (THEORY-1100).
- A new signal. 62% of imported proteins carry a C-terminal “UCYN-A transit peptide”, presumably cleaved, unlike any known organelle signal.
- The host’s dependence is conditional. “When nitrogen compounds are externally provided in culture, B. bigelowii can lose UCYN-A”. So the host’s dependence (D) depends on its environment, while the symbiont’s does not. R and C are high, and D is asymmetric.
UCYN-A’s genome is about 1.4 Mbp, and the partnership is about 91 million years old.