Abstract

A bacterial endosymbiont of marine algae evolved to an organelle , Eukaryotic cells are notably complex—for example, they have various organelles, which are membrane-bound structures with specific functions. Two of these organelles, mitochondria and chloroplasts, which function in respiration and photosynthesis, evolved from the integration of endosymbiotic bacteria to the eukaryotic cell ( 1 ). In marine systems, some nitrogen-fixing bacteria are endosymbionts of microalgae, such as Candidatus Atelocyanobacterium thalassa (UCYN-A), a cyanobacterial symbiont of the unicellular algae Braarudosphaera bigelowii ( 2 ). On page 217 of this issue, Coale et al . ( 3 ) report a close integration of the endosymbiont into the architecture and function of the host cell, which is a characteristic of organelles. These findings show that UCYN-A has evolved from a symbiont to a eukaryotic organelle for nitrogen fixation—the nitroplast—thereby expanding a function that was thought to be exclusively carried out by prokaryotic cells to eukaryotes.

Ramon Massana

Staff scientist

I am microbial ecologists with a deep interest in protist ecology and evolution