From ‘nitrosome’ to ‘nitroplast’: stages in the evolution of nitrogen-fixing organelles from free-living diazotrophs

Abstract

Microbial symbiosis has played an important role in evolution of life on Earth. Lynn Margulis proposed that eukaryotic cells have evolved through a series of intracellular interactions among prokaryotes in the very early stages of life on Earth. There are many current microbial interactions that suggest that complexity in organisms is being driven by symbiosis. For example, plants may establish an endosymbiosis with free-living soil microbes (e.g., Gluconacetobacter diazotrophicus or Burkholderia vietnamiensis) and sequester them in to cellular vesicles termed ‘nitrosomes’ where plants extract nitrogen from the endosymbionts. Similarly, a vesicular ‘symbiosome’ develops in root nodules of many legume family plants. Recently, scientists have discovered a new permanent nitrogen-fixing cell organelle, termed ‘nitroplast’, in the marine alga Braarudosphaera bigelowii. In this mini review we discuss nitrogen-fixing symbioses, and suggest that the ‘nitroplast’ may have been derived from ‘nitrosomes’.