Researchers have recently identified what they call a "nitroplast," a cellular organelle capable of nitrogen fixation. This discovery, likely a result of an evolutionary process that began 100 million years ago, could redefine our understanding of the interactions between bacteria and marine algae.
Previously, it was assumed that these microorganisms merely maintained a symbiotic relationship, but it turns out the microbe has integrated into the algae, becoming an essential part of its metabolism.
This image shows a cell of Braarudosphaera bigelowii dividing into two, with nitroplasts (UCYN-A) appearing in cyan.
Credit: Valentina Loconte
The nitroplast originates from an oceanic bacterium, swallowed by an algal cell millions of years ago. This event is an example of "primary endosymbiosis," a rare process in which a eukaryotic cell (complex cell with a nucleus) envelops a prokaryotic cell (without a nucleus), leading to significant biological evolutions. This process is similar to how mitochondria and chloroplasts, essential for cell respiration and photosynthesis respectively, evolved.
The first indication that the microbe was more than just a simple passenger came in 1998, when scientists isolated a segment of the microbe's DNA from Pacific seawater, discovering its ability to fix nitrogen. Subsequent studies at Kochi University in Japan cultivated algae bearing this microbe, enabling comparative studies that showed the synchronization of their growth.
The findings, published in scientific journals like
Cell and
Science, confirm that UCYN-A (the scientific name of the nitroplast) imports proteins from its host algae, depending on it for its functionality.
This microscopic image shows a marine algae cell with a black arrow pointing to the new organelle, the nitroplast.
Credit: Tyler Coale
This discovery could have significant implications, not only for cell biology but also for our understanding of the evolution of marine ecosystems and their ability to fix nitrogen, a crucial element for life on Earth.