We tend to think of living organisms as distinct entities, each with its own biological identity. They can cooperate, compete, parasitize one another, even live inside one another. But what happens when two organisms, after living together long enough, cease to be two?<\/p>\n
In 2024, a study published in Science documented an extraordinary example of this transformation. A tiny marine cyanobacterium, known as UCYN-A, lives inside a single-celled alga, Braarudosphaera bigelowii. Their relationship dates back some one hundred million years. Over that long period, the bacterium has gradually lost its autonomy, becoming something different: a cellular organelle, named the nitroplast.<\/p>\n
The distinction is more than a matter of terminology. A symbiotic bacterium retains its own organization, even when it depends on its host. An organelle, by contrast, is integrated into the functioning of the cell that contains it. The nitroplast receives numerous essential proteins from the alga, produced according to instructions encoded in the host cell\u2019s nucleus. Even its division is synchronized with that of the alga.<\/p>\n
In return, it performs a fundamental task: it fixes atmospheric nitrogen, converting it into compounds that can be used to build the molecules of life. This ability is found in certain prokaryotes but is absent from eukaryotic cells without such partners. Its incorporation into an organelle therefore represents an evolutionary development of enormous interest.<\/p>\n
This is not the first time something of the kind has happened. Mitochondria, which supply energy to our cells, descend from ancient bacteria. Chloroplasts, which enable plants to carry out photosynthesis, also have bacterial origins. In both cases, once-independent organisms became essential components of other living beings.<\/p>\n
The nitroplast offers scientists an exceptionally rare opportunity to observe another instance of this transformation, known as primary endosymbiosis. Its discovery could also open up new possibilities for agriculture. Understanding how a photosynthetic organism incorporated the ability to fix nitrogen might one day help develop crops that depend less on fertilizers. For now, this is a research prospect, not an available technology.<\/p>\n
But the discovery also raises a more radical question.<\/p>\n
If the mitochondria in our cells descend from autonomous organisms, and if further instances of integration continue to be identified in evolutionary history, then the boundaries of an organism are not necessarily fixed once and for all. They can change over the course of evolution, eventually incorporating what was once another living being.<\/p>\n
Evolution does not proceed solely through competition and the selection of separate organisms. Sometimes it produces innovations through relationships so profound that coexistence becomes mutual dependence and, ultimately, a single biological organization.<\/p>\n
This is precisely what the nitroplast tells us: what we now regard as an integral part of an organism may once, in the distant past, have had a life of its own. The boundary between self and other, which seems so natural to us, is also the outcome of an evolutionary history. And there is no reason to assume that history has come to an end.<\/p>\n","protected":false},"excerpt":{"rendered":"
The Nitroplast and the making of an individual We tend to think of living organisms as distinct entities, each with its own biological identity. They can cooperate, compete, parasitize one another, even live inside one another. But what happens when two organisms, after living together long enough, cease to be two? In 2024, a study […]<\/p>\n","protected":false},"author":1756,"featured_media":902252,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[97],"tags":[],"class_list":["post-902251","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-isobare"],"acf":[],"yoast_head":"\n