In a breakthrough discovery, scientists have unveiled the nitroplast, a previously unknown cell structure found in complex cells. This extraordinary structure possesses the remarkable ability to extract nitrogen from the atmosphere and transform it into a form accessible to the cell. This discovery, outlined in two recent studies, suggests that the nitroplast likely evolved around 100 million years ago, potentially arising from a bacterium incorporated by an algal cell.
Initially, scientists believed that the bacterium and the algae existed symbiotically, with the bacterium supplying nitrogen in a usable form for the algae, while the algae provided shelter for the bacterium. However, research has revealed that the bacterium has undergone a fundamental transformation, evolving into a full-fledged cell structure or organelle, with a metabolism intricately linked to that of the algae.
According to a postdoctoral scholar at the University of California, Santa Cruz (UCSC) and lead author of one of the recent studies, it is an extremely rare occurrence for organelles to emerge from such interactions. This discovery represents only the fourth known example of primary endosymbiosis on Earth, a process where a eukaryotic cell engulfed a prokaryotic cell. The first instance of endosymbiosis is believed to have given rise to complex life itself, leading to the evolution of mitochondria, the cell’s powerhouses, approximately 1.5 billion years ago. All organisms more complex than bacteria owe their existence to this crucial event, including humans.
The second known instance of endosymbiosis occurred roughly 1 billion years ago, resulting in the emergence of chloroplasts, which facilitate photosynthesis and triggered the evolution of plants. The third known event may have given rise to a lesser-known organelle called the chromatophore, a pigment-rich structure found in the skin of cephalopods like squid and octopuses, enabling them to change color for camouflage and communication.
Initially discovered in 1998, the microbe that evolved into the nitroplast was not initially recognized as a true organelle. In subsequent work, a team led by a distinguished professor of marine sciences at UCSC, who authored the second recent study, isolated a short DNA sequence of the microbe from Pacific Ocean seawater. The DNA analysis revealed that the microbe belonged to a nitrogen-fixing cyanobacterium, designated as UCYN-A. Nitrogen fixation refers to the process of converting nitrogen into a form usable by cells.
Concurrently, scientists at Kochi University in Japan successfully cultured the algae that harbor UCYN-A in a laboratory setting. This breakthrough allowed researchers to compare the size of UCYN-A across different species of these algae, which belong to a related group known as diatoms. Their findings, published in the journal ‘The ISME Journal,’ indicated that the growth of UCYN-A and its host cells are synchronized and regulated by the exchange of nutrients, a characteristic typically observed in organelles.
A separate study led by a team of researchers, published in the journal ‘Nature Microbiology,’ provided further confirmation. Their results demonstrated that UCYN-A imports proteins from its host cell, suggesting that the former microbe has gradually lost some of its cellular machinery, relying on its host for certain functions. In essence, the once-independent bacterium has transformed into a specialized component within the host cell’s machinery.
The synchronized replication and inheritance pattern of UCYN-A with its host cell further solidify the classification of the nitroplast as an organelle. This discovery underscores the remarkable adaptability and complexity of life’s evolutionary processes.