Nitroplast

A nitroplast is an organelle responsible for nitrogen-fixing. It arose as a cyanobacteria species living within the eukaryotic alga Braarudosphaera bigelowii. It is the first recorded example of such an organelle, designated in 2024. The cyanobacterium is Candidatus Atelocyanobacterium thalassa, also referred to as UCYN-A, which exists exclusively as an obligate symbiont. Despite being found in measurable quantities throughout the world's oceans, A. thalassa is not known to be free-living in any environment. Unlike typical cyanobacteria, its genome has undergone massive reduction, losing the genes for RuBisCO, photosystem II, and the TCA cycle. Consequently, it possesses no independent means of fixing carbon or generating energy through photosynthesis, rendering it entirely dependent on its host (so far only known to be Braarudosphaera bigelowii and a closely-related unnamed species). This partnership is characterized by a strict metabolic exchange: A. thalassa fixes atmospheric nitrogen into ammonium for the host, while the host provides the essential carbon products the bacterium can no longer produce for itself. While various sublineages are distributed across diverse marine niches—from oligotrophic open waters to coastal regions—every known version of A. thalassa remains confined within a host cell. In the more integrated form, specifically the UCYN-A2 sublineage, the relationship with the alga has progressed so far that the bacterium is now considered to be a true organelle. In these cases, the "bacterium" is imported with nuclear-encoded proteins and its division is synchronized with the host, mirroring the evolutionary history of mitochondria and chloroplasts. This discovery of the first nitrogen-fixing organelle in a eukaryote has major implications for agricultural science, as it demonstrates a biological pathway for potentially engineering crops that do not require nitrogen fertilizer. Members of A. thalassa are spheroid in shape and are 1–2 μm in diameter, and provide nitrogen to ocean regions by fixing non biologically available atmospheric nitrogen into biologically available ammonium that other marine microorganisms can use. There are many sublineages of A. thalassa that are distributed across a wide range of marine environments and host organisms. It appears that some sublineages of A. thalassa have a preference for oligotrophic ocean waters while other sublineages prefer coastal waters. Much is still unknown about all of A. thalassa's hosts and host preferences.

Discovery In 1998, Jonathan Zehr, an ocean ecologist at the University of California, Santa Cruz, and his colleagues found an unknown DNA sequence that appeared to be for an unknown nitrogen-fixing cyanobacterium in the Pacific Ocean, which they called UCYN-A (unicellular cyanobacterial group A). However, UCYN-A seemed to lack the genes needed for photosynthesis, among many others—representing a loss of around 80% of its original genome—that it requires for survival but cannot produce on its own. At the same time, Kyoko Hagino, a paleontologist at Kochi University, was working to culture the host organism, B. bigelowii. Hagino's culturing work proved essential to provide the biological material needed to study the relationship between UCYN-A and B. bigelowii. In 2024 Zehr's laboratory formally identified UCYN-A as a nitrogen-fixing organelle, which they termed the nitroplast. The team demonstrated that many of the proteins required for UCYN-A's own functions, which are absent from its genome, are now encoded in the host's DNA and imported into the organelle; this integration of host-encoded, organelle-targeted proteins is considered a defining hallmark of organelles, paralleling the evolutionary history of mitochondria and chloroplasts.

Ecology

Nitrogen fixation Nitrogen fixation, which is the reduction of N2 to biologically available nitrogen, is an important source of N for aquatic ecosystems. For many decades, N2 fixation was vastly underestimated. The assumption that N2 fixation only occurred via Trichodesmium and Richelia led to the conclusion that in the oceans, nitrogen output exceeded the input. However, researchers found that the nitrogenase complex has variable evolutionary histories. The use of the polymerase chain reaction (PCR), removed the requirement of cultivation or microscopy to identify N2 fixing microorganisms. As a result, marine N2-fixing microorganisms other than Trichodesimum were found by sequencing PCR-amplified fragments of the gene nitrogenase (nifH). Nitrogenase is the enzyme that catalyzes nitrogen fixation, and studies have shown that nifH is widely distributed throughout the different parts of the ocean. In 1989, a short nifH gene sequence was discovered, and 15 years later it was revealed to be an unusual cyanobacterium that is widely distributed. The microbe was originally given the name UCYN-A for "unicellular cyanobacteria group A". In research published in 1998, nifH sequences were amplified directly from water collected in the Pacific and Atlantic Oceans, and shown to be from bacterial, unicellular cyanobacterial nifH, Trichodesmium and diatom symbionts. With the use of cultivation-independent PCR and quantitative PCR (qPCR) targeting the nifH gene, studies found that A. thalassa is distributed in many ocean regions, showing that the oceanic plankton contain a broader range of nitrogen-fixing microorganisms than was previously believed.

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