Biochemical controls: how phytoplankton's fix nitrogen (Introduction)

by David Turell , Wednesday, July 17, 2024, 19:42 (127 days ago) @ David Turell

Symbiosis as usual:

https://www.quantamagazine.org/tight-knit-microbes-live-together-to-make-a-vital-nutrie...

"Nitrogen is fundamental to all life on Earth. Organisms use it to make amino acids and nucleic acids — the building blocks of proteins and DNA — among other vital molecules. Luckily, four-fifths of the atmosphere is nitrogen. Unluckily, in gaseous form it is inert and biologically unavailable: Every nitrogen atom is locked to another with a triple bond, which takes an extraordinary amount of energy to break. Without intervention, cells on land or sea cannot access this atmospheric source.

“'We breathe it in, and we breathe it out, but we can’t do anything with it,” said Bernhard Tschitschko, a microbiologist at the Max Planck Institute for Marine Microbiology. “In life on Earth, nitrogen is one of the elements that controls growth.”

"How, then, do organisms access this indispensable element? They rely on a select few bacteria with a special talent: the ability to convert nitrogen gas (N2) into ammonia (NH3), a process known as fixation, which makes the element available to life. All bacterial species that can break the triple bond of nitrogen gas do it using the same protein: nitrogenase. Every time a molecule of N2 is naturally converted into NH3, anywhere on Earth, it’s because of nitrogenase. The protein’s importance is reflected in how ancient it is: Nitrogenase emerged about 3.2 billion years ago in what researchers have called “one of the most consequential biogeochemical innovations over life’s history.”

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"In a recent paper in Nature, Tschitschko and colleagues reported their discovery of two of these Gamma A organisms — closely related bacteria that live throughout the world’s oceans and supply the food web with nitrogen where Trichodesmium doesn’t. The bacteria don’t work alone: They are lodged firmly inside diatoms, an abundant microscopic phytoplankton, with which they trade nitrogen for housing and energy. The symbiotic relationship — a mutually beneficial collaboration between two independent organisms — is so tight that the bacterium may be on its way to becoming a permanent part of the diatom’s body as a new cellular organelle, according to a DNA analysis.

"The partners’ lives at sea may feel distant from ours, but we have something in common. Most nitrogen on land is fixed by rhizobia bacteria, which live symbiotically in nodules on the roots of legume plants. The Gamma A gene for nitrogenase is related to that found in rhizobia, suggesting an ancient genetic relationship between the two symbiotic partnerships that enable life on land and at sea.

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"This was odd enough, but the researchers still had not laid eyes on the organism in question, only its genome. Using genetic techniques, they tracked the rhizobia DNA to a marine diatom — one of the ubiquitous, photosynthetic microscopic algae of the sea — of the genus Haslea. Inside each diatom were four to eight bacterial cells. The cells turned out to be two bacterial species, which the researchers named Tectiglobus diatomicola and Tectiglobus profundi.

"Haslea diatoms photosynthesize to create energy; then they hand over some of this energy to Tectiglobus, which supplies the diatom with nitrogen.

"This mirrors the relationship between rhizobia and legumes on land, in which bacteria offer nitrogen to the plant in exchange for carbohydrates. Somehow, this nitrogenase gene found its way into two bacterial groups — and both went on to form symbiotic relationships, with very different host organisms, crucial for providing nitrogen to food webs.

"To unpack these twisted histories, the researchers reconstructed evolutionary trees for the rhizobia and Tectiglobus bacteria. The results suggested that both groups acquired the ancient nitrogenase gene from other bacteria through horizontal gene transfer at different points in their evolutionary histories. The authors also speculated that Tectiglobus evolved its symbiotic relationship independently and earlier than its more widely known cousin onshore.

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"It makes sense that a diatom would want to carry an in-house nitrogen source: The ocean is a desert. Nutrients are scarce, and most microbes are in a perpetual state of near-starvation. A photosynthesizing diatom with its own unlimited source of energy, but with a need for nitrogen, offered Tectiglobus a safe and beneficial arrangement.

“'This is the way this one isolated, lonely little diatom can meet its own needs,” said Angelicque White, an oceanographer at the University of Hawaiʻi who wasn’t involved in the work. “These unusual associations break down our simplified description of how ecosystems work. They’re far from land. They’re far from the sources of nutrients. And so these organisms have to adapt in some way.'”

Comment: life is a giant ecosystem on Earth. These symbiotic relationships handle many of our vital processes like nitrogen fixation. The ancestors of these forms that were culled out in their development are part of the same 99.9% dhw always considers unnecessary.