Nitrogen inputs in the ancient ocean
It has long been assumed that cyanobacteria were primarily responsible for fixing nitrogen on early Earth, thereby making nitrogen available to the biosphere. Researchers at the Max Planck Institute for Marine Microbiology in Bremen, Germany, are now showing that sulfur-purple bacteria could have contributed substantially to nitrogen fixation under conditions in the Proterozoic Ocean.
Nitrogen is vital for all forms of life: it is part of proteins, nucleic acids and other cellular structures. Thus, it was of great importance for the development of life on early Earth to be able to convert the nitrogen gas in the atmosphere into a bioavailable form, ammonium. However, it has not yet been clarified who carried out this so-called nitrogen fixation on early Earth and with the help of which enzyme. Today, researchers at the Max Planck Institute for Marine Microbiology in Bremen have shown that under conditions as sterile as in the Proterozoic Ocean, a previously underestimated group of bacteria can fix nitrogen very efficiently.
A “small Proterozoic ocean” in the Swiss Alps
Since the Proterozoic Ocean can hardly be studied directly, researchers Miriam Philippi and Katharina Kitzinger of the Max Planck Institute in Bremen and their colleagues replaced it with a comparable modern habitat: the Alpine Lake Cadagno in Switzerland. Unlike most other lakes, Lake Cadagno is permanently stratified, which means that the upper and lower water layers do not mix.
The purple sulphurous bacteria inhabit the transition zone between the upper oxygenated layer and the lower oxygen-free and sulphurous layer. There, they photosynthesize and oxidize sulfur. “The discovery of fossils of this group of microorganisms shows that they already lived on our planet at least 1.6 billion years ago, during the Proterozoic eon,” said Philippi, first author of the study. “Therefore, this lake and these bacteria represent a system that resembles the Proterozoic Ocean in many ways.” Therefore, it is so well suited for learning more about the processes on the early Earth.
Violet sulfur bacteria fix nitrogen
Using a combination of biogeochemical and molecular analyzes, Philippi and his colleagues found that the purple sulfur bacteria from Lake Cadagno very efficiently fix nitrogen. Nitrogen fixation is the conversion of nitrogen gas, which is not very reactive, into nitrogen compounds that many organisms can use, for example algae. “To our knowledge, this is the first direct evidence of nitrogen fixation by purple sulfur bacteria in nature,” explained co-author Katharina Kitzinger.
“We found that they use the most common enzyme in today’s world, molybdenum nitrogenase, to do this. Although this enzyme is not uncommon, we were very surprised to find it in Lake Cadagno.” Indeed, there is very little molybdenum in the water, just like in the Proterozoic Ocean, which led researchers to believe that non-molybdenum nitrogenases were prevalent on early Earth. “Now we know that molybdenum nitrogenase works very efficiently, even at low concentrations of molybdenum.”
“We thus provide the first indication that sulfur-violet bacteria may have been partly responsible for nitrogen fixation in the Proterozoic Ocean,” Philippi continued. “Until now, it was generally assumed that cyanobacteria then performed most of the nitrogen fixation. We show that the role of sulfur-purple bacteria in this process was probably underestimated.”
The study is published in Nature Communication.
A globally significant microbial process hidden on marine particles
Miriam Philippi et al, Purple Sulfur Bacteria Fix N2 via Molybdenum Nitrogenase in a Low Molybdenum Proterozoic Ocean Analog, Nature Communication (2021). DOI: 10.1038 / s41467-021-25000-z
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Quote: Nitrogen inputs to the ancient ocean (2021, August 6) retrieved August 6, 2021 from https://phys.org/news/2021-08-nitrogen-ancient-ocean.html
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