Until now, the common assumption was that mainly cyanobacteria were responsible for fixing nitrogen from the atmosphere in the early times of our planet and thus introducing it into the biosphere. Researchers at the Max Planck Institute for Marine Microbiology in Bremen now show that sulfur purple bacteria may also have contributed substantially to nitrogen fixation under conditions such as those in the Proterozoic Ocean.
Nitrogen is essential for all forms of life: It is part of proteins, nucleic acids, and other cell structures. Therefore, on the early Earth, it was of great importance for the development of life to be able to use nitrogen from the atmosphere. But who carried out this so-called nitrogen fixation at that time, and with the help of which enzyme, has not yet been clarified. Now, researchers from the Max Planck Institute for Marine Microbiology in Bremen show that under similarly barren conditions as in the Proterozoic ocean, a previously underestimated group of bacteria can fix nitrogen very efficiently.
A “small primordial ocean” in the Swiss Alps
Since the Proterozoic Ocean is no longer available for direct studies, the researchers led by Miriam Philippi and Katharina Kitzinger from the Max Planck Institute in Bremen used a comparable habitat: the Swiss Alpine lake Lago di Cadagno. Unlike most other lakes, Lago di Cadagno is stably stratified, so the upper and lower water layers do not mix. In the transition region between the upper, oxygenated layer and the lower, oxygen-free and sulfide-containing layer, live sulfur purple bacteria. They manage without oxygen, perform photosynthesis and oxidize sulfur. “The finding of fossils of this group of microorganisms indicates that they lived on our planet at least 1.6 billion years ago, in the Proterozoic Era,” said first author Philippi. “So with this lake and with these bacteria, we’re dealing with a system that has a lot in common with the early ocean.” That’s why it’s so well suited to learning more about processes on early Earth.
Sulfur purple bacteria fix nitrogen
Using a combination of biogeochemical and molecular analyses, Philippi and her colleagues discovered that sulfur purple bacteria fix nitrogen very efficiently. Nitrogen ffi¬xation is the conversion of low-reactivity nitrogen gas from the atmosphere to nitrogen compounds that other organisms can use, such as algae.
“As far as we know, this is the first demonstration of nitrogen fixation by sulfur purple bacteria living in the environment. We found that they use the most common enzyme today, molybdenum nitrogenase, to do this. Although this enzyme is not rare, we were very surprised to find it in Lake Cadagno.”
- Co-author Katharina Kitzinger
That’s because there is extremely little molybdenum in the water there — just as there is in the Proterozoic Ocean. That’s why it was assumed that nitrogenases without molybdenum prevailed on early Earth. “So molybdenum nitrogenase functions quite well even at low concentrations of molybdenum.”
“We thus provide the first evidence that sulfur purple bacteria may have been partly responsible for nitrogen fixation in the early ocean,” Philippi added. “Until now, it was mostly assumed that cyanobacteria carried out most of the nitrogen fixation. Now we show that the role of sulfur purple bacteria in this process was underestimated.”
