Scientists from NIOZ (the Royal Netherlands Institute for Sea Research), Utrecht University and the Darwin Center for Biogeosciences have found fossil evidence of symbiosis between water plants and certain types of bacteria in millions-of-year-old sediments. A new technique has been developed to analyze the abundance of these bacteria in sediments of different geological ages. The research contributes to the understanding of global nitrogen cycling since these bacteria play a key role in this process. The team’s findings are published online in the Early Edition of the Proceedings of the National Academy of Sciences USA (October 18).

New tracer for fossil bacteria developed

In large parts of the present-day oceans, cyanobacteria play a key role in the nitrogen cycle as they provide nitrogen for the growth of algae. Until now, it has been difficult to identify these bacteria in sediments and to determine their role in the nitrogen cycle in the geological past. The NIOZ team has developed a new analytical screening technique to analyze fossil components of cyanobacteria in ancient sediments. Screening sediments of different geological ages has revealed that these components of cyanobacteria were preserved well over time. At certain times in Earth’s history, cyanobacteria proliferated and, by providing a source of nitrogen, helped sustain a high primary production by algae in sun-lit surface waters.

Symbiosis found in 49-million-year-old sediments

Components of cyanobacteria were discovered in 49-million-year-old sediments from the Arctic Ocean. At that time, the ocean was covered by a freshwater surface layer that permitted Azolla ferns to grow. At present, Azolla are growing in symbiosis with certain cyanobacteria species. After applying a new tracer technique to compare ancient Arctic Azolla remains with those of present-day Azolla, it turned out that the same cyanobacteria species were already living in symbiosis with this fern 49 million years ago.

Cyanobacteria key organisms in present-day nitrogen cycle

Nitrogen is an essential element for algae to grow. Algae are at the basis of the food web in large parts of the oceans. There is a large reservoir of nitrogen in the marine environment in the form of dissolved nitrogen gas. However, this reservoir can only be tapped by a few microorganisms, mainly cyanobacteria, as they can reduce nitrogen to ammonium, a process referred to as biological nitrogen fixation. Some algae and plants benefit from this by living in symbiosis with these nitrogen-gas-fixing cyanobacteria, thus obtaining a direct supply of ammonium.

Bibliography: Bauersachs T., Speelman E.N., Hopmans E.C., Reichart G.J., Schouten S. and Sinninghe Damsté J.S. (2010). Fossilized glycolipids reveal past oceanic N₂ fixation by heterocystous cyanobacteria. Proc. Nat. Acad. Sci. USA, Early Edition.

This research project was partially funded by the Darwin Center for Biogeosciences.