Fossilized glycolipids reveal past oceanic N2 fixation by heterocystous cyanobacteria

• Published in: Proceedings of the National Academy of Sciences
• Main Authors: Bauersachs, Thorsten, Speelman, Eveline N., Hopmans, Ellen C., Reichart, Gert-Jan, Schouten, Stefan, Damsté, Jaap S. Sinninghe
• Format: Text
• Language: English
• Published: National Academy of Sciences 2010
• Subjects: Physical Sciences; Arctic; Arctic Ocean
• Online Access: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2984197; http://www.ncbi.nlm.nih.gov/pubmed/20966349; https://doi.org/10.1073/pnas.1007526107

N2-fixing cyanobacteria play an essential role in sustaining primary productivity in contemporary oceans and freshwater systems. However, the significance of N2-fixing cyanobacteria in past nitrogen cycling is difficult to establish as their preservation potential is relatively poor and specific biological markers are presently lacking. Heterocystous N2-fixing cyanobacteria synthesize unique long-chain glycolipids in the cell envelope covering the heterocyst cell to protect the oxygen-sensitive nitrogenase enzyme. We found that these heterocyst glycolipids are remarkably well preserved in (ancient) lacustrine and marine sediments, unambiguously indicating the (past) presence of N2-fixing heterocystous cyanobacteria. Analysis of Pleistocene sediments of the eastern Mediterranean Sea showed that heterocystous cyanobacteria, likely as epiphytes in symbiosis with planktonic diatoms, were particularly abundant during deposition of sapropels. Eocene Arctic Ocean sediments deposited at a time of large Azolla blooms contained glycolipids typical for heterocystous cyanobacteria presently living in symbiosis with the freshwater fern Azolla, indicating that this symbiosis already existed in that time. Our study thus suggests that heterocystous cyanobacteria played a major role in adding “new” fixed nitrogen to surface waters in past stratified oceans.