Novel Lipid Biomarkers for Past Oceanic N2 Fixation

Abstract:

Nitrogen-fixing cyanobacteria play important roles in the biogeochemical cycles of aquatic systems. Both heterocystous and non-heterocystous N₂-fixing cyanobacteria are symbiotic with marine diatoms and thrive in low nutrient environments. These associations are significant exporters of carbon to the deep-sea, but suitable tracers for reconstructing their importance in past environments are lacking. We recently analyzed the heterocyst glycolipids (HGs) of the heterocystous Richelia intracellularis symbiont of the marine diatoms Hemiaulus hauckii and H. membranaceus and found unique C₅ glycolipids with C_30-32 carbon chains, a structure different from the C₆ glycolipids detected in freshwater heterocystous cyanobacteria.

We developed a high performance liquid chromatography/ multiple reaction monitoring mass spectrometry (HPLC/MS) method specific for trace analysis of long chain C₅ HGs and applied it to suspended particulate matter (SPM) and surface sediment from the Amazon plume, a region known to harbor marine diatoms carrying heterocystous cyanobacteria as endosymbionts. C₅ HGs were detected in both SPM and sediments demonstrating their biomarker potential. They were not detected in SPM or sediment from freshwater settings in the region. Rather, limnetic SPM and sediments contained C₆ HGs which are established biomarkers for free-living heterocystous cyanobacteria. Glycolipids have been found preserved in sediments of up to 49 Ma old. Our development of the C₅ biomarkers has the potential to improve our knowledge of the contribution of symbiotic cyanobacteria to the paleo-N-cycle.