Darwin Center research has shown that short-chain ladderane lipids, that have been formed during degradation experiments, also naturally occur in the marine environment. Therefore they can possibly be used as biomarkers. The research has been published in Geochimica Cosmochimica Acta.

Anammox and nitrogen

Anammox, the microbial anaerobic oxidation of ammonium by nitrite to produce dinitrogen gas, has been recognized as a key process in both the marine and freshwater nitrogen cycles, and found to be a major sink for fixed inorganic nitrogen in the oceans.

Ladderane lipids and diagenesis

Ladderane lipids are unique anammox bacterial membrane lipids that have been used as biomarkers for anammox bacteria in recent and past environmental settings. However, the fate of ladderane lipids during diagenesis is as of yet unknown.

Oxic degradation experiments to simulate diagenesis

In this study, we performed oxic degradation experiments (at 20–100 ºC) with anammox bacterial biomass to simulate early diagenetic processes occurring in the water column and at the sediment–water interface.

Results

Abundances of C₁₈ and C₂₀ ladderane lipids decreased with increasing temperatures, testifying to their labile nature. The most abundant products formed were ladderane lipids with a shorter alkyl side chain (C₁₄ and C₁₆ ladderane fatty acids), which was unambiguously established using two-dimensional NMR techniques on an isolated C₁₄-[3]-ladderane fatty acid.

Lab results versus natural marine environments

The most pronounced production of these short-chain lipids was at 40 ºC, suggesting that degradation of ladderane lipids was microbially mediated, likely through a b-oxidation pathway. An HPLC–MS/MS method was developed for the detection of these ladderane alteration products in environmental samples and positively tested on various sediments. This showed that the ladderanes formed during degradation experiments also naturally occur in the marine environment.

Suitable biomarkers

Thus, short-chain ladderane lipids may complement the original longer-chain ladderane lipids as suitable biomarkers for the detection of anammox processes in past depositional environments.

Publication

Rush D., Jaeschke A., Hopmans E.C., Geenevasen J.A.J., Schouten S., Sinninghe Damsté J.S (2011). Short chained ladderanes in sediments: oxic biodegradation products of anammox lipids. Geochimica Cosmochimica Acta 75: 1662-1671.