Occurrence and distribution of ladderane oxidation products in different oceanic regimes

[摘要] Ladderane fatty acids are commonly used as biomarkers for bacteria involvedin anaerobic ammonium oxidation (anammox). These lipids have beenexperimentally shown to undergo aerobic microbial degradation to form shortchain ladderane fatty acids. However, nothing is known of the production orthe distribution of these oxic biodegradation products in the naturalenvironment. In this study, we analysed marine water column particulatematter and sediment from three different oceanic regimes for the presence ofladderane oxidation products (C₁₄ ladderane fatty acids) and oforiginal ladderane fatty acids (C₁₈ and C₂₀ ladderane fattyacids). We found that ladderane oxidation products, i.e. C₁₄ ladderanefatty acids, are already produced within the water column of the Arabian Seaoxygen minimum zone (OMZ) and thus only low amounts of oxygen (< 3 μM)are needed for the β-oxidation of original ladderane fattyacids to proceed. However, no short chain ladderane fatty acids weredetected in the Cariaco Basin water column, where oxygen concentrations werebelow detection limit, suggesting that the β-oxidation pathway isinhibited by the absence of molecular oxygen, or that the microbesperforming the degradation are not proliferating under these conditions.Comparison of distributions of ladderane fatty acids indicates that shortchain ladderane fatty acids are mostly produced in the water column and atthe sediment surface, before being preserved deeper in the sediments. Shortchain ladderane fatty acids were abundant in Arabian Sea and Peru Marginsediments (ODP Leg 201), often in higher concentrations than the originalladderane fatty acids. In a sediment core taken from within the Arabian SeaOMZ, short chain ladderanes made up more than 90% of the total ladderanesat depths greater than 5 cm below sea floor. We also found short chainladderanes in higher concentrations in hydrolysed sediment residues comparedto those freely occurring in lipid extracts, suggesting that they had becomebound to the sediment matrix. Furthermore, these matrix-bound short chainladderanes were found at greater sediment depths than short chain ladderanesin the lipid extract, suggesting that binding to the sediment matrix aidsthe preservation of these lipids. Though sedimentary degradation of shortchain ladderane fatty acids did occur, it appeared to be at a slower ratethan that of the original ladderane fatty acids, and short chain ladderanefatty acids were found in sediments from the Late Pleistocene(~ 100 kyr). Together these results suggest that the oxicdegradation products of ladderane fatty acids may be suitable biomarkers forpast anammox activity in OMZs.

[学科分类] 地球化学与岩石