Abstract
In recent years, long chain alkyl diols (LCDs) have been used increasingly to study and reconstruct past sea surface temperatures using the long chain diol index (LDI), which is based on changes in the distribution of 1,15-LCDs. However, the impact of diagenesis on LCDs and the LDI is still poorly constrained. Here we studied the impact of oxygen exposure on LCDs and the LDI, by aerobically incubating biomass of the LCD-producing alga Nannochloropsis oculata for 271 days. The concentrations of extractable free- and bound, residually ester-bound and residually-bound glycosidic ether- or amide-bound saturated fatty acids and LCDs were determined. A significant impact of oxygen exposure was observed for C14, C16 and C18 saturated fatty acids and the C20:5 polyunsaturated fatty acid, as their concentration decreased significantly over time, irrespective of their mode of binding. LCDs, in contrast, increased significantly in concentration over incubation time, e.g. up to a 30-fold increase in concentrations for residually ester-bound LCDs at day 125 compared to concentrations at the beginning of the experiment. This increase in concentration most likely represents a release of LCDs from the insoluble biopolymer algaenan due to the impact of oxygen exposure. Values of the LDI differed strongly depending on the mode of occurrence of LCDs in the biomass. However, despite the large changes in concentration of LCDs in response to oxygen exposure, the LDI remained relatively stable after prolonged degradation. This suggests that oxygen exposure may not have a substantial impact on the LDI of extractable LCDs used for its determination.
Highlights
We mainly focus on Long chain alkyl diols (LCDs), as well as the dominant saturated fatty acids and the C20:5 polyunsaturated fatty acid (PUFA), to contrast their different responses to oxygen exposure
Nannochloropsis oculata biomass was aerobically degraded for 271 days
While saturated fatty acids and the C20:5 PUFA were degraded to a large extent, the concentrations of LCDs increased over time and were probably released from the biopolymer algaenan
Summary
Long chain alkyl diols (LCDs) were first discovered in sediments of the Black Sea (de Leeuw et al, 1981) and since have been studied in a large variety of marine and freshwater environments (e.g. Versteegh et al, 1997; Sinninghe Damsté et al, 2003; Rampen et al, 2007, 2008, 2014a, 2014b; Zhang et al, 2011; Romero-Viana et al, 2012; de Bar et al, 2016; Lattaud et al, 2017a). LCDs in algal cell material of Eustigmatophytes and in the sediment have been shown to occur in different forms, i.e. as free-extractable, extractable-bound (released by hydrolysis of the extract) and residually-bound lipids (released by hydrolysis of the residual biomass left after extraction) (Gelin et al, 1997; Ahmed et al, 2000; Grossi et al, 2001; Hoefs et al, 2002; Shimokawara et al, 2010)
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