The impact of oxygen exposure on long-chain alkyl diols and the long chain diol index (LDI) – a long-term incubation study

Abstract

a b s t r a c tIn 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 freeand 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 C 14 , C 16 and C 18 saturated fatty acids and the C 20:5 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.

Response to Referee 2

“…This suggests that degradation in the water column does not affect the LDI proxy. This is in agreement with Reiche et al (2018) who performed a short-term degradation experiment (< 1 year) and found that the LDI index was not affected by oxic exposure on short timescales. However, the oxygen exposure time on the seafloor is much longer; Rodrigo-Gámiz et al (2016) showed for sediments in the Arabian Sea (deposited under a range of bottom water oxygen conditions) that different LCDs had different degradation rates, which compromised the LDI ratio.…”

Fatty Acids Derivatives From Eukaryotic Microalgae, Pathways and Potential Applications

“…Since lipid extraction techniques mostly used to analyze fatty acid derivatives usually cleave ester linkages but are ineffective toward ether-bound lipids, the cellular abundance of eustigmatophycean LCDs, that are thought to be mostly ether-bound, are likely to be significantly higher than those typically measured. The incubation of N. oculata biomass in the dark, under aerobic conditions, resulted in a sharp increase of LCDs after 100 days, confirming that LCDs can derive from algaenan degradation (Reiche et al, 2018). The chemical structure of algaenans still requires to be fully elucidated, but their high stability against chemical and biological degradation makes these compounds promising for the development of sustainable materials.…”

“…For example, the LCD content in Nannochloropsis spp. can be up to two orders of magnitude lower than that of EPA (Reiche et al, 2018) or monounsaturated and saturated fatty acids (Balzano et al, 2017). The production of substantial amounts of fatty acid derivatives in microalgae is thus more time consuming and risks being economically unviable.…”

Long-chain diols in settling particles in tropical oceans: insights into sources, seasonality and proxies

“…The Cariaco Basin is one of the largest marine anoxic basins (Richards, 1975), located on the continental shelf of Venezuela. The basin is characterized by permanent stratification and is strongly influenced by the migration of the intertropical convergence zone (ITCZ).…”