Abstract
Abstract. Long chain alkyl diols (LCDs) are widespread in the marine water column and sediments, but their biological sources are mostly unknown. Here we combine lipid analyses with 18S rRNA gene amplicon sequencing on suspended particulate matter (SPM) collected in the photic zone of the western tropical North Atlantic Ocean at 24 stations to infer relationships between LCDs and potential LCD producers. The C30 1,15-diol was detected in all SPM samples and accounted for >95 % of the total LCDs, while minor proportions of C28 and C30 1,13-diols, C28 and C30 1,14-diols, as well as C32 1,15-diol were found. The concentration of the C30 and C32 diols was higher in the mixed layer of the water column compared to the deep chlorophyll maximum (DCM), whereas concentrations of C28 diols were comparable. Sequencing analyses revealed extremely low contributions (≈0.1 % of the 18S rRNA gene reads) of known LCD producers, but the contributions from two taxonomic classes with which known producers are affiliated, i.e. Dictyochophyceae and Chrysophyceae, followed a trend similar to that of the concentrations of C30 and C32 diols. Statistical analyses indicated that the abundance of 4 operational taxonomic units (OTUs) of the Chrysophyceae and Dictyochophyceae, along with 23 OTUs falling into other phylogenetic groups, were weakly (r≤0.6) but significantly (p value <0.01) correlated with C30 diol concentrations. It is not clear whether some of these OTUs might indeed correspond to C28−32 diol producers or whether these correlations are just indirect and the occurrence of C30 diols and specific OTUs in the same samples might be driven by other environmental conditions. Moreover, primer mismatches were unlikely, but cannot be excluded, and the variable number of rRNA gene copies within eukaryotes might have affected the analyses leading to LCD producers being undetected or undersampled. Furthermore, based on the average LCD content measured in cultivated LCD-producing algae, the detected concentrations of LCDs in SPM are too high to be explained by the abundances of the suspected LCD-producing OTUs. This is likely explained by the slower degradation of LCDs compared to DNA in the oxic water column and suggests that some of the LCDs found here were likely to be associated with suspended debris, while the DNA from the related LCD producers had been already fully degraded. This suggests that care should be taken in constraining biological sources of relatively stable biomarker lipids by quantitative comparisons of DNA and lipid abundances.
Highlights
Long chain alkyl diols (LCDs) are lipids that consist of a linear alkyl chain with 22–38 carbons, hydroxylated at both the terminal carbon atom and at an intermediate position, and usually saturated or monounsaturated
The Heterocystous Cyanobacteria Cruise (HCC) cruise sailed across tropical Atlantic waters (Fig. 1a) in late summer and was targeted at suspended particulate matter (SPM) from the photic zone collected at the surface, the BWML, and the deep chlorophyll maximum (DCM)
Four of these operational taxonomic units (OTUs) are affiliated with classes that include few LCD-producing species (i.e. Chrysophyceae and Dictyochophyceae), whereas the remaining 23 OTUs belong to taxa in which the presence of LCDs has never been assessed
Summary
Long chain alkyl diols (LCDs) are lipids that consist of a linear alkyl chain with 22–38 carbons, hydroxylated at both the terminal carbon atom and at an intermediate position, and usually saturated or monounsaturated. The marine biological sources of LCDs are still not fully clear because, in contrast with the widespread occurrence of LCDs in the sediment, few marine taxa have been shown to contain these lipids. Eustigmatophyceae contain C30 1,13-, C30 1,15-, and C32 1,15-diols (Volkman et al, 1992; Rampen et al, 2014a), but they comprise mostly freshwater species, and only a few rare marine representatives from the genus Nannochloropsis are known (Andersen et al, 1998; Fawley and Fawley, 2007). The widespread occurrence of LCDs in the marine environment, despite the restricted abundance and distribution of known marine LCD producers, suggests that these compounds may be produced by unknown phytoplankton species. C26−32 diols have been occasionally detected in other plants (Buschhaus et al, 2013) This suggests that vegetal debris may in principle source LCDs in seawater
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