ALBERT

Description: 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 tropical North Atlantic 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 to 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 in other phylogenetic groups, were significantly correlated with C30 diol concentrations. However, it is not clear whether some of these OTUs might indeed correspond to LCD-producers or whether these correlations are just indirect. 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 to 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.