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.
Size-fractionated iron concentrations in the water column of the western North Atlantic Ocean
