Intact polar diacylglycerol biomarker lipids isolated from suspended particulate organic matter accumulating in an ultraoligotrophic water column

Intact polar diacylglycerols (IP-DAGs) are essential components of cell membranes. Because they are structurally diverse and hypothesized to represent primarily living cells, they are potential molecular markers for a recent contribution by microbial communities to various carbon reservoirs. This study employed a novel molecular networking approach to investigate the evolution of IP-DAG structural diversity with depth in an ultraoligotrophic environment of the western South Pacific Ocean to test the hypothesis that particle transport to depth is rapid enough to preserve the IP-DAG biomarker signature of the photosynthetic community. IP-DAG profiles of several cultured cyanobacteria and photosynthetic picoeukaryotes were used as templates for constructing molecular networks to compare and interpret IP-DAG signatures of suspended particles isolated from a water column depth profile. Analysis of corresponding genetic community composition data for the same field samples was used to connect IP-DAG structures with their likely biological sources. Our data show that, although most IP-DAG classes associated with photosynthetic organisms were not observed below the euphotic zone, several other IP-DAG classes in deep samples might provide interesting targets for future studies seeking to examine the in situ contribution of deep sea microbes to suspended particulate organic matter (POM). Overall, the results represent the deepest water column IP-DAG dataset to date and demonstrate the utility of molecular networking for analyzing and visualizing complex environmental datasets.