Microbial contributions to suspended POM accumulation in an ultraoligotrophic water column, investigated using intact polar diacyglycerol biomarker lipids

New insights into aggregation processes and predicted climate-influenced shifts in microbial size classes have prompted a re-evaluation of the contribution of picophytoplankton to carbon fixation and subsequent organic matter export to depth, necessitating the development of new methods that may allow the quantitative determination of the scale of that contribution. Intact polar diacylglycerols (IP-DAGs) are essential components of both bacterial and eukaryotic cell membranes. Structurally diverse and hypothesized to represent primarily living cells, IP-DAGs therefore have the potential to act as molecular markers for recent carbon flow in microbial communities, but this potential has yet to be sufficiently tested. This study aimed to address this in several ways: first, we characterized the IP-DAG profiles of several cultured organisms, including cyanobacteria and photosynthetic picoeukaryotes, with the goal of using these profiles as templates for examining environmental samples. Second, we investigated the distribution of IP-DAGs in the ultraoligotrophic Tonga Trench region in the western South Pacific, to address the extent to which these IP-DAG signatures can be used as biomarkers for organic matter derived from surface ocean communities versus that produced in situ by microbes deeper in the meso- to bathy-pelagic zones. Finally, as an additional validation measure, we compared the IP-DAG composition of the field samples to corresponding genetic community composition data. Together the results represent the deepest IP-DAG dataset collected to date, and provide new insight into the identity of IP-DAG structures that can potentially be used as indicators of export contributions from bacterial and eukaryotic picophytoplankton.