Lipidomic analysis of trophic structure and microbial community dynamics over a diel time-series in Monterey Bay

Here we use lipid biomarkers to elucidate the microbial dynamics of the biological carbon pump in highly productive waters over the continental margin. In June 2019, particulate lipids (>0.2 μm) in the water column were sampled every 12 hours for 4 days on a process study cruise to the Monterey Bay Canyon lead by MBARI. Meta-lipidomic analysis was conducted using high-resolution accurate-mass MS paired with UHPLC, a suite of authentic standards, msⁿ spectral libraries, and a combination of bioinformatics pipelines for compound annotation and statistical analysis. Similar to a recent open ocean study, we observed an increase in storage molecules called triacylglycerides (TAGs) in the surface ocean over the day and a decrease at night. This suggests a build-up of TAGs as phytoplankton photosynthesize and a decrease as phytoplankton shunt energy towards cellular repair, respire, or succumb to grazing at night. Wax esters, which are storage molecules unique to zooplankton, composed a small percentage of the meta-lipidome. While wax esters were relatively more abundant in the surface ocean at night, it was not statistically significant. Larger volume filtration would have been more appropriate for sampling zooplankton and their diel vertical migration. C18 polyunsaturated fatty acids associated with phytoplankton decreased with depth, reflecting microbial degradation of phyto- derived organic matter and a shift in the community towards heterotrophic bacteria below the euphotic zone. The ratio of chlorophyll to chlorophyll degradation products in the euphotic zone decreased throughout the study, consistent with a decline in bloom health with time. The suite of pigment degradation products suggests top-down mechanisms of bloom decline such as grazing and viral infection may have been at play. Future iterations of this work will integrate environmental DNA analyses to provide a clearer picture of community structure and evaluate the used of lipid biomarkers for taxonomy.