Lipid and 13C signatures of submicron and suspended particulate organic matter in the Eastern Tropical North Pacific: Implications for the contribution of Bacteria

• Submicron particulate organic matter (X-POM) was compared to typical suspended POM.
• Fatty acids from IPLs carry a snapshot of δ13C values of freshly-synthesized biomass.
• IPL δ13C values vary up to 5‰ over depth, polar lipid classes, and POM size classes.
• X-POM appears to carry a relative 13C-enrichment deriving from bacterial metabolisms.
• The OMZ hosts a complex mixture of sinking particle sources and in situ metabolisms.

The contribution of bacterial biomass to total particulate organic matter (POM) in the ocean, including exported material, is poorly constrained. To examine potential signatures for the presence and export of bacterioplankton and their detrital remains, here we provide a detailed compound-specific 13C characterization of fatty acids from membrane polar lipids obtained from a water column profile in the Eastern Tropical North Pacific. POM of submicron size (0.2–0.7 μm; “X-POM”) was sampled and analyzed separately from the size class typically collected as “suspended” POM (0.7–53 μm; “L-POM”). The distributions of polar head group classes, specific fatty acid side chains, and natural 13C contents all vary, both between particle size classes and with depth in the water column. In general, the polar lipids in submicron material – and by inference, lipids of bacterial origin – have higher 13C content than polar lipids from larger POM and are equally abundant. Lipid signatures from the photic zone appear to be partially conserved in the suspended pool during transit down the water column. However, bacterial heterotrophy and possibly chemoautotrophy partially overprint these surface signatures. In addition, active metabolisms in the oxygen minimum zone (OMZ) appear to mediate the disaggregation of POM transported from the surface, thus adding complexity to the pathways of mid-water carbon flux and providing additional organic substrates to the OMZ and below. This “substrate injection” may provide important fuel for the denitrification and anammox reactions. Finally, examination of 13C content in polar lipids provides a basis for new interpretation of depth-related variations in δ13C values of bulk suspended POM.