Phospholipid ester-linked fatty acids composition of size-fractionated particles at the top ocean surface

Surface microlayer and subsurface water samples were collected at an oligotrophic Mediterranean site during a diel cycle. The composition of phospholipid ester-linked fatty acids (PLFA) was determined in size-fractionated particles (0.2–0.7 μm; 0.7–200 μm) in order to characterize the major contributors of organic matter to different size fractions. GF/F-retained particles (0.7–200 μm) from the surface microlayer were consistently enriched in PLFA relative to the underlying water. Molecular markers revealed a substantial difference in biological assemblages in both particle sizes. The larger particles were dominated by dinoflagellates, cyanobacteria, microzooplankton and attached bacteria, whereas particles filtered through GF/F and collected on 0.2 μm porosity Durapore filters (0.2–0.7 μm) were mostly bacteria and heterotrophic flagellates. Bacterial PLFA associated with 0.7–200 μm particles were more abundant than those in the 0.2–0.7 μm particles. Specific markers in the branched series appeared more representative of bacteria of smaller particle size.