The compositional heterogeneity of particulate organic matter from the surface ocean: An investigation using flow cytometry and DT-MS

In this study the morphological and chemical heterogeneity of particulate organic matter (POM) in the surface ocean was investigated by the application of flow cytometry, direct temperature-resolved mass spectrometry (DT-MS), and multivariate statistics. In order to obtain a diverse collection of surface-ocean POM samples, POM was collected from a station in Great Harbor, Woods Hole, MA, U.S.A. (on a monthly basis) and on transects across the Mid-Atlantic Bight (in March 1996). Small-particle (< 53 μm) POM was separated from large-particle (> 53 μm) POM by filtration through a nylon screen. Aliquots of the small-particle POM were then further separated, via flow cytometric sorting into phytoplankton and detrital pools. Broad-band molecular-level characteristics of these POM subclasses were obtained through DT-MS (16 eV, El⁺) and were explored via principal component and discriminant analyses. For both the Woods Hole (WHTS) and the Mid-Atlantic Bight (MAB) data sets, the statistical treatments separated POM samples primarily according to their class and subclass distinctions; therefore, this paper focuses on the molecular-level differences among large-particle and small-particle POM, phytoplankton and detritus. Small-particle POM is found to be more enriched in protein, phytosterols, diglycerides and chlorophyll than large-particle POM, which is more enriched in pentose polysaccharide fragments, C(16:0) fatty acid, and cholesterol. At the WHTS site, large-particle POM is also enriched in chitin and appears to contain a significant grazer biomass component. In the MAB, the corresponding POM fraction appears more phytodetrital. Phytoplankton particles are enriched in protein, chlorophyll and lipids relative to detritus. Detritus is enriched in selected polysaccharides and appears to be composed of phytodetritus and/or fecal material.