“Cellular” vs. “detrital” POM: a preliminary study using fluorescent stains, flow cytometry, and mass spectrometry

The mass of nonliving particulate organic matter (POM) in marine waters can be significantly greater than that in living cells, yet very little is known about the composition and ultimate fate of this “detrital” POM. In recent marine chemistry studies, flow cytometry has been used to provide further insights into the composition, source, and fate of chlorophyll-fluorescing POM (which includes living algal cells) vs. detrital POM within natural waters. In this study, we extend such morphological characterization of POM beyond the use of chlorophyll autofluorescence and particle size or light scatter. Instead, nucleic acid staining with SYTO-13 and flow cytometry sorting were used to identify and isolate “cellular” and “detrital” subclasses of POM from samples collected in the Lafayette River and the Elizabeth River (subestuaries in the lower Chesapeake Bay). The resulting subclasses, as well as aliquots of unsorted POM sample, were then analyzed by direct temperature-resolved mass spectrometry. We show that “cellular” and “detrital” subclasses differ significantly from each other, and that detrital POM appears chemically distinct from the bulk POM. We then suggest further applications for flow cytometry approaches in aquatic organic biogeochemistry.