Rapid Abiotic Transformation of Marine Dissolved Organic Matter to Particulate Organic Matter in Marine Surface and Deep Waters.

When aquatic samples are filtered, the particulate organic matter (POM) is retained on filters and the organics in the filtrate are considered dissolved organic matter (DOM). POM is composed of membrane enclosed particles like cells or fecal pellets, and hydrogels. POM can be quantified as particulate organic carbon (POC) and nitrogen (PON), basic parameters of the pelagic ocean that are used to analyze its ecology and biogeochemistry, from the vertical transport of organics to the ecology of microbes and filter feeders. Here we report the rapid conversion of DOM in the filtrate that had passed through GFF filters (<0.7um) into POM retained by the same type of filter. We filtered seawater from different sources and locations (phytoplankton culture, marine surface and deep water (1000-2000m)) and measured POC and PON. After the initial sample filtration yielding the conventional POC/PON sample (POM-1) we re-filtered the filtrate yielding POM-2, re-filtering its filtrate we obtained POM-3 and so on till POM-i. For cultures and surface water the POM-2 to POM-i were about 10 percent of POM-1; from POM-2 on, with successive re-filtration the POM-i tended to decrease slightly. For deep water from the Gulf of Mexico the POM-2 was often higher than POM-1. POM adsorbed to the GFF filter contributed little to the measured POM. Bacteria that had passed the POM-1 GFF filter could not explain the results. We suggest that POM-2 and higher are mainly composed of gels that are abiotically formed from dissolved organics as a result of hydrologic stress when the sample is passing through the GFF filter. Experiments with transparent exopolymer substances (TEPs) yield similar results. Our results underline the relative importance of gels in the oceanic carbon cycle, specifically in deep waters. We discuss methodological and ecological implications of our results.