Abundance, size distribution, and stable carbon and nitrogen isotopic compositions of marine organic matter isolated by tangential-flow ultrafiltration

Tangential-flow ultrafiltration was used to isolate particulate and high-molecular-weight dissolved material from seawater collected at various depths and geographic regions of the Pacific and Atlantic Oceans, Ultrafiltration proved to be a relatively fast and efficient method for the isolation of hundreds of milligrams of material, Optical and electron microscopy of the isolated materials revealed that relatively fragile materials were recovered intact. Depth-weighted results of the size distribution of organic matter in seawater indicated that similar to 75% of marine organic carbon was low-molecular-weight (LMW) dissolved organic carbon (< 1 nm), similar to 24% was high-molecular-weight (HMW) dissolved organic carbon (1-100 nm), and similar to 1% was particulate organic carbon (> 100 nm). The distribution of carbon in surface water was shifted to greater relative abundances of larger size fractions, suggesting a diagenetic sequence from macromolecular material to small refractory molecules, The average C:N ratios of particulate organic matter (POM) and HMW dissolved organic matter (DOM) were 7.7 and 16.7, respectively. Differences in C:N ratios between POM and HMW DOM were large and invariant with depth and geographic region, indicating that the aggregation of HMW DOM to form POM must be of minor significance to overall carbon dynamics, The stable carbon isotope composition (delta(13)C) Of POM averaged -22.7 parts per thousand in surface water and -25.2 parts per thousand in subsurface water. Several possible explanations for the observed isotopic shift with depth were explored, but we were unable to discern the cause. The delta(13)C Of HMW DOM samples was relatively constant and averaged -21.7 parts per thousand, indicating a predominantly marine origin for this material. The delta(15)N values of POM were highly variable (5.8-15.4 parts per thousand), and the availability of nitrate in surface waters appeared to be the major factor influencing delta(15)N values in the equatorial Pacific, In the upwelling region nitrate concentrations were relatively high and delta(15)N values of POM were low, whereas to the north and south of the upwelling nitrate concentrations were low and delta(15)N values were high. The delta(15)N values of HMW DOM reflected the same trends observed in the POM fraction and provided the first such evidence for biological cycling of dissolved organic nitrogen (DON). Using the observed delta(15)N values and an estimate of meridional advection velocity, we estimated a turnover time of 0.3 to 0.5% day(-1) for HMW DON, These results suggest a major role for DON in the upper ocean nitrogen cycle.