Radiocarbon in marine bacteria: Evidence for the ages of assimilated carbon

It is generally accepted that marine bacteria utilize labile, recently produced components of bulk dissolved organic matter. This interpretation is based largely on indirect measurements using model compounds and plankton-derived organic matter. Here, we present an assessment of the relative proportions of modem and older dissolved organic carbon (DOC) utilized by marine bacteria. Bacterial nucleic acids were collected from both estuarine (Santa Rosa Sound, FL) and open-ocean (eastern North Pacific) sites, and the natural radiocarbon signatures of the nucleic acid carbon in both systems were determined. Bacterial nucleic acids from Santa Rosa Sound were significantly enriched in radiocarbon with respect to the bulk DOC and were similar to the radiocarbon signature of atmospheric CO2 at the time of sampling, indicating that these bacteria exclusively assimilate a modem component of the estuarine bulk DOG. In contrast, bacterial nucleic acids from the oceanic site were enriched in C-14 relative to the bulk DOC but depleted in C-14 with respect to modem surface dissolved inorganic carbon (DIC) and suspended particulate organic carbon (POCsusp). This suggests that open-ocean bacteria assimilate both modem and older components of DOG. The distinct radiocarbon signatures of the nucleic acids at these two sites (i.e., +120 +/- 17% estuarine vs. -34 +/- 24% oceanic) demonstrate that natural C-14 abundance measurements of bacterial biomarkers are a powerful tool for investigations of carbon cycling through microbial communities in different aquatic systems.