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

It is generally accepted that marine bacteria utilize labile, recently prod uced components of bulk dissolved organic matter. This interpretation is ba sed largely on indirect measurements using model compounds and plankton-der ived organic matter. Here, we present an assessment of the relative proport ions of modem and older dissolved organic carbon (DOC) utilized by marine b acteria. Bacterial nucleic acids were collected from both estuarine (Santa Rosa Sound, FL) and open-ocean (eastern North Pacific) sites, and the natur al radiocarbon signatures of the nucleic acid carbon in both systems were d etermined. Bacterial nucleic acids from Santa Rosa Sound were significantly enriched in radiocarbon with respect to the bulk DOC and were similar to t he radiocarbon signature of atmospheric CO2 at the time of sampling, indica ting that these bacteria exclusively assimilate a modem component of the es tuarine bulk DOG. In contrast, bacterial nucleic acids from the oceanic sit e 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 par ticulate organic carbon (POCsusp). This suggests that open-ocean bacteria a ssimilate both modem and older components of DOG. The distinct radiocarbon signatures of the nucleic acids at these two sites (i.e., +120 +/- 17% estu arine vs. -34 +/- 24% oceanic) demonstrate that natural C-14 abundance meas urements of bacterial biomarkers are a powerful tool for investigations of carbon cycling through microbial communities in different aquatic systems.