Fate of organic carbon released from decomposing copepod fecal pellets in relation to bacterial production and ectoenzymatic activity

Fecal pellets were produced by Acartia tonsa fed C-14-labeled diatom, cryptophyte, and dinoflagellate diets, and were incubated in 1.2 mum-filtered Long Island Sound seawater. Based on the C-14 label, the decrease in fpOC (fecal pellet organic carbon), the release and fate of dissolved organic carbon (DOC) and particulate organic carbon (POC), as well as bacterial production and enzymatic activity, were followed over a 96 h period. fpOC decreased by 9, 14, and 19% d(-1) in diatom, cryptophyte, and dinoflagellate pellets, respectively. There was a fast, possibly passive, leakage of DOC from pellets from all 3 diets within a few hours after egestion, which may not have been utilized by attached bacteria. Bacterial production rates were 17, 12, and 31 pg C pellet(-1) h(-1), on diatom, cryptophyte, and dinoflagellate pellets, respectively. These were 5 orders of magnitude higher than production rates of free-living bacteria, indicating that copepod fecal pellets are hot spots of pelagic microbial production. The high production was caused primarily by high initial bacterial abundances. Accordingly, production and growth were entirely uncoupled in diatom pellets. There were no increases in abundance of attached bacteria on any of the 3 diets, indicating that the produced bacterial cells were released from the fecal pellets. Attached bacteria had a higher ectoenzymatic activity than free-living bacteria, but their production and ectoenzymatic activity were uncoupled and they only assimilated a minor fraction of the released DOC. DOC was therefore released favoring free-living microbes. The chitinase activity, which increased several-fold, was coupled to the production of attached bacteria; thus, chitin may play an important role in bacterial production on copepod fecal pellets.