Microbial decomposition of large organic particles in the northwestern Mediterranean Sea: an experimental approach

Sediment trap particles, as well as particulate organic material including particles larger than 10 pm collected by in situ pumps (ISP) and fresh corp ses of the gelatinous zooplankton species Thalia democratica, were collecte d in the northwestern Mediterranean Sea from April to July 1995, and incuba ted (after mixing with 0.2 pm filtered seawater) under laboratory condition s with their own bacterial assemblages for 6 to 24 d in batches under oxic conditions and in the dark. Particulate (POC > 0.7 mu m), dissolved (DOC < 0.7 mu m) and colloidal (0.02 < COC < 0.7 mu m) organic carbon contents, as well as bacterial abundance and production, were quantified over time. In all experiments, total organic carbon (TOC = POC + DOG) decrease covaried w ith an increase in bacterial abundance and production, bacteria being the m ain mediators of particle decomposition. We found that COC accounted for 19 to 31% of DOC immediately after particle dilution in 0.2 pm filtered seawa ter, and always for less than 9 % at the end of the experiments. As organic colloids comprised less than 7 % of DOC in the 0.2 pm filtered seawater us ed to dilute the particles, this result suggests that COC was mainly produc ed from particle decomposition. Assuming that bacterial populations were th e sole decomposer of organic matter in the batches, the results gave bacter ial growth efficiencies (BGE) in the range of 3 to 21%, indicating that dec omposition of these particles significantly produce CO2 through bacterial r espiration. The results showed that bacteria degraded 16 to 87% of the init ial amount of POC within the first 48 h, whereas only 6 to 22 % of POC was degraded in the second stages. Our data and modeling work based on such sho rt incubation times suggest that salp bodies are composed of 1 labile and 1 refractory organic fraction, whereas both ISP- and trap particles are comp osed of 2 labile and 1 refractory organic fraction. A 1G-model (for salp) a nd a 2G-model for other particles was able to satisfactorily reproduce the data sets.