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Although the sediment of the ORC box showed an increase in βG, CSEA, Eh, and pH (Figs. 1–3), the lack of evidence for a OM decrease (Fig. 3) may be explained by the excessive sedimentary organic load. An additional reason for the lack of CHO degradation could be due to the microcosms’ operating conditions: OM, once mobilized, is not removed (because of the closed system) and may sink back to the sediment. However, the overall functional and structural response observed using ORC compounds (both in the water and in the sediment compartments) supports the evidence for an increased OM degradation in the sediment compartment. In particular, the significant effect recorded in the water compartment suggests the occurrence of a strong benthic–pelagic coupling (ORC socks were positioned within the sediment). This implies a mobilization of OM in the sediment that, once released into the water column, may stimulate a prompt response by the bacterial community. This is in accord with Fabiano et al. (2003) who found that bacterial response and OM degradation during bioremediation of sediment was more evident in the overlying water due to the benthic–pelagic flux of dissolved carbon mobilized in the sediment.
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