The deep-sea floor has long been considered a 'food desert' but recent observations suggest that episodic inputs of relatively fresh organic matter (phytodetritus) occur and that benthic processing of this material may be rapid. Although the responses of the total community in terms of oxygen consumption and of some individual benthic groups have been identified, the quantitative role of the different groups in the short-term response remains largely unknown. We examined the short-term response in major benthic compartments in an in situ experiment in the NE Atlantic (2170 m water depth) using 13 C-enriched diatoms as a tracer of labile carbon. Within 35 h, 6 mg C m -2 was processed by the benthos, with the majority of the processed carbon recorded as respiration (45%). Among the fauna retained on a 300 µm sieve, Foraminifera were rapid consumers which, together with Bacteria, accounted for 50% of the processing. Therefore, although Bacteria dominate long-term carbon mineralization (as suggested by their general dominance in the benthic biomass), some faunal components, in this case Foraminifera, may play a central role in the rapid initial processing of fresh organic carbon in deep-sea sediments.

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The short-term benthic response to an input of fresh organic matter was examined in vastly contrasting benthic environments (estuarine intertidal to deep-sea) using 13 C-labeled diatoms as a tracer of labile carbon. Benthic processing was assessed in major compartments through 13 Cenrichment in ⌺CO 2 , in bacteria-specific phospholipids and in fauna tissue. A rapid response was evident in all environments. Under warm bottom water (14-18°C), similar quantities of the added carbon were respired within 24 hours in shallow and deep-sea sediments. However, the speed and magnitude of respiration were strongly reduced under low bottom water temperature (4-6°C), both in a shallow and a deep-sea site. Rapid carbon respiration even in deep-sea sediments almost devoid of fauna highlights the key role of bacteria, the most ubiquitous benthic component, in this short-term respiration of fresh organic matter. However, when present, fauna rapidly ingest algal material, thereby increasing the amount of carbon processed and directly extending carbon flow pathways.