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The cycling of carbon (C) by benthic organisms is a key ecosystem function in the deep sea. Pulse-chase experiments are designed to quantify this process, yet few studies have been carried out using abyssal (3500-6000 m) sediments and only a handful of studies have been undertaken in situ. We undertook eight in situ pulse-chase experiments in three abyssal strata (4050-4200 m water depth) separated by tens to hundreds of kilometers in the eastern Clarion-Clipperton Fracture Zone (CCFZ). These experiments demonstrated that benthic bacteria dominated the consumption of phytodetritus over short (similar to 1.5 d) time scales, with metazoan macrofauna playing a minor role. These results contrast with the only other comparable in situ abyssal study, where macrofauna dominated phytodetritus assimilation over short (2.5 d) time scales in the eutrophic NE Atlantic. We also demonstrated that benthic bacteria were capable of converting dissolved inorganic C into biomass and showed that this process can occur at rates that are as high as the bacterial assimilation of algal-derived organic C. This demonstrates the potential importance of inorganic C uptake to abyssal ecosystems in this region. It also alludes to the possibility that some of the C incorporation by bacteria in our algal-addition studies may have resulted from the incorporation of labeled dissolved inorganic carbon initially respired by other unstudied organisms. Our findings reveal the key importance of benthic bacteria in the short-term cycling of C in abyssal habitats in the eastern CCFZ and provide important information on benthic ecosystem functioning in an area targeted for commercial-scale, deep-sea mining activities.

Biogenic reefs constructed by the tube-building ecosystem engineer Lanice conchilega (Terrebilidae, Polychaeta) have profound structuring impacts on the benthic environment in that they alter the biogeochemical and physical properties of the sediment. This study provides new insights into the functioning and effects on food webs of L. conchilega reefs in intertidal sediments using linear inverse models to quantify the carbon flows in the food webs in the presence and absence of the tubeworm. The inverse food web models were based on an empirical dataset from 2 study sites, which provided biomass and stable isotope data, and information on general physiological constraints from the literature. Results of the model showed that the carbon input into reef food webs (mean ± SE; 191 ± 50 mmol C m -2 d -1 ) is ca. 40 times higher compared to bare sand areas (5 ± 2 mmol C m -2 d -1 ) and is mainly derived from organic matter (OM) in the water column. Most of the OM input towards these reefs is consumed by suspension-feeding macrofauna, particularly L. conchilega ; however, the worm is not an important source of carbon for other macrofaunal organisms. The ratio of OM input to primary production indicates that the OM needs to be produced in an area at least 15 times larger than the reef area, demonstrating significant OM ‘focussing’ within the reef food web. The reef structures created by L. conchilega act as a trap for OM, resulting in an overall higher macrofaunal biomass and much more diverse food webs than in the absence of the tubeworm.