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Surprising habitats in the Congo lobe region

 

In addition of being a large repository of terrestrial organic matter originating from the Congo River, the lobes of the Congo deep-sea fan host peculiar ecosystems which associate bacterial mats and Vesicomyidae bivalves. The functioning of these ecosystems is still mysterious with regards to the interactions between the bacteria and the bivalves and their overall relation to the organic substrate on which they live. When looking at the picture below (Fig. 1), the precise delimitation of the habitat hosting bivalves and bacteria is striking. Given the well-delimited shape and the concentration of organisms inhabiting a relatively small patch, one would expect to find a larger quantity of substrate within the limits of the assemblage.

Picture of the peculiar bacteria-bivalve ecosystem

Fig. 1: Picture of the peculiar bacteria-bivalve ecosystem, with the position of the cores. CT14 was 1 meter away from the habitat.

Recent work performed during two Master 1 research internships in ISTeP (Anabel Aboussou-UPMC) and LSCE (Chabbha Berrached-CNRS/CEA/UVSQ) showed a more contrasted pattern.
As expected from the population density of the organisms, the DOU (Diffusive Oxygen Uptake) calculated for two locations inside (CT3, CT11) and outside (CT14) the bivalve-bacteria assemblage shows much larger values inside (Fig. 2A). The DOU reflects the overall ecosystem functioning, and is linked to the intense catabolism of both the bivalves and bacteria which use the organic substrate as the ultimate fuel. This would further support the hypothesis of higher organic carbon (OC) content inside than outside the habitats. However, the average OC concentration of the two areas (Fig 2B) are similar (it could even be larger outside the habitat) indicating little difference in quantity of sedimentary OC between the interior and the exterior of the assembalges.

Figure 2: A) average diffusive oxygen uptake (from onboard oxygen microprofiles) indicating the overall metabolism of the ecosystem (red is inside the ecosystem, blue is outside) B) average organic carbon concentration in each cores down to 12cm.

Figure 2: A) average diffusive oxygen uptake (from onboard oxygen microprofiles) indicating the overall metabolism of the ecosystem (red is inside the ecosystem, blue is outside) B) average organic carbon concentration in each cores down to 12cm.

Then, a question arises: why does the inside of the habitat display a much larger catabolic activity with a similar or lower quantity of substrate?
It could be the quality of the substrate which is different. Yet, little difference was found when comparing the oxygen Index (OI) or the hydrogen Index (HI) derived from the Rock-Eval analysis, indicating that the differences of organic matter type found inside and outside of the habitat is not so obvious. To provide more information on the characterization of the organic substrates, their palatability, the composition of the labile fractions, other biomarkers are currently being investigated (e.g Master of Haolin Wang and PhD of E. Stetten at LECOB/IsTep-UPMC): these data could help unravel the present mystery.