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WP5 Ecosystems

 
 

Web_WP5_vesicosCold seep ecosystems fuelled by methane rich fluids are among the most recently discovered marine habitats; the first such environment was described just 25 yr ago and more than 30 sites are now described in tectonically active settings. Dense biological communities are sustained by local primary production based on microbial oxidation of methane and sulphide, or chemosynthesis. Novel environments are thus likely to be found where in other unusual geological or oceanographic constructs (e.g., with oxygen minimum zones, trenches) or deep-sea fans in relation to elevated organic matter inputs. Typical cold-seep fauna such as symbiont-bearing bivalves have also been recognised driven by rapid recycling of organic matter like whale falls or sunken woods that can serve as stepping stones for cold-seeps or hydrothermal vent fauna.
The last step in the sedimentary metabolic pathways is methanogenesis which involves a group of strictly anaerobic archaea, called methanogens. They use a small number of low molecular weight substances for the biosynthesis of methane which in turn are formed by microbial fermentation from more complex organic substances during the early diagenesis. The dominant pathway for methane production is still questionable. However, the diversity and distribution of methanogens remain poorly described in marine sediments characterized by high incoming carbon fluxes such as Congo Lobes. Another important process which is related to methanogenesis is the anaerobic methane oxidation mediated by consortia of archaea probably able to reverse methanogenesis and mainly sulfatereducing bacteria. The formation of hydrogen sulfide constitutes a second energy source for chemoautotrophic organisms living on the seafloor.
The rare information available from previous investigations of the Congo lobes, revealing high metabolic demand, patches of chemosynthesis-based taxa such as large vesicomyid bivalves, dense aggregations of polychaetes and microbial mats low diversity of the nematode community with dominance of opportunistic genera, indicate the occurrence of biological features exceptional at this abyssal depth. Nevertheless, only visual observations by ROV were made and no samples are available. The species forming those communities need therefore to be described and characterized, and the community structure, spatial patterns and ecosystem functioning, particularly the microbial processes that sustain these communities and the contribution of chemosynthesis-based production in the food web are still unknown.

Web_WP5_tapisChemosynthesis-based communities occur at active cold seeps in the Congo basin where microbial and faunal communities formed highly diverse benthic habitats such as microbial mats and vesicomyid bivalve beds also observed in the deep Congo lobes. Comparison in terms of taxonomic composition, nutritional pathways and genetic connectivity with a will give a global view of chemosynthesis-based benthic production linked to an active channel and increase our knowledge on the contribution of the heterogeneity of reducing habitats to regional biodiversity. The increase in beta diversity across seep habitat types that have been recently demonstrated to be significant sources of heterogeneity on continental margins will be likely further increase by the description of these new reducing habitats.

This task will contribute to the following questions:
‐ What is the diversity and the distribution of microbial communities involved in the methane cycle ?
‐ How do the geochemical gradients impact the microbial communities structure and activities in the sediment of the recent and aged Congo lobes ? (in interaction with task 4)
‐ How does microbial community structure shape the biodiversity and spatial patterns of macrofauna communities? How do microbial communities contribute to the nutrition of the upper trophic levels, through symbiosis or free living microbe ingestion?
‐ How do the organic matter inputs by the channel in the terminal lobes contribute to the spatial patterns of the benthic fauna community structure? Are there related to subsurface or deep geological structures? What are the driving forces shaping the biological assemblages (in relation with other tasks)
‐ How different are the chemosynthesis-based community from the adjacent sediment one?
Is the benthic metabolism higher when linked to chemosynthesis production?
‐ What are the similarities and differences of the benthic community structure with the
“typical” cold‐seep communities linked to methane‐rich fluids on pockmarks?
‐ What is the contribution in terms of biodiversity to the regional seep community, and what
is the degree of genetic exchanges between populations in the different seep habitats of the
Congo fan?
‐ What is the larval supply to the Congo fan? How does it compare with the regional
community of settled organisms in terms of biodiversity?

Team:

Coordinator: Karine OLU