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Title: Microbial communities in sinking and suspended particles and their influence on the oceanic biological carbon pump
Author: Duret, Manon
ISNI:       0000 0004 7656 3248
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2018
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Export of photosynthetically produced organic matter, from the sunlit to the dark ocean, in the form of sinking particles represents the major mechanism of the biological carbon pump that removes CO2 from the atmosphere. Most of the organic matter bound in sinking particles undergoes microbial remineralisation while traversing the water column, thereby causing CO2 and inorganic nutrients to be released. Increasing evidence indicates that most remineralisation does not occur directly on sinking particles, but rather on suspended particles and dissolved organic matter resulting from their disaggregation and solubilisation. Most particulate organic carbon in the mesopelagic ocean is bound to suspended particles, which represent a major substrate for heterotrophic organisms. Despite their crucial importance, suspended particles and their associated microbial communities have been largely overlooked in favour to sinking particles. This thesis presents the first comparison of diversity and functionalities between microbial communities associated with suspended and sinking particles. Using amplicon sequencing of small-subunit ribosomal RNA genes on particles collected with a marine snow catcher deployed in the Southern Ocean, this thesis demonstrates that prokaryotic communities associated with suspended and sinking particles differ significantly. Particle-associated remineralising bacteria showed a clear preference for either particle-type likely relating to differential organic matter composition. Suspended particles from the upper-mesopelagic were predominately composed of prymnesiophytes and soft-tissue animals, while more efficient carbon export from diatoms was indicated by their prevalence in sinking particles. Eukaryotic sequences associated with suspended and sinking particles were largely dominated by heterotrophic protists, highlighting their major contribution to particulate organic matter remineralisation in the upper-mesopelagic. Finally, remineralisation activities, as well as nitrogen and sulphur cycling, were investigated by comparing metatranscriptomes of various particle-types collected in the North Atlantic. Free-living, small sinking and small suspended particle-associated microbes appeared most active in the remineralisation of simple organic compounds, while large suspended particles acted as the main venue of complex organic matter remineralisation. Additionally, actively expressed genes related to anaerobic processes in small particles corroborate recent postulations that marine particles may serve as oxygen-deficient microniches, and hence, may be key to redox cycling of elements in the ocean. Overall, this dissertation highlights differences between suspended and sinking particles as well as their potential biogeochemical implications in the ocean and provides further insights into constraints shaping the oceanic biological carbon pump.
Supervisor: Lam, Tin Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available