Rhubarb@NOCS

Tuesday 3rd of April Liz Sargent will present: Assessing the direct contribution of TRICHODESMIUM TO export

 Trichodesmium, a colonial marine cyanobacterium, is integrally involved in ocean biogeochemical cycling as it is a significant supplier of fixed nitrogen to the warm surface ocean.  Recent reports have suggested that Trichodesmium is also important in the subsurface layer, and actively fixes nitrogen in the deep chlorophyll maximum (DCM); however, the role Trichodesmium plays in the biogeochemistry of deeper waters has yet to be described.  This study focuses on Trichodesmium’s involvement in the direct export process.  Contrary to previous expectations, results suggest that despite its buoyancy this organism is a constituent of sinking material. Sampling on research cruises in the eastern subtropical and tropical Atlantic, and in the Gulf of Mexico showed Trichodesmium was commonly present below 100 m in three forms: tufted colonies, free filaments, and free filaments included in aggregations with other organisms/faecal matter.  The Marine Snow Catcher (MSC), a 100 L messenger-operated PVC closing water bottle, and in situ Stand Alone Pumping Systems (SAPS) were used to collect sinking particles, which were imaged and preserved for post-cruise assessment.  All sub-DCM MSC collections between 80-250 m in areas where Trichodesmium was a significant counterpart of the surface population included negatively buoyant colonies sinking at 12 - 120 m d^-1, as well as free filaments; SAPS collections also revealed the presence of free filaments in low concentrations as deep as 500 m. Further microscopic analysis of these colonies will allow for the elucidation of the mechanism of sinking in Trichodesmium, such as gas vacuole collapse, as well as aiding in describing its involvement in the export flux of POC and PON.