Carbon - trace metal interactions in the oceanic twilight zone

Marine microbes are an important control on carbon (C) sequestration depth and biogeochemical cycling of nutrients and trace metals in the global ocean. The biological carbon pump (BCP) is the set of processes by which inorganic carbon (CO$_\textrm2$) (along with nutrients and trace metals) is fixed into organic matter via photosynthesis by autotrophic phytoplankton and the C, nutrients and trace metals sequestered away from the atmosphere generally by transport into the deep ocean. Most (\textasciitilde80 %) of the organic C produced by autotrophic phytoplankton is remineralised (returned to the dissolved inorganic inventory from the particulate organic form) in the surface ocean and the inorganic CO$_\textrm2$ is available for release back into the atmosphere. The depth at which remineralisation occurs is important, as the deeper the remineralisation depth of the C the increased likelihood of long term storage in the deep water and sediment. The sequestration of C is primarily dependent on flux attenuation and remineralisation of organic matter in the mesopelagic or ‘twilight’ zone (100-1000 m), where much of the downward particle flux is attenuated via zooplankton and bacterial respiration, replenishing dissolved nutrients and trace metals back into the water column. Understanding the controls on the BCP in the twilight zone is important to understand the transfer efficiency of C sequestration and the regulation of atmospheric CO$_\textrm2$. Oceanic regions such as the Southern Ocean have inefficient BCPs as the phytoplankton are unable to fully utilise available nutrients, restricting their growth and drawdown of C due to limited access to micronutrients such as iron (Fe). Iron is a scare resource in these regions and low concentrations of bioavailable Fe exert significant controls on global phytoplankton productivity, species composition and therefore ecosystem structure and the C cycle. Iron is not only an important micronutrient for phytoplankton growth but also for heterotrophic bacteria, limiting bacterial secondary production and abundance. Two focused and inter-related processes which influence Fe cycling and consequently C cycling in the mesopelagic were investigated. Firstly, differentiating the biotic and abiotic factors on Fe cycling in the twilight zone and the (de-) coupling of Fe and macronutrients at depth. Secondly, to investigate Fe and C (co-) limitation of mesopelagic bacteria. This researched performed shipboard experiments and subsequent laboratory work to evaluate the relative remineralisation rates of C, Fe and silica (Si) from live and detrital phytoplankton cells resuspended in upper mesopelagic waters. Iron consistently transferred from the particulate fraction into the dissolved fraction from both live and detrital cells, this transfer was dominated by the abiotic movement of extracellular adsorbed particulate iron into the dissolved fraction (de- absorption). The live phytoplankton cells remained viable throughout the incubations and continued to respire C whilst the detrital cells potentially leaked dissolved organic C which was subsequently taken up and respired by bacteria with minimal secondary bacterial production. Limited dissolution of Si occurred from the live viable cells with the detrital cells showing more Si dissolution potential. The remineralisation length scales of Fe, C and Si were thus decoupled in the upper mesopelagic as Fe resulted in the shortest remineralisation length scale due the abiotic transfer of extracellular Fe into the dissolved pool, which could resupply biota potentially alleviating Fe limitation. Intracellular pools of Fe (along with C and Si) would be exported to deeper depths with a slow remineralisation rate if processes such as grazing or cell lysis do not act to break cells up and speed up remineralisation processes. Heterotrophic bacterial production was Fe and C (co-) limited in the mesopelagic above the ferricline. An increase in cell abundance of very large high nucleic bacteria when combined Fe and C were added to mesopelagic waters from 150 and 500 m supported a large (1-2 order of magnitude) increase in bacterial production indicating the (co-) limitation of a sub-population of the free-living bacteria at depth. The controls on ferricline depth and mesopelagic standing stocks of Fe (from winter mixing, scavenging, Fe associated with sinking material and the de-absorption of Fe into the water column) will be important in determining the extent of ocean Fe C (co-) limitation of mesopelagic bacterial growth and production and will be a driver in bacterial community composition at depth. Nutrient limitation in the mesopelagic bacteria has potentially important consequences if it also reduces the overall rate of remineralisation and thus both generates a potential reinforcing feedback on the maintenance of a deep ferricline and increases the remineralisation depth and hence long-term storage of carbon in the ocean.