Aggregate turnover at the base of the euphotic zone – Influences of diatom morphology on carbon export

The world’s oceans play an important role in sequestering carbon over timescales of 105 years and more. For the carbon to be transferred from the atmosphere to the deep ocean where it is stored, it first has to be fixed in the form of particulate organic matter. This is mostly done by microalgae like diatoms, which fix CO2 through photosynthesis. When particle abundances are high (for example during a phytoplankton bloom), diatoms and other organic and inorganic matter aggregate and sink through the water column to the deep sea. Only a small fraction of the carbon fixed by phytoplankton reaches the seafloor because it is getting degraded by microorganisms and higher animals while it is sinking through the water column. Therefore, understanding the factors that influence degradation and carbon release is important for predicting and analyzing local as well as global carbon fluxes. One factor that has been neglected so far is the effect of phytoplankton morphology on carbon export. For this thesis, I studied two species of diatoms exhibiting common but contrasting morphologies: chain-forming and non-chain-forming, and studied the aggregation process and other down-stream factors affected by aggregate morphology: size, sinking velocity, carbon and nitrogen content, and microbial degradation rate. The process of aggregate formation and the appearance and structure of the respective aggregates strongly suggested that in chain-forming diatoms physical aggregation via the entangling of chains is the predominating process, whereas in non-chain-forming diatoms, TEP was the main factor causing aggregation. This had large influences on the settling behavior and the carbon content, thus affecting the amount of bacterial colonization and aggregate degradation