Primary production in the oceans depends on, among other nutrients, the presence of iron in the seawater. Iron does not dissolve well in seawater and needs to be bound by a soluble substance to be bioavailable: iron-binding organic ligands. These ligands are a diverse mix of organic substances and relative contributors are understudied. This dissertation aims to gain insight in sources of iron-binding organic ligands and to study the effect of microbial processes through seaborne and laboratory study.We find that in the Mediterranean Sea sources of these ligands are diverse, and sometimes different from the sources of iron. In the Arctic Ocean, iron and organic material from surrounding rivers is transported across the surface. Among which humic substances, bacterial breakdown products known to bind iron, which are an important part of the ligand pool here. This transport is expected to be highly sensitive to climate change which changes the discharge of organic material in river catchments.In laboratory study we find that viral infection of phytoplankton is hampered by low iron concentrations, limiting the effectiveness of nutrient cycling through the viral breakup, or lysis, of phytoplankton cells. We also show that viral lysis increases the concentration of ligands, dependent on the phytoplankton host species. Bacteria are found to modify parts of the ligand pool, and pose a possible loss factor of ligands.Finally, we identify important considerations for methodology, as no approach currently applied is capable of elucidating the composition of the iron-binding organic ligand pool alone.
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