The ocean is about as productive as land for photosynthesis and represents an important carbon sink. In the oceans, Fe, Zn, Co, Mn, and other transition metals are present at trace concentrations. Iron is known to limit the growth of phytoplankton and nitrogen fixation in large areas of the ocean and thereby determine the cycles of the major elements of life. While being present at trace concentrations, speciation experiments show that nutrient trace-metals are also bound to various organic ligands which can determine their biological availability. Strong ligands for iron and other metals can be released by bacteria and are suspected to play a central role in interactions with phytoplankton. Our current research focuses on the bloom-forming oligotrophic subtropical nitrogen-fixing cyanobacterium Trichodesmium and its bacterial epibionts.