Microbial iron cycling on Trichodesmium colonies : laboratory culture studies of Trichodesmium and associated model organisms

Iron is an important micronutrient to the colonial cyanobacterium Trichodesmium, a photosynthetic diazotroph. Trichodesmium is known to be limited by iron, but field studies of iron acquisition by this organism have been complicated by the fact that natural colonies have a large associated bacterial community. The research presented in this thesis will focus on understanding iron bioavailability to Trichodesmium and the associated heterotrophic bacteria and how this may affect iron cycling in the colony environment. The bioavailability of a selection of iron sources to Trichodesmium and two representative associated bacteria, Microscilla marina and Silicibacter TrichCH4B is investigated in Chapter 2. The representative bacteria are shown to acquire iron from a diverse selection of iron complexes while Trichodesmium is shown to access iron from only the most labile iron sources. A model of iron uptake on Trichodesmium colonies is created using Trichodesmium- and bacteria-specific iron uptake rates, and results suggest that bacteria are potentially responsible for the majority of iron uptake in a colony environment. The iron acquisition pathways used by Trichodesmium to acquire two forms of bioavailable iron, inorganic iron and ferric citrate complexes, are investigated in Chapter 3. Trichodesmium is shown to utilize two different iron acquisition pathways, extracellular reduction of inorganic iron and a semi- specific iron transport system for ferric citrate. Both mechanisms may give Trichodesmium a means to compete for iron in the colony environment. In Chapter 4, the Trichodesmium-associated Roseobacter Silicibacter TrichCH4B is shown to have an uptake system specific for the utilization of heme complexes and the functional characterization of this system is described. Further investigation and identification of heme uptake systems in the Roseobacter clade, an abundant bacterial lineage in the ocean, and the detection of heme genes in environmental samples suggest that heme utilization may be wide spread in the marine environment and may be a pathway for iron recycling in the ocean. Based on these studies of iron bioavailability and utilization, iron must be efficiently cycled in a Trichodesmium colony, a unique oceanic particle environmen