Abstract: Trichodesmium are colonial marine N2-fixing cyanobacteria that contribute significantly to new nitrogen pools in the oligotrophic tropical and subtropical oceans. Due to the high iron requirement for diazotrophic growth and the low availability of iron in oceanic waters, iron availability has been proposed to be an important limiting factor for Trichodesmium growth and N2 fixation. Currently little is known about the mechanisms employed by Trichodesmium to acquire iron. Because Trichodesmium colonies are large enough to support a diverse array of associated species, we assert that Trichodesmium iron acquisition can only be fully understood by viewing the colonies as an integrated whole. In particular, we hypothesize that bacteria living in close association with Trichodesmium colonies may play a key role in facilitating colony iron acquisition, through the production of Fe chelating agents (eg. siderophores). Herein we propose laboratory and field investigations to explore this hypothesis within the larger context of mutualistic associations in nutrient-poor environments, using Trichodesmium colonies as unique and ecologically significant model systems. Our long-term objective is to gain an understanding of the biological and chemical interactions between Trichodesmium and its associated organisms and to use this knowledge to increase our understanding of nutrient uptake and recycling in oligotrophic oceans. This project is a multidisciplinary collaboration between marine microbiologists, co-PI Margo Haygood (SIO) and subcontractors and collaborators John aterbury (WHOI) and Eric Webb (WHOI); and chemists, co-PI Katherine Barbeau (SIO) and collaborators Alison Butler (UCSB) and Lihini Aluwihare (SIO). Specific goals include characterizing the microbial community associated with Trichodesmium colonies, and investigating the ability of the community to produce iron complexing compounds by using cultivated strains, close relatives of community members, or molecular approaches. With this information, radiotracer studies of Trichodesmium uptake of dissolved and colloidal iron, including iron complexes with marine siderophores from Trichodesmium-associated bacteria, will be carried out both in the laboratory and in the field. Molecular and uptake studies will be carried out with both cultivated and natural populations. Cultivation of non-axenic and axenic Trichodesmium strains will allow us to compare siderophore uptake and colloidal iron dissolution with and without the presence of bacteria. Collection of natural populations will be carried out in conjunction with ongoing programs at the Bermuda Atlantic Time Series station (BATS), where our preliminary work was done, and the Hawaii Ocean Time-series study (HOT), which will allow us to test the generality of our observations with Pacific populations, as well as on cruises of opportunity. This project will promote teaching, training and learning, and will broaden the participation of under-represented groups. Support is requested for a post-doctoral researcher and a graduate student to participate in the proposed research, providing valuable interdisciplinary training. The project will also host a student each summer through the UCSD STARS program, a challenging eight-week research program targeted for members of under-represented groups. In addition, travel money is requested for project participants to take an active part in minority outreach activities through UCSD's Office of Graduate Student Research.