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» Bacterial associates of the cyanobacterium Trichodesmium

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Scanning Electron Micrograph of Trichodesmium community culture K-11-131 collected from the South Pacific in 2007 and maintained in continuous culture to present. (Joint Program Student, Laura Hmelo)

Determination of potential metabolic pathways from epibiotic microbes associated with Trichodesmium cyanobacteria communities

Benjamin Van Mooy, Associate Scientist, WHOI

Sonya Dyhrman, Associate Scientist, WHOI

Joint Genome Institute, Department of Energy- CSP2010

Cyanobacteria of the genus Trichodesmium are nitrogen-fixing phototrophs that can periodically form intense blooms spanning hundreds of square miles in subtropical and tropical waters (Westberry and Siegel 2006, Capone et al., 1997). Capable of fixing at least 80 Gt N y-1 of atmospheric nitrogen, these microbes thrive in extremely low-nutrient (oligotrophic) marine environments and are understood to be a major source of organic carbon and reduced nitrogen, creating hotspots of microbial activity in surface waters (Capone et al., 1997; Letelier and Karl, 1996). Trichodesmium can exist in various “puff” or “tuft” morphotypes and are well-known for harboring epibiotic bacteria, covering Trichodesmium filaments in high density. These epibionts had for many years been hypothesized to be taking advantage of these organic exudates, however, the community composition of these epibionts has not been characterized using cultivation-independent molecular methods. Employing Small Subunit Ribosomal RNA (SSU rRNA) gene clone library sequencing methods, we have gathered strong evidence that the Bacteria colonizing the surface of Trichodesmium are not random, rather, they are specific ecotypes, actively growing and adapted to the unique environment that this host provides. In addition, the most abundant epibiont clones found in our libraries are either rare or not observed in the Sargasso Sea or GOS picoplankton metagenomic datasets or other picoplankton SSU rRNA surveys. Together, our findings suggest that the Trichodesmium host is the foundation of an ecosystem harboring unique copiotrophic microbial species relative to the surrounding oligotrophic water column. Specific Aim: To determine if the ecological success of Trichodesmium in the oligotrophic environment is a cooperative metabolic effort among a community of microbes physically associated with this cyanobacterial host.

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