Characterization of bacterial epibionts on the cyanobacterium Trichodesmium

Interest in Trichodesmium, a nitrogen-fixing genus of Cyanobacteria, has been fueled by its prominent role in the marine nitrogen cycle. However, it is often overlooked that Trichodesmium occur in the ocean as colonies and are only one member of a complex microbial consortium. In the present study, we used 16S rRNA gene sequences to phylogenetically classify the communities associated with 2 morphological types of Trichodesmium colonies, tufts and puffs, which were collected at the Bermuda Atlantic Time Series (BATS) site in the Sargasso Sea. Both Trichodesmium morphotypes were most closely related to T. thiebautii at >= 99% identity. Non-cyanobacterial sequences from both types of colonies were dominated by Flavobacteria, Sphingobacteria, and Alphaproteobacteria. However, the epibiotic communities possessed significantly lower diversity than bacterioplankton; major seawater planktonic taxa, such as the SAR11 clade and Archaea, were conspicuously absent. Moreover, several epibiotic taxa appeared to be novel. Among the tuft or puff 16S rRNA clone libraries constructed in the present study, epibionts sharing common operational taxonomic units at the 97 percent sequence identity (PSI) threshold or higher were not observed, presenting the possibility that these 2 morphologies could select for different epibiotic communities. While our data are representative of a single sample point in time and space, these data suggest that Trichodesmium possess an epibiotic microbial community of relatively low diversity, distinct from that observed in bacterial picoplankton.