Antagonistic interactions among marine pelagic bacteria

Recent studies suggest that bacterial abundance and species diversity in th e ocean's water column are variable at the millimeter scale, apparently in response to the small-scale heterogeneity in the distribution of organic ma tter. We hypothesized that bacterium-bacterium antagonistic interactions ma y contribute to variations in community structure at the microscale. We exa mined each of the 86 isolates for their inhibition of growth of the remaini ng 85 isolates by the Burkholder agar diffusion assay. More than one-half o f the isolates expressed antagonistic activity, and this trait was more com mon with particle-associated bacteria than with free-living bacteria. This was exemplified by members of the cz subclass of the class Proteobacteria ( alpha -proteobacteria), in which production of antagonistic molecules was d ominated by attached bacteria. We found that gamma -proteobacteria (members of the orders Alteromonadales and Vibrionales) are the most prolific produ cers of inhibitory materials and also the most resilient to them, while mem bers of the Bacteriodetes were the organisms that were least productive and most sensitive to antagonistic interactions. Widespread interspecies growt h inhibition is consistent with the role of this phenomenon in structuring bacterial communities at the microscale. Furthermore, our results suggest t hat bacteria from pelagic marine particles may be an underutilized source o f novel antibiotics.