## 4 Discussion  
Our initial goal was to search for, and identify, potential AHL QS molecules from bacteria of the genus _Marinobacter_ that could be involved in cell-cell signaling of potential importance to population-dependent colonization of algal or other surfaces such as marine snow. As \(\gamma\)-proteobacteria are common producers of AHLs we were surprised to find only a very few examples in the literature [35] of a _Marinobacter_ isolate as a possible AHL producer and no examples from our own screening of 40+ laboratory cultures of _Marinobacter_ species. Searching the available _Marinobacter_ genomes also failed to provide any evidence for the presence of AHL receptors or biosynthetic machinery. Thus, we conclude that most, if not all, members of this genus do not produce, nor utilize, an AHL-based QS system. In the absence of a clearly defined QS system, we looked for alternate means by which bacterial populations in a particular ecological niche could be controlled and this led us to search for the production of bacteriocin-like substances by _Marinobacter_. Thus, when two sibling colonies of _M. algicola_ DG893 were plated near each other on a soft nutrient agar plate, the colonies expanded isotropically until a well-defined distance was reached when colony expansion halted at the interface between the two colonies, forming a demarcation line (DL) or zone of mutual inhibition. Using a range of _Marinobacter_ isolates, including our model _M. algicola_ DG893, we repeatedly observed intercolony inhibition between sibling colonies (Table S1). It should be noted that intercolony inhibition was not restricted to \"siblings\" but was also observable between different species of _Marinobacter_. However, it is not clear that the mechanism is the same and this will be the subject of further investigation. \begin{table}
\begin{tabular}{l l l l} \hline
**Node** & **Annotation** & **Acc. Number** & **Score** \\ \hline EDM46611.1 & Glyceorophosphoryl diester phosphodiesterase & MDQ893\_19429 & Query \\ EDM49251.1 & FAD dependent oxidoreductase & MDQ893\_07635 & 0.890 \\ EDM47432.1 & Glyceorophosphoryl diester phosphodiesterase & MDQ893\_00180 & 0.877 \\ gpsA & Glycerol-3-phosphate dehydrogenase (GPDH) & MDQ893\_06880 & 0.807 \\ EDM47089.1 & Glycerol kinase & MDQ893\_11889 & 0.756 \\ EDM45841.1 & Glycerol kinase & MDQ893\_05159 & 0.756 \\ EDM46512.1 & Phosphodiesterase/alkaline phosphatase D & MDQ893\_20089 & 0.754 \\ EDM47357.1 & Flagellar secretion chaperone protein Fils & MDQ893\_01080 & 0.724 \\ EDM46845.1 & Extracellular nuclease & MDQ893\_17732 & 0.721 \\ EDM47042.1 & Predicted phosphatase & MDQ893\_02110 & 0.712 \\ EDM47259.1 & Chemotaxis protein histidine kinase & MDQ893\_18974 & 0.681 \\ \hline \end{tabular}
\end{table} 
Table 1: _M. algicola_ glycerophosphoryl diester phosphodiesterase (GDPD) potential protein interactions based on the STRING database. Superficially similar phenomenon between genotypically identical colonies growing on solid media where two types of behavior (merging vs. formation of DL) have been described by many authors [55; 56; 57; 58; 59] and a number of mathematical models that mirror this behavior have been presented. In most of these examples, colonies are found to merge when the agar concentration was low and to form a DL when the agar concentration was high, with nutrient depletion being the primary driver. Thus, it was proposed that rapidly expanding colonies (i.e., those growing on low percentage agar and moving rapidly via swimming or swarming) never have a chance to experience nutrient depletion before merging, while those expanding more slowly on high percentage agar have time to deplete the nutrients between colonies and thus stop growing at the interface, forming a DL. However, only in a few cases was a biochemical mechanism to account for this latter effect have been elucidated, such as _P. dendritiformis_, which produces and releases a protein termed \"sibling lethal factor\" (SIf), resulting in the killing of cells at the interface between colonies [49; 50], or _M.