## 4 Discussion
xanthus_, which uses the type VI secretion system (T6 SS), based on cell susceptibility to an immunity protein TsxEI [60]. Very recently, a non-lethal sibling rivalry has been described in _P. mirabilis_, a robustly swarming pathogenic bacterium, where an Ids self-recognition system selectively induces non-self cells into a growth-arrested lifestyle incompatible with cooperative swarming [61]. In order to determine if the inhibition we saw here was the result of a mutually lethal mechanism such as has been described for a \"sibling lethal factor\", we employed a variety of tests to determine the viability of cells in the inhibition zone. However, unlike the situation with \"sibling lethal factor\", all the data points to the cells at the inhibition zone remaining viable and metabolically active. Simple depletion of nutrients at the interface seems unlikely to be the cause of this effect since qualitatively and quantitatively similar inhibition effects are observed when sibling colonies of DG893 were grown on minimal media (ASW succinate) or rich media (MB) and the effect was reversed from that of previous studies in that a DL is formed by rapidly expanding colonies on low percentage agar. Indeed, we did not generally observe merging between sibling colonies of DG893 no matter what media was used. As the rival expanding colonies never come in contact with each other and nutrient depletion seems an inadequate explanation, some form of non-lethal chemical communication between the rival colonies seems likely. In an effort to search for chemical cues, extracts of the inhibition zone between colonies revealed the presence of a single protein that was absent in the agar from expanding non-interacting areas of the colonies. In gel trypsin, digestion followed by mass spectral analysis identified the single dominant protein as a glycerophosphoryl diester phosphodiesterase (GDPD) similar to _Escherichia coli_ periplasmic phosphodiesterase (GlpQ, NCBI entry WP_007154889.1). Here, GDPD is clearly being secreted into the external media, however, its role in the process remains unclear. Secretion of related phosphodiesterase into the media have also been reported during phosphate starvation in _B. subtilis_[62]. It is also notable that other phosphodiesterase such as DipA from _Pseudomonas aeruginosa_ have previously been implicated in flagellum-mediated swimming and swarming [63]. Secreted GDPD could thus be considered as a type of bacteriocin in that it is a biologically active protein complex that may display an allelopathic mode of action towards the same or closely related species. However, GDPD has also been connected with bacteriocin resistance in _Enterococcus_[64]. As a relatively large protein, GDPD is likely to be the result of a signal rather than being a signal itself and, therefore, is probably only one part of a coordinated response to threat, population density, or nutrient depletion. Indeed, it is clear that the protein itself does not inhibit motility. If GDPD is not the signal, what then is the chemical signal that turns off motility? We hypothesize that environmental cues detected by bacteria are transduced into intracellular second messengers, such as cyclic di-GMP, (p)pGpp, or c-AMP, that in turn initiate the appropriate cellular responses, including a reduction or loss of motility [65]. Indeed, previous work [66] has established that cyclic di-GMP levels are correlated with the transition between motile and biofilm phases, therefore, further work will seek to determine cyclic di-GMP levels to help elucidate the mechanism underpinning this switch in behavior. Finally, \"sibling rivalry\" seems to be a widespread phenomenon among the _Marinobacter_ genus, potentially enabling cells to discriminate between siblings. To our knowledge, this is the first documentation of \"sibling rivalry\" in marine bacteria, the mechanism of which differs from that previously reported. In _M. algicola_ DG893, the inhibition is clearly non-lethal and appears to operate via reduction in motility.