Photobacterium phosphoreum

Photobacterium phosphoreum illuminates the ocean in ways Vibrio fischeri cannot. While V. fischeri achieves its spectacular bioluminescence through symbiosis with squid, P. phosphoreum produces light as a free-living organism in open seawater. Both use quorum sensing to coordinate light production, but their ecological strategies diverge completely. P. phosphoreum demonstrates that the same core mechanism—population-density-dependent bioluminescence—can serve different purposes in different ecological contexts.

In the open ocean, P. phosphoreum's bioluminescence likely serves attractant functions rather than the camouflage role it plays in squid light organs. Glowing bacteria may attract zooplankton that ingest them; the bacteria survive passage through planktonic guts and benefit from dispersal to new nutrient-rich environments. This 'bait' hypothesis suggests bioluminescence evolved as a dispersal mechanism—light attracts vehicles that carry bacteria to new habitats. It's biological advertising: make yourself visible to get consumed and transported.

P. phosphoreum also causes fish spoilage, producing the characteristic 'fishy' smell of decomposing seafood. The bacterium grows on fish surfaces under refrigeration, eventually reaching quorum densities that trigger both bioluminescence and production of volatile amines. Glowing fish in markets or refrigerators indicates P. phosphoreum contamination and imminent spoilage. The connection between light production and spoilage chemistry isn't coincidental—both are quorum sensing-regulated, ensuring they activate simultaneously when population density makes them worthwhile.