Glowing bacteria is just waiting to be eaten...

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Some marine bacteria glow to increase the likelihood that they will be eaten, according to a study published in a recent issue of the Proceedings of the National Academy of Sciences.

Margarita Zarubin and coauthors carried out four sets of experiments to test the long-standing hypothesis that non-symbiotic marine bacteria glow in order to make themselves more visible to predators and be consumed, thereby augmenting their propagation and dispersal.

In the first experiment, the authors placed a dialysis bag containing bioluminescent Photobacterium leiognathi in one corner of a tank and another bag containing a dark mutant form of the bacteria in the other corner.

They noted significant changes in the zooplankton composition around the bags in as little as 15 seconds. Decapods and mysids were found to be attracted to the glowing bacteria, while there was no significant effect on copepods.

In the second experiment, the authors found that zooplankton such as brine shrimp (Artemia salina) began to glow very soon after swimming in a liquid culture of bioluminescent bacteria. The luminescence was found in both the guts and on the exoskeleton and appendages where bacteria had attached themselves.

The third experiment found that the Ringtail cardinalfish (Nectamia annularis) readily consumed glowing zooplankton, while non-glowing prey were rarely consumed.

The last experiment showed that the bacteria remained viable after passage through the guts of the zooplankton and fish.

Taken together, the results provide evidence for the hypothesis that bioluminescent bacteria are viewed as favourable food items by zooplankton because the bioluminescence signals a rich food source (glowing is bioenergetically expensive). When ingested by zooplankton, the bacteria make it glow and become more visible to predators (fish).  In turn, by surviving digestion in the guts of both zooplankton and fish, the bacteria gain a nutrient-rich, sheltered environment for proliferation as well as an efficient means of dispersal.

For more information, see the paper: Zarubin, M, S Belkin, M Ionescu and A Genin (2012) Bacterial bioluminescence as a lure for marine zooplankton and fish. Proceedings of the National Academy of Sciences of the United States of America 109, pp. 853–857.

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