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 Bacterial bioluminescence, a mechanism for dispersion/propogation in free-living bacteria 

Bioluminescence has obvious advantages in symbiotic bacteria but its role in free-living bacteria is less certain, some theories such as anti-oxidative activity, enhanced DNA repair and UV resistance have been put forward as explanations but none are conclusive. The ecological function in propagation and dispersal was put forward 30 years ago (known as the bait hypothesis), the bacterium, by glowing, visually mark the presence of a food particle for fish in order to get into their nutritious guts but has received little attention.

The Objectives of this study was to test the following key points of the bait hypothesis;

1. Visual attraction of zooplankton to bacterial bioluminescence
2.  Promotion of luminescence in zooplankton by contacting or ingesting luminous bacteria
3. Attraction of zooplanktivorous fish to glowing prey
4. Survival of bacteria gut passage in both zooplankton and fish

The attraction of zooplankton to bacterial bioluminescence was tested by adding two dialysis bags of bacteria, P. leiognathi, in to a large tank of seawater. One bag containing luminous bacteria was placed at one corner of the tank, while on the other side there was a mutant strain (incapable of luminescence). Significant changes in zooplankton distribution were noticeable within 15 minutes, decapods and mysides were found almost exclusively over the corner with luminous bacteria, however copepods showed no significant attraction to either corner. Similarly, no difference was noted between non-motile organisms (which served as an internal control).


Fig 1  - Luminescent Artemia



The brine shrimp Artemia salina were used to show luminescence. Artemia became luminescent after swimming in a liquid culture of P. leiognathi after just 10 seconds, this was noticed from the guts (due to ingested bacteria) and the exoskeleton, where bacteria had attached. Artemia was used because they are readily eaten by zooplankton, easy to handle and lack evasive behaviour. The promoted glow in Artemia dramatically affected its risk of being preyed on by the nocturnal fish Apogon annularis in a recirculating laboratory flume in the dark. (Fig 1&2).

Fig 2 - Non-luminescent Artemia

            Fecal pellets of Artemia which had ingested luminous bacteria showed luminescence; indicating that P. leiognathi had survived the passage through the guts. High concentrations of viable P. leiognathi were found also in the feces of fish A. Annularis that had fed on luminious Artemia, in which the abundance of luminescence cells in there fecal pellets was fiver orders of magnitured high than that of fish fed on non-luminous Artemia.

This study shows experimental evidence for some key steps of the bait hypothesis, showing benefits of bioluminescence in non-symbiotic bacteria. Though the authors hint that this benefit of bioluminescence is most applicable in food-deprived environments, such as the deep sea, where food availability decreases with depth. However, this experiment did not differentiate between the luminescence generated when zooplankton ingested the bacteria or whether they attached externally via the exoskeleton. Also, the use of a non-marine zooplankton, Artemia, may have influenced bias. As well, the authors also make the assumption that the mechanism by which organisms becomes luminescent is the same, Artemia may not be applicable for all zooplankton.

I chose to review this study after beginning to read up on the symbiotic bacteria lectures, bioluminescence has obvious advantages when in a mutualistic relationships, such as in Vibryo fischeri and Euprymna scolopes. I however struggled to think of ways in which it may benefit free-living marine bacteria and so chose to look in to this, as there must be a reason some free-living bacteria are selecting for bioluminescence.

If anyone is interested in reading the paper, heres a link:
http://www.pnas.org/content/109/3/853

Zarubin, M., Belkin, S., Ionescu, M., & Genin, A. (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(3), 853–7. doi:10.1073/pnas.1116683109