Bioluminescent bacteria purposely glow to be consumed!

Bioluminescence is an ocean wide phenomenon and is present in many organisms, some of the main organisms being bacteria. Luminescent bacteria are responsible for several occurrences such as behavioral alterations, predation, communication and signaling, amongst others. This paper looks at how bacterial bioluminescence can serve as a ‘bait’ to attract predatory organisms, and how this can benefit the bacteria. The ability of bioluminescence in bacteria is energetically costly, and therefore the benefits of luminescence must outweigh these costs.

Luminous bacteria can exist in symbiotic relationships (for example, in the bobtail squid Euprymna scolopes from Colin’s lectures), as saprophytes (on marine snow for example), and as parasites. Some suggested reasons for why bacteria possess the bioluminescence characteristic are for advanced repair of DNA, protection against UV radiation, and for dispersal purposes.

Dispersal using bioluminescence is based on the bait hypothesis. The bait hypothesis is where the glowing of the bioluminescent bacteria attracts zooplankton and/or fish in order to gain access into their gut. These bacteria are able to survive the digestive processes in the fish. They do this because the gut of a fish is rich in nutrients on which the bacteria thrive, and it is safe from other predators. Another reason why bioluminescent bacteria adopt the bait hypothesis is because by entering the gut of a fish, their dispersal abilities increase dramatically by the high motility of the fish. When they are excreted, the chances of the bacteria being in a less favourable environment are low due to their wide tolerance of environments, therefore the new area they occupy can be exploited (see diag below).

In this particular study, the bacteria used were the luminous Photobacterium leiognathi strain MI1. From this, a mutant was also produced which failed to produce a measurable light emission, which was named the dark mutant. This dark mutant was created by mutagenising the wild type with N-methyl-N¹-nitro-N-nitrosoguanidine. The clones which then failed to glow were isolated, identified, and pure cultures were produced. The zooplankton used were small brine shrimp Artemia salinaas they are readily eaten by zooplanktivourous fishes, they are easy to handle and they lack evasive behaviour. The fish species chosen for the study was the nocturnal zooplanktivorous Apogon annularis, as it is a visual predator with excellent light sensitivity.

In the initial experiment, two dialysis bags were placed in opposite corners of a large tank of seawater; one containing the luminescent P. leiognathi, and the other containing the dark mutant P. leiognathi. A variety of zooplankton were then added to the tank and left for 15 minutes in the dark. The results showed that the decapods and mysids were found almost entirely where the bag containing the luminescent bacteria was. The copepods showed no significant attraction, and the internal control of non motile organisms such as fish eggs also showed no attraction. Having swum in a liquid medium containing the bacteria, A. salina also became bioluminescent after only 10 seconds. Long exposure photography showed that the P. leiognathi were visible in the gut of A. salina, as well as being seen externally. The chances of predation by A. annularis dramatically increased after the consumption of the luminescent bacteria also turned the mysid A. salina luminescent. Almost all luminescent mysids were actively hunted by the fish, whereas the non luminescent mysids were only consumed on the rare occasion of one passing near the fish’s head. The final experiment was to determine whether the bacteria survived passing through the guts of fish and/or zooplankton. To do this, the faecal pellets from A. annularis were analysed and showed strong bioluminescence, indicating that the bacteria survived this process. Strong bioluminescence was also observed in the faecal pellets of the mysids A. Salina. The abundance of luminous cells in A. annularis faeces inceased fivefold in the specimens which had consumed luminescent mysids, compared to those who consumed non luminescent mysids.

Bioluminescence in bacteria can be used as a visual cue, as it has the potential to travel further than chemical or mechanical cues. The zooplankton take a great risk when approaching bioluminescent bacteria, as their own predators may also be attracted to the area. However, the presence of this bacteria indicates ephemeral patches rich in nutrients where, if successfully consumed without predation occurring, benefits the zooplankton greatly. This experiment showed that the bioluminescence of non symbiotic bacteria is a visual attractant for organisms higher up the food chain. When consumed by these organisms, the bacteria are in an area where they are sheltered and surrounded by nutrients in the gut, enabling them to thrive and proliferate. Once these bacteria are excreted, they will have travelled a lot further than their natural capabilities would allow due to the high mobility of the fish. Due to their wide range of ecological tolerances, they can take advantage of the new areas they are introduced to.

Zarubin, M., Belkin, S., Ionescu, M. & Genin, A. (2011) Bacterial bioluminescence as a lure for marine zooplankton and fish. PNAS. 109 (3): 853-857.