Why glow if you don’t have eyes to see?
Glowing takes energy. Down in the deep ocean, energy is in short supply, so why would bacteria do this? Bacteria don’t have eyes. It’s not like they’re going to be able to use it to find stuff. And these bacteria are not living in another organism, so it’s not as though they’re glowing in some sort of mutual trade with a host.
These bacteria only glow when they’re in large numbers, close together (quorum sensing), however. This gives a clue to what might being going on. A new paper by Zarubin and colleagues conducts several experiments to test the hypothesis that these deep sea bacteria are glowing because they want to be eaten.
You might think getting eaten is not a productive thing to do. The idea is: bacteria light up when they’re in large enough numbers to signal decent food. The bacteria themselves might not be the food, so much as the article they’re attached to.
The bacteria use the insides of their consumers as a way to disperse themselves throughout the ocean. It’s already been shown that a fairly large number of these glowing bacteria can survive passage through the gut. But that alone doesn’t provide enough a strong test of the hypothesis that the bacteria glow to advertise themselves as bait.
First, the team tested whether animals preferred glowing bacteria by putting two bags in a big tank of predators. One bag contained glowing bacteria; another contained same species, but with a mutation that prevented the glowing. Decapod and mysid crustaceans went almost all for the glowing bacteria. But it’s not a universal attractor; copepod crustaceans ignored both bags of bacteria.
Brine shrimp (Artemia) would start to glow after swimming in these bacteria, and their guts started to glow after the shrimp ate the bacteria.In the picture below, you can see Artemia in plain light, and after 30 seconds in the dark. The light is dim, but they do indeed glow.
There is a problem here, though: they switched the species eating the bacteria. They don’t say whether they tested if Artemia were attracted preferentially to the glowing bacteria. You can show a plausible chain of events, but to “close the loop” on this story, you’d have to use the same bacteria eaters all the way through. The authors justify this partly by convenience (Artemia are easy to rear in large numbers) and partly by saying that this allows them to see the effect better. Brine shrimp don’t have escape behaviour. Thus, this removed possible confounds of an interaction between the glowing and any movements caused by escape responses. They also say that one of the mysid species glows after contacting the bacteria. They don’t show data for that, or give any citations, however. Their convenience came at the cost of ecological plausibility.
The glowing Artemia are much more likely to be eaten by fish – about ten times more likely. They tested this by putting Artemia in tanks with ring-tailed cardinal fish (Apogon annularis, pictured), which is nocturnal. And after the cardinalfish eat these brine shrimp, the bacteria do fine. They make it all the way through the fish’s digestive system, and they make the resulting feces also glow (though probably not brightly). The authors also tested the feces of other bacteria eaters – the Artemia and mysids – and they also tend to glow.
What I’d like to see next is some indication of whether the zooplankton are getting any nutritional value from eating these bacteria. Are the bacterial consumers being tricked into wasting time consuming “empty calories” that will just pass through their guts without benefit? If so, why haven’t the zooplankton wised up to this? I mean, how embarrassing would it be to be punked by bacteria? Or is these a “selfish herd” sort of situation, where a small proportion of group members are lost, but the risk to individuals is so low? And is there any manipulation of the plankton behaviour by the bacteria, similar to the way large parasites often work?
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 109(3): 853-857. DOI: 10.1073/pnas.1116683109
Apogon annularis picture from here.