Now, why in the world would bacteria emit light? It probably isn't because bacteria need to find their way-around in the dark - so, what's going on? We of course have absolutely no idea as to the "real" reason; but, we can make some pretty good guesses based on the science we already know. It turns out that the bacteria which can emit light, usually are involved in a give-and-take (symbiotic) relationship with some other kind of organism - like a fish, or squid, for example. Several species of bacteria in the genera Vibrio and Photobacterium form symbiotic relationships with certain fish and squid. The place on or in the fish or squid which is occupied by these bacteria is special - the bacteria won't be anywhere else on or in the fish or squid. This place is called, for obvious reasons, the light-emitting organ. At the same time, these bacteria are also capable of living by themselves (i.e., free-living) in sea-water. Here comes the interesting part - when free-living, they DO NOT emit light! BUT, whenever they occupy a fish or squid's light-emitting organ, they shine like a new dime! We don't exactly understand all that is going on, but we do know that some of these bacteria, when packed tightly around one another, emit light, while others apparently receive chemical signals from the place they occupy in the fish or squid, and respond to these signals by light emission.
It is therefore believed, that a fish or squid with such bacteria around, will have a kind of "guiding light" which will attract whatever it is that this fish or squid eats - therefore, maybe helps with predation success. I guess if you were a little fish or something else swimming around in the darker depths of the ocean, and saw a light up ahead, you'd probably think about checking it out. Possibly a Big mistake!
One fascinating thing is, that we have learned how to use luciferase to help us find out how genes work (molecular genetics), and how to measure oxygen levels and available energy in the form of ATP, in soil and water (to examine existing ecological conditions for different life-forms). For example, the Jamaican beetle (a bug bug) named the "kittyboo", has three different kinds of luciferase (made from three different genes in the beetle (remember that genes are the functional units of genetic material [DNA] called chromosomes, from which all things that are used or made inside a cell,derive). We can use these luciferase genes to determine how OTHER genes work! We can place the luciferase gene next to (right up against it) the end of the gene of interest (let's call it the Hooray! gene) - sort-of like the following:
We arrange it such that the luciferase gene WILL NOT work, unless the gene to which it is attached, works. Then this tandem-gene genetic material is added to bacterial cells, and we hope that the stuff ends-up inside the bacteria.
Now, we want to know: (1) were we successful in getting this genetic
stuff inside the bacterial cell; and, (2) if inside, does our
Hooray! gene work?. So, we grow our
little critters (remember, they're dividing about every 20 minutes), and
add luciferin. Luciferin will enter the cells and float around inside.
If our Hooray! gene is working, not only will the ultimate protein
product of this gene appear, but also, luciferase will be made!
Therefore, in the presence of luciferin, the enzyme luciferase, and
molecular oxygen, the bacterial cells which have a working Hooray! gene
inside them, will GLOW!. Then, all we need do, is isolate the
glowing bugs.... In this way we can specifically select the cells we
want - we just look for those with flashlights! With this procedure, we
can test all kinds of things about what makes the Hooray! gene, or
ANY gene we choose for that matter, function properly.