Bacteria. Help! - Need help with... augh... Bacteria. (Oct/09/2005 )
Ok, here goes...
A population of bacteria are very sensitive to light. They live in a lake that is in a cave, so they flourish, Slowly, however, a hole begins to erode in the cave's roof. As the days pass, light begins to filter in and the cave starts to be dimly lit. The bacteria begin to die. While the erosion is taking place, however, two individual bacteria in the soil above ground fall into the lake. These bacteria cannot survive in the dimness of the poorly-lit cave, so they die immediately. Days later, the hole has opened up so that light floods the cavern. Nevertheless, the bacteria that were once dying, are now flourishing, with a population larger than they had ever had before the erosion. What two things must have happened?
They mutate to adapt to the new environment haha 
1. You were assigned this question as a homework assignment.
2. You failed to pay attention in class or to read assigned chapters.
Seriously, though, if the question is indeed worded as "must have happened", it is a poor question. Scientists seeking to answer this question would not use "must have happened", given the observations included. There are, in fact, several things that could have happened to lead to the observations made. In fact, the two bacteria dropping into the lake could be completely irrelevant to the sudden fitness of the lake-dwelling bacterial population.
Nevertheless, I suspect what the professor is looking for is that the gene or genes responsible for resistance to light were released from the bacteria that fell into the lake when they died and lysed, were taken up by a member of the dying population, and were incorporated into its genome. The progeny of this new, fitter bacterium (now resistant to light) replaced the population of the light-sensitive bacteria.
It is unclear to me the purpose of the statement "with a population larger than they had ever had before the erosion". The professor might be suggesting that the gene or genes that confer light resistance also confer a phenotype of higher population density, or that they are geneticlly linked to a locus that does, and these linked genes were incorporated as well.
But, as I said, spontaneous mutation(s) in a bacterium of the original population could also explain these observations, thus the two bacteria that fell in the lake would be irrelevant to the change observed...
It also seems clear that if the bacteria from the Dark Side below evolved in the cave in the absence of light then they may well have had a chemolithotrophic mode of nutrition.
It is unclear whether what killed the bacteria from the Bright Side above is multifactorial or not, IE, whether it was the "foodstuffs" and/or the waste products of the Dark Side, hostile pH, osmotic strength, sulphurous compounds etc or simply the absence of light. It does say that they cannot survive the dimness of the cave, suggesting it is simply the lack of light that kills them, but it also says that they die immediately, suggesting a toxic or otherwise hostile environment snuffing them out. But it also implies that the "immediate" death is a consequence of the lack of light, rather than anything else.
So lets go with the lack of light being the key factor, then clearly the two bacteria from the bright side are photosynthetic (whether green, blue-green or purple). Thus, not only do the bacteria from the Dark Side gain new genes that overcome their sensitivity to light but they also gain genes allowing photosynthesis and so, with their combined chemolithotrophic and photosynthetic nutritional pathways, they attain a larger population than ever before.
An alternative is that the light poisoned the internal biochemistry of the Dark dwellers and by absorbing the DNA and photosynthetic machinery of the Light dwellers they simply replaced their old mode of nutrition with a new one. The light continues to interfere with their old mode but, with an inexhaustible supply of free photons, the new mode of nutrition is simply more efficient than the old one.
I don't think that there are any red herrings in the question. Spontaneous mutations would not overcome the poisoning of a chemical energy pathway by light and provide an entire, new form of nutrition as well in the form of photosynthetic pigments and a new respiratory electron transport chain etc. This might be too much adaptive fitness to ascribed to a few point mutations.
That's all assuming that this little tableau isn't just a joke with a corny punchline, of course.
Cheers.
~
Dear Merrick,
Well, first things first.... It is not "....augh... bacteria". Thay are gr8 to work with and do wonders.
To answer your question, I would guess that two events could have occured (thought it need not be ONLY two events)
One is a mutation that has rendered the light-sensitive bacteria to become resistant. This is very possible, considering that there is now a new challenge for the bacteria, with the light filtering in. The most probable time when this would have happened would be when light first started filtering in and sub-lethal doses of light would have given a chance for the bacteria to mutate. But remember that the numbers would have been abysmally low and were selected over time with increasing light. One example for this phenomenon is the rampant emergence of MDR bacteria in hospitals.
The second is natural gene transfer, most probably by transformation, from some BGA to make the mutated bacteria photosynthetic. That would explain the larger population after the erosion than before it. BGA would be found in plenty, either as spores or as germinated cells in these kinds of areas.
Hope this helps.
A population of bacteria are very sensitive to light. They live in a lake that is in a cave, so they flourish, Slowly, however, a hole begins to erode in the cave's roof. As the days pass, light begins to filter in and the cave starts to be dimly lit. The bacteria begin to die. While the erosion is taking place, however, two individual bacteria in the soil above ground fall into the lake. These bacteria cannot survive in the dimness of the poorly-lit cave, so they die immediately. Days later, the hole has opened up so that light floods the cavern. Nevertheless, the bacteria that were once dying, are now flourishing, with a population larger than they had ever had before the erosion. What two things must have happened?
I hope you passed the exam.
I would have thought of sporulation in a case like this.