What is the reason for bacteria growth at the interface between oil and water?

[prime-factor]

In my experiment with R.rubrum bacteria, which can grow phototrophically in anaerobic conditons, and by fermentation in dark anaerobic conditions, I am noticing that the bacteria are growing at the interface between the oil and the water. The experimented is testing:

photoautotrophic growth: bacteria in sterile water, covered with oil in closed test-tube. Placed in sunlight.

photoheterotrophic growth: bacteria in sterile water with 1g fructose, covered with oil in closed test-tube. Placed in sunlight.

fermentation: bacteria in sterile water with 1g fructose, 1g sodium bicarbonate(electron acceptor) left in dark cupboard.

Why might this be?

 

[Moonbear]

We can't do your assignment for you, so here are some added questions to help you on your way.

First, did you see the growth under all conditions, or only some? What information does that provide for your interpretation of the results?

Second, do you understand WHY each of the types of growth tested used the substrates in question?

Third, do you have controls in your experiment?

Fourth, you mention starting with sterile water; what about the oil? Was that also sterile at the start of the experiment?

Fifth, have you confirmed that it is the bacteria of interest growing and not some other organism that is a contaminant?

You may think of more questions on your own too, but these are the type of questions you should consider to interpret your results.

 

[prime-factor]

Thanks.

I have researched the different types of growth, and the substrates used in each.
The controls, one for each experiment, are showing no growth. To my knowledge, the oil was not sterilized.

I am also finding that the bacteria (Rhodospirillum rubrum) are growing inside the oil.