Final answer:
A positive 'O' tube indicates that the MO can ferment lactose, changing the broth color from red to yellow. In terms of water potential, adding solute to the side with a higher water level can equalize the water levels due to the osmotic effect.
Step-by-step explanation:
A positive 'O' tube in the context of microbial testing, such as the most probable number (MPN) method, indicates that the microorganism (MO) is able to ferment lactose. The MPN method is often used to estimate the concentration of viable microorganisms in a sample.
When lactose broth is used as the medium, successful fermentation by the microorganisms will result in the broth changing color from red to yellow due to the production of acid during lactose fermentation. Each tube represents a different dilution of the original sample, which allows for the calculation of the most probable number of bacteria present in the sample.
If we consider water potential in a different biological context, adding solute to the side of the tube with lower water potential can indeed equalize the water level. The addition of solute would lower the water potential because of the osmotic effect, whereby water moves from an area of higher water potential to an area of lower water potential.
To equalize the water levels, solute should be added to the side with the higher water level, which would reduce its water potential and allow water to move back to the other side until equilibrium is achieved.