Forgive me if a similar question has been asked, I can't find it.
How would tiny creatures, ants, plankton, bacteria, for example, be affected if the Earth had twice as much mass. What are the gravitational limits to which a creature no larger than the size of an ant would be able to survive on earth?
The main limitation gravity imposes on animals is the square/cube law. Simply, physical-strength scales with cross-sectional area (length squared), whereas mass scales with volume (length cubed). Since cubes grow faster than squares, animals simply can't be very large (à la Pacific Rim). The ones that are large have thick legs and must move slowly. Oppositely, animals that are very small enjoy very little effect from the square-cube law, and can have thin legs and fast motion.
The effect of increased gravity is to make the effect of the square-cube law worse. Animals that were viable (elephants) aren't anymore, whereas smaller animals (dogs) are, and these become the new largest critters.
However, at the small end (insects, bacteria), the effects are prettymuch negligible. Obviously, a planet with higher gravity might have different surface conditions--pressure and suchlike--but it's reasonable to assume your life forms are evolved for any differences.
Probably yes.
Well, I don't know any experiment done in microorganisms in gravity stronger then Earth's, probably because such environment (a rocky planet/moon, with more gravity then Earth so we can land a probe and do experiments) don't exist in your solar system.
What we have is the data about the experiments with microorganisms in micro gravity done in the ISS. The current understanding tells that micro gravity alters two primary functions in bacteria, mass transfer and motility. See, in microorganisms the molecular transfer between cells are done by convection and diffusion, the first one is driven by gravity. In a micro gravity environment convection stops, which limits the bacteria to diffusion to maintain mass transfer.
In the case of mobility, bacteria have show to increase mobility in micro gravity environment, which leads to more capability to reach nutrients in the environment, at least in liquid cultures. Gravity has show to cause a severe nutrient depletion in comparison to the cultures in micro gravity.
So I think is sure to think that in a environment with higher gravity that Earth's, the effect will be more to "Oh god' we can't find food" then "Jesus we are been squeezed to death by your own weight", at least in the microscopic world.
