Most animal cells and single-celled organisms have very little color, and their intracellular organelles are almost totally invisible under simple bright field illumination.
The single-celled algae called phytoplankton are so dense that our instruments disappear from sight a mere few feet after we plunk them into the water.
An example of this would be microbes that survive from the chemical processes that take place between particular minerals, such as the single-celled micro-organism, archaea.
