Complex sentence: He looked under the microscope at the bacteria surrounding the blue-green mold and noticed that many were dead or dying due to the mold preventing the bacteria from making new cell walls and reproducing.
Split into simpler sentences: He looked under the microscope at the bacteria surrounding the blue-green mold and noticed that many were dead or dying. This later turned out to have been due to the mold preventing the bacteria from making new cell walls and reproducing.