A researcher observed a reduction in the number of chromosomes during cell division for a microbe, he concludes that the microbe must be

That microbe must be Eukaryotic. Eukaryote cells reduce their chromosome numbers in half when producing gametes in the process of meiosis. This enables eukaryotes to reproduce sexually. Prokaryotic cells are not able to produce gametes because they do not have the ability to undergo the process of meiosis.