Algae is a term used for certain species from several unrelated groups, and include some simple plants as well as some protists.
Bacteria are single-celled and have no nucleus. But these organisms have tough cell walls and the ability to reproduce rapidly, allowing them to adapt to and live in every environment on earth.
Microflora literally means "microscopic plants," or plants too small to be clearly viewed with the naked eye alone. Diatoms are now classified as plants, and thus are included in this category.
Plankton is a broad term for a variety of very small fresh and saltwater pelagic organisms, in the form of eggs, larvae or adult. These include some plants, fish, cephalopods, jellyfish, crustaceans, starfish and corals as well as animals like rotifers, all providing a crucial source of food.
Simple-structured eukaryotes that are not quite animals, but not quite plants. They include groups such as foraminifera and radiolarians, which are used as paleoclimate indicators of temperature, sea level, ocean depth and salinity.