The kingdom Protoctista is defined as eukaryotic microorganisms that are neither animals, plants, nor prokaryotes. It includes all nucleated algae, water molds with flagella, slime molds, slime nets, and protozoa. Protoctista show remarkable diversity in cell organization, division, and life cycles. They are grouped together due to shared organelle structure like mitochondrial cristae. Characteristics include being mostly autotrophic via photosynthesis, though some can switch between auto- and heterotrophic states, and inhabiting aquatic environments like salt water, freshwater, moist soil, or other organisms' tissues.