All unicellular eukaryotes come under Kingdom Protista and are mostly aquatic. Chrysophytes, dianoflagellates, euglenoids, slime moulds and protozoans are few categories under this kingdom. Since they are eukaryotes, they contain a well-defined nucleus and other membranous organelles. Some protists also have a flagellum or a cilium. They reproduce both asexually through cell fusion and sexually through zygote formation.
Chrystophytes include diatoms and golden algae or desmids. They are microscopic and float in great numbers in both fresh water and saline water and are mostly photosynthetic. Diatom cell walls are embedded with silica, making them indestructible.
Dianoflagellates are colourful protists found both in marine and fresh water. Depending on the main pigments in their cells, they are yellow, green, brown, blue or red. Dianoflagellates usually have two flagella: a longitudinal flagella and a transverse flagella. Their names indicate how both the flagella are positioned.
Euglenoids have two flagella, one short and the other long one. Instead of a cell wall, euglenoids have a protein-rich layer called pellicle that makes their bodies flexible. Euglenoids are phototrophic autotrophs in the presence of sunlight, since they also have chloroplasts. However, they behave as heterotrophs in the absence of sunlight.
Slime moulds are saprophytic protists. They form a huge aggregation called plasmodium. However, during unfavourable conditions, this plasmodium differentiates and forms spores that possess true walls. These spores are so resistant that they can survive for years under adverse conditions.
Protozoans are heterotrophs and live as predators or parasites. They can be classified into Amoeboid protozoans, flagellated protozoans, ciliated protozoans and sporozoans. Amoeboid protozoans are found in fresh water, marine water as well as moist soil and have pseudopodia to move and capture their prey. Some amoeboids like Entamoeba are parasites. Flagellated protozoans have flagella and can be either free-living or parasitic. Some parasites like Trypanosoma cause sleeping sickness. Ciliated protozoans like Paramoecium can be found in almost all places where there is water. The coordinated movement of the cilia steers the water containing food into their gullet. The gullet is a cavity that opens to the outside of the cell surface. Sporozoans like Plasmodium have an infectious spore-like stage in their life cycle, and cause malaria in humans.