Plant-like Protista

The Kingdom Protista comprises eukaryotic organisms that are predominantly single-celled or colonial in organization. It includes both autotrophic ("plant-like") organisms generally known as algae and heterotrophic ("animal-like") forms generally known as protozoa. The algal forms are treated here.

Euglena viridis, a typical euglenoid flegellate.

The Euglenoid flagellates are remarkably similar to the heterotrophic protozoan flagellates and are considered to be closely related to them. Unlike the protozoan forms, however, the euglenoids are photosynthetic and never ingest particulate food. While photosynthetic, these organisms are not strictly heterotrophic as they typically require sources of B-vitamins. This requirement restricts them to aquatic environments characterized by very high organic content - such as barnyard pools. The cells tend not to be readily preserved and the group has essentially no fossil record.

The organisms can be classified with great precision based on cell shape (round or centric vs. elongate or naviculate) and the sculpturing patterns on the cell walls or tests. The group is predominantly marine (phytoplankton), although there are numerous freshwater forms. Diatoms are used for time dating and correlation and, in the case of freshwater forms, for reconstructing ancient environments.

Dinoflagellate cell walls, as in this specimen of Gonyaulax, are made up of a series of plates. The number and arrangement of the plates permits considerable precision in identifying various types and the cells are readily preserved in sediments. Dinoflagellates are widely used for dating/correlation and in the reconstruction of marine environments. They are particularly useful in oil exploration. The cells have a distinct equatorial furrow and a specialized, whip-like flagellum encircles the cell in this furrow. A second flagellum of a different type trails the cells (it is anchored in the furrow, and the cells have a very erratic swimming patterns as a result of the action of this oddly-arranged pair of flagella. Dinoflagellates often secrete toxic metabolites and blooms of these organisms are known as red tides. The toxins result in high fish mortality when populations bloom.