Protozoan Diversity and Biogeography

Abstract

Abstract Protozoa is an informal name used to describe a polyphyletic group of single‐celled eukaryotic organisms (protists) that feed mostly by heterotrophy. Over 26 000 morphospecies of protists have been named, spanning all modern eukaryotic supergroups. They are incredibly diverse in terms of physiology and lifestyle, which has enabled them to colonise a wide range of moist environments, including marine, freshwater and extreme habitats. Many species have evolved specialised anatomical structures for feeding, motility and exchange of genetic material. They can be free‐living, parasitic or symbiotic, or can transition between these states. Some are also capable of forming dormant cysts or spores, which enable them to resist environmental stress. Species were traditionally defined based on morphological features and modes of nutrition, which do not always correlate with phylogenetic similarity; thus, it is difficult to ascertain their true biodiversity and biogeography. However, some appear to be abundant and widespread in nature, while others are rare and limited to very specific environmental niches. Key Concepts ‘Protozoa’ is not a formal phylogenetic taxon but is loosely used to describe diverse unicellular eukaryotic organisms (protists) with a mostly heterotrophic lifestyle. Protists are found in virtually all moist environments on Earth, including fresh and marine water, soil, extreme environments and the fluids of plants and animals. Protists may be free‐living, parasitic or symbiotic, or may transition between these lifestyles. In order to persist under stressful conditions, some species may form dormant cysts or spores. These structures also enable protists to survive passive transport from one location to the next. Protists may have a variety of specialised anatomical structures for feeding, locomotion and the exchange of genetic material, including extrusomes, pseudopodia and cilia. The taxonomy of protists was traditionally based on morphological features and nutritional mode but, has since been revised based on biochemical and genetic similarities. Many of the previously described morphospecies are actually clusters of several phylogenetically distinct entities. As a group, protists are incredibly abundant and geographically widespread. Common species may be present in densities of tens of thousands of cells per litre of water (or gram of soil), for example. However, some species are relatively rare. Despite their capacity for global distribution, most species display some degree of endemism.