Protists Overview

Protists

• Eukaryotic organisms that didn't fit into Animalia, Fungi, or Plantae kingdoms historically
• Single-celled eukaryotes that live in a variety of aquatic and terrestrial environments
• Some are microscopic, unicellular
• Some are large, multicellular species, like kelps
• Modern molecular genetics shows some protists are more closely related to animals, plants, or fungi than to other protists
• Protist lineages have been reassigned to different kingdoms due to close relationships found through molecular genetics
• Protists display a great diversity in morphology, physiology, and ecology
• Over 100,000 described species; exact number of undescribed species unknown
• Many exist in symbiotic relationships with other organisms
• Few characteristics common to all protists
• Mostly aquatic (freshwater, marine, damp soil, snow)
• Some are parasites infecting animals or plants
• Some feed on dead organisms or waste, contributing to decay
• Cells among the most complex of all cells
• Many microscopic and unicellular
• Some multicellular, form colonies, or multinucleated single cells with diverse morphologies
• Sizes range from less than a micrometer to 3 meters
• Cell membranes or cell walls
• Glassy silica shells, or covered in pellicles of interlocking protein strips for flexible armor
• Most are motile (flagella, cilia, pseudopodia)
• Can move toward light by coupling locomotion with light-sensing organs
• Can be aerobic or anaerobic
• Photosynthetic protists (photoautotrophs) have chloroplasts to obtain energy
• Heterotrophs consume organic materials
• Obtain energy through phagocytosis (engulfing food particles)

Protist Reproduction

• Mostly asexual reproduction (binary fission, multiple fission, budding)
• Sexual reproduction involving meiosis and fertilization common in some
• New variations of progeny may result, better suited to surviving in changed environments
• Sexual reproduction is often associated with periods of nutrient depletion or environmental change
• Cysts are resting, protective stages that allow for survival during challenging conditions

Protist Diversity

• Relationships are better understood with DNA sequencing
• Morphological similarities may be analogous, not recent common ancestry
• Classification scheme groups eukaryotes into six supergroups (Excavata, Chromalveolata, Rhizaria, Archaeplastida, Amoebozoa, and Opisthokonta)
• Monophyletic nature is supported by evolutionary traits and genetic analysis

Human Pathogens

• Many protists are pathogenic parasites (malaria, African sleeping sickness, waterborne gastroenteritis)
• Infect other organisms to survive
• Some parasitic species prey on plants, effecting massive destruction of food crops
• Plasmodium species (specifically P.falciparum) are the primary causative agents of malaria and lead to high mortality rates, particularly in African children

Trypanosomes

• Parasites cause African sleeping sickness ( T. brucei)
• Evade immune systems by changing surface glycoproteins during each infectious cycle
• Symptoms affect the nervous system
• T. cruzi is another species causing Chagas disease (Latin America), affecting heart and digestive systems

Plant Parasites

• Some protists (oomycetes) are plant parasites destroying crops (downy mildew on grape vines [ Plasmopara viticola], late blight of potato [ Phytophthora infestans])
• Result in significant crop losses and famines

Protists as Food Sources

• Essential sources of nutrition for other organisms
• Some consumed directly in form of plankton
• Others (e.g., photosynthetic protists) are producers providing nutrition for other organisms in food webs

Protist Importance

• Essential primary producers in aquatic environments
• Contribute to carbon fixation and food webs
• Crucial decomposition agents (saprobes/saprophytes, feed on dead organisms)
• Play a vital role in nutrient cycling

Fungi

• Eukaryotic organism
• Diverse group of organisms
• Mostly multicellular (Some yeasts are unicellular)
• Cell walls contain complex polysaccharides (chitin and glucan), not cellulose
• Membrane-bound nucleus
• Non-photosynthetic
• Heterotrophic (absorb nutrients)
• Complex cellular organization
• Can be asexual or sexual reproduction
• Fungi produce spores for dispersal
• Important decomposers increasing nutrient availability
• Play roles in mutualistic symbiotic relationships, such as mycorrhizae with plant roots, plant parasitism, and lichens
• Some are pathogenic, causing diseases in plants and animals
• Commercial enzymes and antibiotics can be produced
• Important in food and beverages(e.g. fermentation of grains produce beer, and fruits to wine)
• Involved in ecological roles and cycles