Chapter 28Protists

Most Eukaryotes are Single-celled Organisms

Protist is the informal name of the group of mostly unicellular eukaryotes

Eukaryotic cells have organelles; more complex than prokaryotic cells

Most protists are unicellular, but there are some colonial and multicellular species

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Eukaryotic systematics have caused the classification of protists to change significantly

Protists exhibit more structural and functional diversity than any other group of eukaryotes

Protists, the most nutritionally diverse of all eukaryotes, include

Photoautotrophs, which contain chloroplasts

Heterotrophs, which absorb organic molecules or ingest larger food particles

Mixotrophs, which combine photosynthesis and heterotrophic nutrition

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Some protists reproduce asexually, while others reproduce sexually, or by the sexual processes of meiosis and fertilization

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Endosymbiosis in Eukaryotic Evolution There is now considerable evidence that

much protist diversity has its origins in endosymbiosis

Endosymbiosis is the process in which a unicellular organism engulfs another cell, which becomes an endosymbiont and then organelle in the host cell

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Five Supergroups of Eukaryotes Our understanding of the relationships

among protist groups continues to change rapidly

One hypothesis divides all eukaryotes (including protists) into five supergroups

1. Excavata

2. Chromalveolata

3. Rhizaria

4. Archaeplastida

5. Unikonta© 2011 Pearson Education, Inc.

Protists Roles in Ecological Communities

Protists are found in diverse aquatic environments

Protists often play the role of symbiont or producer

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Symbiotic Protists Some protist symbionts benefit their hosts

Dinoflagellates nourish coral polyps that build reefs

Wood-digesting protists digest cellulose in the gut of termites

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Some protists are parasiticPlasmodium causes malariaPfiesteria shumwayae is a dinoflagellate

that causes fish killsPhytophthora ramorum causes sudden

oak death

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Photosynthetic Protists Many protists are important producers that

obtain energy from the sun In aquatic environments, photosynthetic

protists and prokaryotes are the main producers

In aquatic environments, photosynthetic protists are limited by nutrients

These populations can explode when limiting nutrients are added

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Biomass of photosynthetic protists has declined as sea surface temperature has increased

If sea surface temperature continues to warm due to global warming, this could have large effects on

Marine ecosystemsFishery yieldsThe global carbon cycle

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