Chapter 28 Protists

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 © 2011 Pearson Education, Inc.

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 © 2011 Pearson Education, Inc.

Some protists reproduce asexually, while others reproduce sexually, or by the sexual processes of meiosis and fertilization © 2011 Pearson Education, Inc.

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 © 2011 Pearson Education, Inc.

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 Excavata Chromalveolata Rhizaria Archaeplastida 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 © 2011 Pearson Education, Inc.

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 © 2011 Pearson Education, Inc.

Some protists are parasitic Plasmodium causes malaria Pfiesteria shumwayae is a dinoflagellate that causes fish kills Phytophthora ramorum causes sudden oak death © 2011 Pearson Education, Inc.

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 © 2011 Pearson Education, Inc.

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 ecosystems Fishery yields The global carbon cycle © 2011 Pearson Education, Inc.