CHAPTER 28 LIVING SMALL Small unicellular groups of eukaryotes are informally known as protists 28 1 MOST EUKARYOTES ARE SINGLE CELLED ORGANISMS Protists along with plants animals and fungi are classified as Eukaryotes Eukaryotic cells have a nucleus membrane enclosed organelles and are more complex Eukaryotic cells also have a well developed cytoskeleton STRUCTURAL AND FUNCTIONAL DIVERSITY IN PROTISTS Protists exhibit more structural and functional diversity than the eukaryotes we are most familiar with Most protists are unicellular although there are some colonial and multicellular species Mixotrophy photoautrophy and heterotrophy have all arisen independently in many different protist lineages FOUR SUPERGROUPS OF EUKARYOTES Eukaryotes are broken into 4 super groups but it is not known which diverged first from Prokaryotes Morphological and DNA data have undermined hypothesis about the evolution of Eukaryotes in the past and will continue to do so in the future The four supergroups of Eukaryotes are EXCAVATA SAR CLADE ARCHAEPLASTIDA and UNIKONTA ENDOSYMBIOSIS IN EUKARYOTIC EVOLUTION There is abundant evidence that much of protists diversity has its origins in endosymbiosis a relationship between two species in which one organism lives inside the cells or cells of another organism Structural biochemical and DNA sequence data indicate that mitochondria and plastids are derived from prokaryotes that were engulfed by ancestors of eukaryotic cells Studies indicate that mitochondria arose from an alpha proteobacterium Recent genomic studies indicate that the host in the Endosymbiotic relationship was an Achaean Results from mtDNA sequence analyses also indicate that the mitochondria of protists animals fungi and plants descended from a single common ancestor suggesting that mitochondria arose once PLASTID EVOLUTION A CLOSER LOOK In addition to evidence supporting the notion that mitochondria are descended from a bacterium that was engulfed by an Achaean cell there is also evidence that a lineage of heterotrophic eukaryotes acquired an additional endosymbiont a photosynthetic cyanobacterium According to this hypothesis this plastid bearing lineage gave rise to two lineages of photosynthetic protists red algae and green algae This hypothesis is supported by evidence Just like gram negative cyanobacteria red algae and green also have two plasma membranes and the transport proteins are also homologous On several occasions red algae and green algae underwent secondary endosymbiosis which means that they were ingested in the food vacuoles of heterotrophic eukaryotes and became endosymbionts themselves Nucleomorphs a tiny vestigial nucleus provides evidence for this hypothesis 28 2 EXCAVATES INCLUDE PROTISTS WITH MODIFIED MITOCHONDRIA AND PROTISTS WITH UNIQUE FLAGELLA Excavata a clade that was originally proposed based on morphological studies of the cytoskeleton Some members have an excavated feeding groove on one side of the cell body This clade includes the diplomonads pabasalids and euglenozoans Molecular data indicates that each of these 3 groups is monophyletic and evidence supports the monophyly of the excavate super group DIPLOMONADS AND PARABASALIDS The protists in these two groups lack plastids and have highly modified mitochondria Most diplomonads and parabasilids are found in anaerobic environments Diplomonads have reduced mitochondria called mitosomes and lack an Electron transport chain which prevent them to use oxygen to extract energy Instead they get energy from anaerobic biochemical pathways Many diplomonads are parasites Structurally they have 2 nuclei and multiple flagella extensions of cytoplasm Parabasilids also have reduced mitochondria called hydrogenosomes and generate some energy anaerobically EUGLENOZOANS The main morphological feature that distinguishes protists in this clade is the presence of a rod with either a spiral or a crystalline structure inside each flagellum The 2 major groups are kinetoplastids and euglenids KINETOPLASTIDS These protists have a single large mitochondria that contains an organized mass of DNA called a kinetoplast Kinetoplastids in the genus Trypanosoma infect humans and cause sleeping sickness Trypanosomes can evade immune response with an effective bait and switch defense This allows it to change surface proteins and prevent the host from developing immunity EUGLENIDS A Euglenid has a pocket at one end of the cell from which one or two flagella emerge Some are mixotrophs performing photosynthesis when sunlight is available but absorb organic nutrients when no sunlight is around Many are also phagocytotic 28 3 THE SAR CLADE IS A HIGHLY DIVERSE GROUP OF PROTISTS DEFINED BY DNA SIMILARITIES This super group is proposed recently based on whole genome DNA sequence analyses These studies resulted in 3 major clades stramenopiles alvelolates and rhizarians which form a monophyletic super group Some say that the stramenopiles and alveolates originated when a common ancestor engulfed a single celled photosynthetic red alga There is controversy surrounding this clade because some species lack plastids or remnants in nuclear DNA STRAMENOPILES Major subgroup of SAR clade include some of the most photosynthetic organisms on the planet Their name refers to their characteristic flagellum which has numerous hair like projections There are three subgroups diatoms golden algae and brown algae DIATOMS A key group of photosynthetic protists diatoms are unicellular algae that have a glass like wall made of SiO2 embedded in an organic matric The wall provides effective protection from the crushing jaws of predators They are among the most diverse group of Protists and among the most abundant photosynthetic organisms in the ocean and in lakes affecting the global CO2 levels The reason is that the Diatoms absorb CO2 during their lifetimes and when they die they sink to the ocean floor effectively pumping CO2 down GOLDEN ALGAE Color results from yellow and brown carotenoids The cells of golden algae are typically biflagellated All are photosynthetic but some are mixotrophic Mixotrophs absorb dissolved organic compounds or ingest food particles by phagocytosis Most are unicellular but some are colonial If the environment worsens some foRm protective cysts that can survive for decades BROWN ALGAE Largest and most complex algae All are multicellular and most are marine Color is due to carotenoids in their plastids Many of the species are commonly called