Water molds.

Protists

Unit 2 - Microbiology

Lecture Outline

  • Eukaryotic Origins and Endosymbiosis

  • Defining Features of Eukarya

  • Overview of Protists

    • How are the main groups defined?

  • Key Features to Know

    • Multicellularity

    • Photosynthetic capabilities

    • Locomotion methods

    • Nutritional modes

Eukaryotic Origins

  • Differences from Prokaryotes:

    • Presence of a complex cytoskeleton

    • Compartmentalization (nucleus and organelles)

  • Origin of Eukaryotes:

    • First appearance in microfossils approximately 1.5 billion years ago (BYA)

Structure of a Protist Cell

  • Components:

    • Cytoplasm

    • Chloroplast

    • Plasma membrane

    • Nucleus

    • Mitochondria

    • Smooth endoplasmic reticulum

    • Rough endoplasmic reticulum

    • Golgi apparatus

    • Ribosomes

    • Lysosomes

Origin of Compartments

  • The nucleus and endoplasmic reticulum evolved from infoldings of prokaryotic cell membrane.

Endosymbiosis

  • Definition: A process where many organelles evolved from a symbiotic relationship between an ancestral eukaryote and a bacterial cell.

  • Organelles Involved:

    • Mitochondria:

    • Originated from aerobic bacteria engulfed by larger eukaryotic cells

    • Chloroplasts:

    • Derived from small photosynthetic bacteria that were engulfed by larger eukaryotes

    • E.g., Chloroplasts come from a single line of cyanobacteria.

  • Importance of Hosts:

    • Hosts are not monophyletic; for instance, brown algae have engulfed red algae that already contained chloroplasts, demonstrating secondary endosymbiosis.

Evidence for Endosymbiosis

  • DNA inside mitochondria and chloroplasts:

    • Circular DNA resembling that of bacteria in both size and character

  • Ribosomes in mitochondria similar to bacterial ribosomes, susceptible to antibiotics

  • Replication method:

    • Chloroplasts and mitochondria replicate via binary fission, not mitosis.

  • Mitosis is a process that evolved in eukaryotes, not prokaryotes.

Mitosis in Eukaryotes

  • Prokaryotic Genetic Structure:

    • Prokaryotes possess genes on a single DNA molecule.

  • Eukaryotic Genetic Structure:

    • Eukaryotes have multiple chromosomes

    • Mitosis and cytokinesis evolved to ensure the separation of chromosomes and other cell contents during cell division.

Overview of Protists

  • Diversity:

    • Protists are the most diverse among the four eukaryotic kingdoms.

    • They are classified based on exclusion from fungi, plants, or animals.

    • Varying aspects: unicellular, colonial, and multicellular forms

    • Most are microscopic; some species are quite large.

    • All modes of nutrition are represented.

Protists Across Supergroups

  • Classification of Protists:

    • Protists are not monophyletic (not forming a clade)

    • They are paraphyletic and span multiple clades across all five eukaryotic supergroups:

    • Excavata

    • SAR

    • Archaeplastida

    • Amoebozoa

    • Opisthokonta

Cell Surface in Protists

  • Protists exhibit varied cell surface structures:

    • Plasma membrane

    • Extracellular matrix (ECM) in some

    • Unique structures: silica shells in diatoms and foraminifera

  • Cysts:

    • Dormant cells with a resistant outer covering, used in disease transmission.

Locomotion in Protists

  • Types of Locomotion:

    • Flagella: One or more flagella present.

    • Cilia: Shorter and more numerous than flagella.

    • Pseudopodia:

    • “False feet” used primarily by amoebas but also by other protists.

Nutrition in Protists

  • Types:

    • Autotrophs:

    • Some are photosynthetic, while others are chemoautotrophic.

    • Heterotrophs:

    • Phagotrophs: Ingest particulate food matter.

    • Mixotrophs: Capable of both phototrophy and heterotrophy.

Reproduction in Protists

  • Asexual Reproduction:

    • Primary mode of reproduction; includes

    • Mitosis producing equal-sized daughter cells

    • Budding, which results in one smaller daughter cell

    • Schizogony: cell division after several nuclear divisions, producing several individuals

  • Sexual Reproduction:

    • Some protists reproduce sexually as a routine practice, while others do under stress.

    • Meiosis: evolved as a major eukaryote innovation.

    • Involves the union of haploid gametes produced by meiosis.

    • Advantage: facilitates frequent genetic recombination.

Protists as a Bridge to Multicellularity

  • Transition from single cells to colonies to true multicellularity has occurred multiple times via convergent evolution, indicating that this is not a homologous trait.

  • Fostering Specialization:

    • Multicellularity has greatly influenced the history of life.

Key Groups of Protists to Know

  • Familiarize yourself with various protist groups across different supergroups:

    • Excavata

    • Diplomomads

    • Parabasalids

    • Euglenozoans

    • SAR

    • Stramenopila (Brown algae, Diatoms, Oomycetes)

    • Alveolata (Dinoflagellates, Apicomplexans, Ciliates)

    • Rhizaria (Radiolaria, Foraminifera, Cercozoa)

    • Archaeplastida

    • Rhodophyta (Red algae)

    • Chlorophyta (Green algae)

    • Land plants

    • Amoebozoa

    • Opisthokonta

    • Choanoflagellates

Excavata

  • Comprising diplomonads, parabasalids, and euglenozoans

  • Shared cytoskeletal features and DNA sequences among these groups.

Diplomonads

  • Characteristics:

    • Unicellular

    • Multiple flagella for movement

    • 2 haploid nuclei

    • Example: Giardia (a parasite)

    • Functionally absent mitochondria.

Parabasalids

  • Live in symbiotic relationships, like those in termite guts with cellulose-degrading bacteria.

  • Key Examples:

    • Trichomonas vaginalis (a sexually transmitted infection)

  • Features include an undulating membrane and flagella; possess semi-functional mitochondria.

Euglenozoa

  • Characteristics:

    • Change body shape while swimming, transitioning between being elongated and rounded due to lack of cell walls.

  • Among the earliest eukaryotic organisms with mitochondria - Includes free-living euglenids and parasitic kinetoplastids.

Euglenids

  • Approximately 1/3 of euglenids possess chloroplasts and are autotrophic; others lack chloroplasts and are heterotrophic.

  • Reproductive method: asexual via mitosis.

  • Notable features:

    • Two anterior flagella (unequal in length)

    • Contractile vacuoles to collect excess water

    • Stigma for movement towards light

    • Small multiple chloroplasts, likely evolved from an ancestral symbiotic relationship through ingesting green algae.

Parasitic Kinetoplastids

  • Second major group in Euglenozoa characterized by a unique, single mitochondrion

    • Important diseases caused by Trypanosomes:

    • African sleeping sickness (transmitted by tsetse fly)

    • Leishmaniasis (transmitted by sand fly)

    • Chagas disease (contact with contaminated urine/blood of infected wild animals)

Stramenopila

  • Includes Brown algae, Diatoms, and Oomycetes

  • Characterized by very fine hairs on their flagella.

Brown Algae

  • Recognized as the conspicuous seaweeds of northern regions.

  • Life cycle features alternation of generations involving sporophyte (multicellular diploid) and gametophyte (multicellular haploid) stages.

  • Distinction: Brown algae are not plants.

Diatoms

  • Taxonomically classified under Phylum Chrysophyta

  • Photosynthetic and unicellular, recognized by their unique double silica shells.

  • Movement through structures called raphes containing grooves lined with vibrating fibrils.

Oomycetes

  • Commonly referred to as