Eukaryotic microbes and endosymbiosis overview

Flashcards covering the key concepts of eukaryotic microbes, protists, and endosymbiotic origins (primary and secondary), including evidence, examples, and terminology.

Protists

Protists are a historical, informal term used for microbial eukaryotes. This group is considered "non-monophyletic," meaning it does not include a common ancestor and all its descendants. It is similar to how the term "invertebrate" is used for animals without a backbone; it describes what they are not, rather than a cohesive evolutionary group. This vast and diverse group primarily comprises single-celled organisms, though some are multicellular or colonial, and they are typically found in aquatic or moist environments.

Microbial Eukaryotes

Eukaryotic organisms that are typically unicellular and microscopic, encompassing a vast diversity of forms, nutritional strategies, and ecological roles. Protists fall under this descriptive category. They represent a significant portion of eukaryotic diversity and play crucial roles in ecosystems as producers, consumers, and decomposers.

Monophyletic Group

A taxonomic group that consists of a common ancestor and all of its evolutionary descendants. Also known as a clade, these groups are the basis of modern phylogenetic classification, representing true evolutionary lineages.

Non-Monophyletic Group

A taxonomic group that does not include all descendants of a common ancestor. This classification often groups organisms based on shared characteristics that are either ancestral (symplesiomorphies) or independently evolved (homoplasies), rather than on true evolutionary relatedness. This category includes paraphyletic groups (which include a common ancestor but not all descendants, like 'reptiles' without birds) and polyphyletic groups (which do not share an immediate common ancestor but are grouped by convergent traits). The term 'protists' is an example of a non-monophyletic grouping.

Protist Nutritional Strategies

Protists exhibit diverse nutritional strategies, including photoautotrophy (producing food from light), chemoheterotrophy (obtaining energy and carbon from organic food sources), and mixotrophy (combining both photoautotrophic and chemoheterotrophic methods).

Photoautotrophs

Organisms (like plants, algae, and some protists) that utilize light as their primary energy source and convert inorganic carbon (e.g., carbon dioxide) into organic compounds for their carbon needs, essentially performing photosynthesis.

Chemoheterotrophs

Organisms (like animals, fungi, and some protists) that obtain energy by oxidizing chemical compounds and require pre-formed organic compounds as a source of carbon to build their biomass.

Mixotrophs

Organisms that can derive energy from light (like photoautotrophs) and also utilize organic carbon to build biomass (like chemoheterotrophs). They can sometimes use light to convert inorganic carbon to organic carbon, demonstrating metabolic flexibility and adapting to varying environmental conditions.

Phycobilins

Accessory photosynthetic pigments found in some protists and in cyanobacteria. These pigments, such as phycoerythrin (red) and phycocyanin (blue), capture light energy at specific wavelengths, particularly in the green and yellow regions of the spectrum, which chlorophyll a may not efficiently absorb, thereby enhancing the overall efficiency of photosynthesis in deep-water environments or under certain light conditions.

Endosymbiosis Theory

A major evolutionary mechanism proposed for the origin of eukaryotic organelles like mitochondria and chloroplasts. This theory, most notably championed by Lynn Margulis, posits that critical eukaryotic organelles were once free-living prokaryotes that were engulfed by a host cell. Rather than being digested, they established a mutually beneficial symbiotic relationship, eventually becoming permanent and indispensable cellular components, such as mitochondria for energy production and chloroplasts for photosynthesis.

Origin of the Eukaryotic Nucleus

While the exact mechanism is debated, strong genomic evidence suggests that the eukaryotic nucleus, which houses the cell's genetic material, shares significant evolutionary ties with archaea. Specifically, many eukaryotic nuclear genes exhibit strong sequence similarity to genes found in archaeal genomes, pointing towards an archaeal ancestor or gene transfer from archaea contributing to the development of the proto-eukaryotic cell's informational machinery and nuclear components.

Mitochondria Origin according to Endosymbiosis Theory

According to the Endosymbiosis Theory, mitochondria, the primary sites of aerobic respiration in eukaryotic cells, originated when a proto-eukaryotic cell engulfed an aerobic alpha-proteobacterium. Instead of being digested, this bacterium established an endosymbiotic relationship, providing the host with efficient energy production in exchange for protection and resources, eventually evolving into the mitochondrion.

Characteristics Supporting Mitochondrial Bacterial Origin

Further supporting their bacterial ancestry, mitochondria replicate by binary fission, contain a double membrane (the inner derived from the bacterial membrane, the outer from the host's engulfing vesicle), possess their own circular DNA genome distinct from the nuclear DNA, and have ribosomes that are more akin to prokaryotic ribosomes (70S) than eukaryotic cytoplasmic ones (80S).

Primary Endosymbiosis

This foundational event in eukaryotic evolution involves a single engulfment where a eukaryotic cell directly takes in a prokaryotic cell (like an aerobic bacterium leading to mitochondria, or a cyanobacterium leading to chloroplasts in red and green algae). The engulfed prokaryote survives, thrives within the host, and over evolutionary time, its genetic material is partially transferred to the host nucleus, and it becomes an essential, non-removable organelle, mutually beneficial to both.