Notes on Biodiversity: Microscopic Organisms

Definition: Microscopic eukaryotic organisms that do not belong to the plant, animal, or fungus kingdoms, exhibiting considerable diversity in form and function.

Historical Context:

• First appeared in the fossil record approximately 2.1 billion years ago, which is more recent than prokaryotes, which emerged around 3.5 billion years ago.

• The evolution of protists marks a significant developmental stage in the history of life on Earth.

• Typically unicellular, though some are multicellular and exhibit complex structures.

Diversity of Protists:

• Major categories include:

  • Protozoans: These are primarily heterotrophic and exhibit varying levels of mobility and complexity.

  • Phytoplankton: These are autotrophic organisms crucial for aquatic ecosystems, forming the foundation of the marine food web.

  • Slime molds: These exhibit unique life cycles and behaviors, often resembling fungi at certain growth stages.

Protozoans

• Nutritional Mode:

  • Obtain nutrients primarily by consuming other organisms either through ingestion or absorption.

  • They play an essential role in ecosystems by regulating populations of bacteria and other microorganisms.

• Examples:

  • Plasmodium: A single-celled parasite responsible for malaria, which is transmitted through the bites of infected mosquitoes.

  • Giardia: An intestinal parasite affecting humans and many animals, leading to giardiasis, often resulting in severe gastrointestinal symptoms.

  • Paramecium: Characterized by hair-like cilia for movement, it utilizes these to capture food particles in aquatic environments.

  • Amoeba: Known for its agile and changing body shape, it can engulf food through a process called phagocytosis, exhibiting fascinating adaptability in various environments.

Phytoplankton

• Characteristics:

  • Aquatic protists that perform photosynthesis, contributing significantly to global oxygen production.

  • Found in both freshwater and saltwater environments, playing a foundational role in aquatic ecosystems.

  • Serve as a crucial food source for a wide array of marine and freshwater organisms, from zooplankton to large fish species.

• Examples:

  • Algae: Include unicellular forms, such as diatoms and green algae, as well as multicellular forms like seaweed that resemble plants in structure and function.

  • Diatoms: Notable for their unique silica cell walls, which come in various shapes and sizes; they are vital to both food webs and biogeochemical cycles.

  • Dinoflagellates: Mostly photosynthetic but can also engage in heterotrophy; they are known for their bioluminescence and the occurrence of harmful algal blooms, referred to as "red tides" that produce toxins affecting marine life and human health.

Slime Molds

• Definition: Protists that share characteristics with fungi but possess sexual reproduction and other traits more typical of protists.

• Habitat: Commonly found in damp forest environments where they decompose organic matter.

• Cell Wall Composition:

  • Made of chitin, which is different from fungi that typically have cell walls composed of cellulose.

• Feeding Mechanism:

  • Actively ingest food, similar to animals, with a unique ability to move toward nutrients, showcasing a level of complexity in behavior and responsiveness.

• Behavior:

  • Demonstrate interesting social behaviors, including aggregating together to form multicellular structures when food is limited.

Comparisons: Protists and Bacteria

• Similarities:

  • Both can be unicellular and microscopic.

  • Both may be photosynthetic or heterotrophic.

• Differences:

  • Bacteria: Prokaryotic, always unicellular, always microscopic, and lack membrane-bound organelles.

  • Protists: Eukaryotic, can be unicellular or multicellular, can range from microscopic to macroscopic, and possess complex cellular structures.

Review Question

• Scenario: A microbiologist identifies a new organism from a water sample sent for analysis.

• Characteristics: (a) unicellular, (b) microscopic, (c) has a nucleus, (d) consumes nutrients.

• Answer: The organism is classified as a protist due to the presence of a nucleus, which is a defining characteristic that distinguishes it from prokaryotic bacteria, indicating its eukaryotic nature.

Viruses

• Classification:

  • Viruses are not categorized as living organisms due to their inability to thrive independently; they require a host cell to reproduce.

• Structure:

  • Typically comprised of a viral envelope with a recognition spike for host binding, a nucleic acid core (either DNA or RNA), and a protective protein coat known as a capsid.

• Infection:

  • Viruses can infect both animal and plant life, causing a variety of diseases.

• Examples of Viruses:

  • Tobacco Mosaic Virus: The first virus discovered, causing significant agricultural damage.

  • HIV: A retrovirus that targets the human immune system, leading to the development of AIDS and severe immune deficiency.

  • Bird Flu: A highly contagious virus affecting avian species and can be fatal to humans in some instances.

  • Respiratory Syncytial Virus (RSV): A leading cause of hospitalization among infants and young children, causing bronchiolitis and pneumonia.

Vaccines

• How Vaccines Work:

  • Vaccines introduce a dead or weakened pathogen, or a piece of its genetic material, into the body to stimulate an immune response.

  • They prompt white blood cells to produce antibodies against the pathogen, ensuring a rapid immune response upon future exposure.

  • Vaccination has proven to be one of the most effective public health measures in preventing infectious diseases by establishing herd immunity in populations.