Amoebae or amoebas are single-celled microorganisms of phylum Amoebozoa. Testate amoebae build unique shells of silica and other organic materials. An amoeba is a favorite of microbe seekers as it flows into diverse shapes created by its pseudopodia or false feet.
Above: Some testate amoebae (A, B, C - Heleopera sphagni; D, E - Hyalosphenia papilio; F, G - Centropyxis aculeata; H, I - Arcella discoides; J, K - Difflugia pyriformis) showing the shells (1) enclosing the cytoplasm (2) and the pseudopodia (3) protruding through the aperture. Adapted from Leidy (1879). Source ResearchGate.
About Amoebae
Amoebae or amoebas are a varied group of one-celled organisms within the kingdom Protista. They're characterized by their shape-shifting capabilities, allowing them to extend portions of their cell membrane to form pseudopodia for movement and capture of food.
Amoebas are often found in water bodies and moist terrestrial environments. Primarily freshwater organisms of lakes and ponds, they can also be found in saltwater and soil.
They can differ widely in their habits. Some species live independently, while others are parasitic. Despite these differences, they all share key features, such as fluid-like cytoplasm and a nucleus containing genetic material.
Testate or Shelled Amoebae
Free flowing amoebae are called naked amoebae. Testate amoebae are different. They form an external shell, or test, made of organic or inorganic materials. Silica is a favorite building material and some testate amoeba shells are made entirely of diatom silica skeletons.
This protective casing with its masterful structure integrity helps these vital organisms maintain a stable internal environment. They are protected from predators and changes in the environment, such as pollution or low oxygen levels.
Now this is truly amazing. The shells of testate amoebae are not just thrown together like a bunch of stuff. They're intricately designed with pores for the amoeba's pseudopodia to extend through. In this way the amoeba easily moves and captures food.
Testate amoebas include:
Arcella: Known for its dome-shaped shells.
Euglypha: Features complex, elongated tests made of organic materials.
Centropyxis: Characterized by its unique, rounded tests.
Difflugia: Recognized for its ornate, often mineral-based shells.
Each of these genera showcases different shell shapes and textures, adapted for their particular lifestyles and environments.
Materials Used for Their Shells
Testate amoebae construct their shells using found materials such as silica, organic debris, or fine particles in the environment. The choice of materials can vary greatly among different species and even within a species, depending on local conditions.
Composition includes:
Silica: Testate amoebae such as Difflugia produce siliceous shells from soluble silica found in water. Amoebas extract soluble silica from the water and precipitate it to form solid structures.
Organic materials: Species like Euglypha build intricate tests from organic debris. They secrete sticky substances to bind debris and minerals, solidifying to form the protective shells.
Mineral particles: Some testate amoebas actively collect sand grains and other particles, using adhesion mechanisms to make them into cohesive shells. This also helps camouflage them from predators and other microbe seekers.
Environments of Testate Amoebae
Testate amoebae are water lovers. They live in a variety of environments, often freshwater bodies, such as streams, ponds, and marshes, as well as damp soils or decaying organic matter. They comprise up to half of all microbial biomass in peatlands.
The presence of testate amoebae is a sign of a healthy ecosystem, as they are important to nutrient cycling and decomposition of matter. Like the amoebae, many microbes are decomposers and without them we would have a horrific world.
Testate amoebae help maintain the microbial food web as both predators and prey. They feed on bacteria and decaying organic materials, ensuring bacterial populations remain in check. Recycling of nutrients fosters plant growth and benefits other microorganisms.
Feeding, Breathing, Reproducing
Testate amoebas are heterotrophs, meaning they cannot produce their own food. Omnivorous microbes, they primarily consume bacteria, algae and organic detritus. They use their pseudopodia to engulf food particles through a process called phagocytosis.
In phagocytosis the amoeba surrounds its food with pseudopodia to create a food vacuole. The vacuole fills with digestive enzymes to quickly break down the food. The amoeba can absorb resulting nutrients. One testate amoeba can eat thousands of bacterial cells per day.
After digestion, any excess or undigested materials are expelled from the amoeba through exocytosis. The waste is moved to the edge of the cell, which opens up to release it into the surrounding environment. This provides nutrients for other life forms.
To breathe, testate amoebae rely on diffusion to exchange gases across their cell membrane. They absorb oxygen from water and release carbon dioxide, vital to plant health. Testate amoebae contribute up to 13 % of total carbon and nitrogen mineralization in upland forest soils.
Reproduction is most often asexual through binary fission, where one amoeba splits into two identical daughter cells. Under favorable conditions, a single testate amoeba can rapidly multiply, leading to significant population growth.
Historical Existence on Earth
Testate amoebas have an evolutionary history of hundreds of millions of years. The fossil record indicates the presence of testate amoebas has been consistent throughout various geological periods, showing their resilience and adaptability to changing conditions.
In environmental microbiology, testate amoebae are valuable research subjects. Their presence can act as bioindicators, shedding light on water quality and overall ecosystem health. Testate amoebae are harmless to humans.
Facts About Testate Amoeba
Diversity: There are numerous species within the testate amoeba group, each adapted to specific environmental niches.
Environmental Indicators: Due to their sensitivity to environmental changes, testate amoebas are often used as bioindicators for assessing water quality and sediment health.
Intricate Shells: Many testate amoebas have beautifully crafted shells that can be observed under microscopes, showcasing patterns and designs unique to each species. Their shells have been found preserved in sedimentary rocks.
Some testate amoebas demonstrate remarkable morphological diversity, which helps them adapt to various habitats. They can endure harsh conditions, such as drying out and temperature fluctuations.
Research on testate amoebas can reveal insights into prehistoric ecosystems and ancient climates.
Testate amoeba are vital components of their ecosystems. They contribute to the decomposition of organic matter and nutrient cycling while exhibiting the unique adaptation of shell formation.
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