Question

Cryptobiotic Tardigrades and Life Tardigrades, also called water bears or moss piglets, are small animals that can grow to about \(0.5 \mathrm{~mm}\) in length. Terrestrial tardigrades (pictured here) typically live in the moist environments of mosses and lichens. Some of these species are capable of surviving extreme conditions. Some tardigrades can enter a reversible state called cryptobiosis, in which metabolism completely stops until conditions become hospitable. In this state, various tardigrade species have withstood dehydration, extreme temperatures from \(-200{ }^{\circ} \mathrm{C}\) to \(+150{ }^{\circ} \mathrm{C}\), pressures from 6,000 atm to a vacuum, anoxic conditions, and the radiation of space. Do tardigrades in cryptobiosis meet the definition of life? Why or why not?

Short answer

Tardigrades in cryptobiosis do not meet all life criteria, but are considered living due to the reversible nature of the state.

Step-by-step solution

Definitions

We need to establish what the definition of life is. Typically, life is defined by several characteristics including metabolism, growth, reproduction, response to stimuli, and adaptation to the environment.

Analyze Tardigrades' Cryptobiotic State

In their cryptobiotic state, tardigrades halt their metabolism completely, which means they are not growing or reproducing. They are not responding to stimuli in the usual way since they are in a suspended state.

Compare with Life Characteristics

Now, compare the characteristics of life with the cryptobiotic state. Metabolism is paused in cryptobiosis, which means tardigrades do not exhibit one of the primary characteristics of life in this state. However, they are capable of resuming life processes when conditions are favorable.

Conclusion Drawing

While in cryptobiosis, tardigrades do not meet all the usual definitions of life, as their metabolism is suspended. However, since this state is reversible and they resume life functions afterward, they are still considered living.

Key concepts

Tardigrades

Tardigrades, affectionately known as water bears or moss piglets, are fascinating microscopic creatures. These tiny invertebrates can reach a size of about 0.5 mm and are often found in moist habitats, like mosses and lichens. Tardigrades are renowned for their ability to adapt to incredibly harsh environments, captivating scientists and nature enthusiasts alike.
Tardigrades have several astonishing features, including their robust exoskeleton and unique claws that assist in mobility. They play vital roles in ecosystems, such as decomposing organic material and forming a basic component of the food chain for some small predators. Their ability to survive where few others can make them an intriguing subject of biological studies.

Metabolism

Metabolism is a fundamental characteristic of life, describing the chemical reactions within organisms that allow them to grow, reproduce, and maintain their structure. At the core of metabolism is the conversion of energy from external sources to fuel biological processes. This includes breaking down nutrients to generate energy and synthesizing compounds needed for cellular structures.
For tardigrades, metabolism is typically active during their normal life processes, enabling them to thrive in their damp environments. However, during cryptobiosis—a unique survival strategy—tardigrades significantly alter their metabolism. They effectively pause it, conserving energy and resources, awaiting favorable conditions. This remarkable ability to halt metabolism without sustaining damage is rare and highlights the incredible adaptability of tardigrades.

Characteristics of Life

The characteristics of life involve several critical processes: metabolism, growth, reproduction, response to stimuli, and adaptation. These processes form the framework for defining what it means to be "alive."
In the cryptobiotic state, tardigrades defy some of these characteristics. For instance, while their metabolism and reproduction are halted, the potential for these processes to resume indicates a reversible suspension of life activities, not the cessation of life itself. Tardigrades truly test the boundaries of life's definition, revealing the flexibility and adaptiveness of living organisms.
Understanding these livings traits helps us appreciate how life can persist across a wide array of conditions, echoing the resilience seen in nature's designs.

Extreme Conditions Survival

Tardigrades are experts in extreme conditions survival. Their ability to endure temperature extremes from -200°C to 150°C, immense pressures, anoxic environments, and even the vacuum of space is nothing short of extraordinary.
This resilience is attributed to cryptobiosis, a survival mechanism where tardigrades lose nearly all water content, shrinking into a near-dormant state. In this form, they can survive without nutrients, water, or air for several years. When conditions improve, they rehydrate and rapidly return to their active form.
This extreme adaptability not only safeguards them but makes tardigrades a prime subject in astrobiology studies, seeking to understand life's potential on other planets and the true limits of biological endurance.