TARDIGRADES: The walking dead
Ever wondered to yourself what the strongest animal in the world was? King of the jungle; the lion? The mighty giant of the ocean; the blue whale? Fortunately for us, a cute and squishy micro-animal holds the number 1 spot as strongest animal in the world. This animal is the one and only Tardigrade. The tardigrade is known as an extremophile species. Extremophiles are organisms that can survive in harsh conditions, such as extreme temperature, pressure, and chemical concentrations. Environments with such extreme conditions are believed to be ‘non-habitable’. Organisms in these environments adapt by changing their physiology and metabolism in order to survive.
What are they?
The first discovery of tardigrades was in 1773 by Johan Goeze, a German pastor. These small, micro-animals are segmented with 8 legs. Each leg has 4-8 claws (refer to figure 1) which in fact resembles those of a bear, hence also being known as ‘water bear’. The body length can vary between 1mm to below 0.1mm. There are no respiratory organs in the water bear allowing for gas exchange to take place all over the body. These micro-animals have tubular mouths which is surrounded by stylets, which is used as a ‘piercer’. This allows them to feed on plant cells, algae, and small invertebrates where they gather the organism’s body fluids and cell contents. All tardigrades species will possess buccopharngeal mechanisms, which help differentiate the different species. Tardigrades are found in various locations, from the deep ocean to the Antarctic ice to the Himalayan Mountains at heights from 18,196ft.
Living at the extreme:
Tardigrades have been around for millions of years. Fossil records approximate to around 530 million years. Therefore they have in fact outlived dinosaurs. Tardigrades are very flexible in the environments they are found in. Scientists found that tardigrades can live 10 years without water or food, showing that movement is not necessary. A scientist found a tardigrade in dried moss, showing that even though nutrients were not obtained they can survive. Therefore if the perfect location is found, tardigrades can just stay there for a decade then move to an area with better nutrients.
Surviving an icy prison:
85% of tardigrades bodies are water. From this 85%, tardigrades can make the water content decrease by 82% leaving them with 3% water in their bodies. This is an adaption that allows the tardigrades to survive in the freezing cold. Simple biology tells us that when water is frozen, it will expand. Therefore with 85% water content in the body, it will result in the tardigrade ‘bursting’ due to the large expansion. Therefore tardigrades dehydrate themselves to only 3% water being present to prevent the water expanding.
In order to prevent desiccation, tardigrades will put themselves in a state known as cryptobiosis. This is a state that is brought about by organisms being completely dehydrated or put in declining temperatures, where their metabolic activity becomes reduced to an undetectable level and the organism is dormant. Trehalose, an osmolyte compound (refer to figure 3), is used to prevent damage caused by dehydration to the organism’s membranes. Experiments have also been conducted where they have frozen a tardigrade in temperatures of -200°C. When warmed up, the organisms survived their icy prison where they also laid viable eggs. Out of 19 eggs, 14 hatched. When they were frozen in temperatures of -272°C then warmed back up, they managed to survive for a few more minutes. Therefore resuscitating themselves and coming ‘back from the dead’.
Bearing the scorching heat and radiation waves:
Tardigrades coming back from the dead happens in more ways than one. Being boiled does not seem to be as deadly for the tardigrades as it would be for most organisms. In 1842, Doyère, a French scientist found that a tardigrade in their tun state could survive at 125°C for a few minutes. The tun is a cryptobiotic or anhydrobiotic state where metabolic activities can be reversed (refer to figure 4). For the tardigrade, the tun is where they curl up into a little ball, similar to the armadillo.
In 1927, a scientist known as Gilbert Rahm heated a tardigrade to temperatures up to 151°C. Once they were brought back to life after being left to cool down, they survived for 15 minutes. When the temperature increases, proteins and cell membranes separate and chemical reactions are put on halt. Like being frozen, the sugar trehalose needs to be present and synthesis in order to replace lost water and prevent further damage of their membrane. Exposing the tardigrades to extreme radiation can also show how tardigrades are mighty and unbeatable. Scientists found that Gamma rays could kill tardigrades at levels of 5,000-6,000 Gy, meanwhile for humans 7.5 Gy would kill. All of the above being said, it should be stated that it is costly to be in their tun state and poses some risks. When completely dried out, whether in scorching heat or trying to survive in the frozen conditions they become inactive and are no longer able to avoid dangers, such as predators, presented around them.
To infinity and beyond:
After years and years of research humans built technology in order to protect people in outer space. There is a lack of pressure, resulting in air being forced out of your lungs. There would be an expansion in gases being dissolved in body fluids, which would results in us being inflated. Veins and eardrums would burst, and blood would also begin to boil. Therefore, we humans would become unconscious in approximately 15 seconds without protection. Although, this is no deadly matter for the tardigrades. An investigation was conducted in 2007 where tardigrades in their tun state, were placed on a satellite dish then taken to outer space. The comic on the right shows in a comedic manner how the tardigrades use their tun state through cryptobiosis to survive. Blasting them into space left them unprotected to the extreme UV radiations from the sun and outer spaces almost ‘perfect vaccum’. The results showed many had survived the trip, some females even laid eggs which were normal in health. Evidently tardigrades can produce viable embryos even in the harshest of conditions. Surviving in low pressures as well as intense radiation shows why these animals are known as the master of survival.
Coming back from the dead may just be a fantasy to us mere humans and all other organisms but not for the mighty tardigrades. Humans can’t go a month without food, tardigrades can go a whole decade without food. Freeze them, desiccate them, boil them, starve them, pressurize them or better yet throw them into outer space; the water bears will survive and be back for more. Tardigrades are known for their effective indestructibility on Earth showing why they are the ‘Masters of survival’. Survival is their strength, so wish them luck as they will be here even after we are gone.