Stress is the driver of evolution
All organisms are naturally influenced by a number of stress factors, such as climate change, radiation, predator-prey relationships, parasites, diseases, and competition within and between species. As a result, the ability to respond to stress is an important feature of all living systems and, conversely, the development of species and ecosystems as a whole is not possible without such natural stressors.
Water is the source of life. The human body is made of 60% water and a loss of only 3% leads to deterioration of the motor capacity and the functions responsible for regulating body temperature. Unlike humans, tiny chironomid larvae of the Polypedilum vanderplanki genus, Hinton 1951 [Diptera, Insecta] when in stress conditions slow down their body and enter a life-saving deep sleep – a phenomenon known as anhydrobiosis. The anhydrobiosis, as a form of cryptobiosis, is a stage in the life of some organisms in which the level of metabolic activity is reduced (reversibly) to an immeasurable level, in order for the organism to survive in unfavorable environmental conditions – in this case dry conditions.
The tardigrades, which are relatively well known for the process of anhydrobiosis, but also microbes, fungi and some plants also survive the drought in this way. However, the P. vanderplanki species is important because it is the largest animal (8 mm long body of the larva) that can survive via anhydrobiosis through complete desiccation of its body.
Picture 1. P.vanderplanki lifecycle (Source: Anhydrobiosis Research Group, National Institute of Agrobiological Sciences, 2009).
Sleeping P. vanderplanki larvae have so far been found mainly in central Africa near the equator, where temperatures are expected to be relatively high all year round and the dry season can last several months without a drop of rain. The P. vanderplanki larvae are found in small puddles on rocks where the water evaporates quickly at higher temperatures, and in order to survive in such an unstable environment, the larvae form a tubular mud formation called a tubular nest where they sleep. As the water evaporates, the larva dries up, gradually protected in the tubular nest, and when the water content in the body finally falls to 3% of body weight, the larva falls asleep waiting for the next rainfall.
But how do they actually survive? Amazingly, these larvae have adapted to convert their tissues into glass in order to cope with stress (high temperature and drought). First, their tissues accumulate high concentrations of a type of sugar called trehalose, which is vitrified into glass. The result is a wrinkled, small, vitreous unit that can remain in this state for a long time, defying stress. As soon as the wet season begins, the larvae rehydrate and continue for the rest of their life cycle: pupal stage metamorphosis, followed by adult metamorphosis, mating and laying eggs. An important aspect of their life cycle is that they must go through a drying process. In other words, they are so adapted to this strategy that they cannot live without it.
Surprising is the fact that the dormant larvae can survive decades of anhydrobiosis and show cross-resistance to a variety of environmental stresses, including temperature fluctuations, high doses of ionizing radiation and organic solvents. Given the level of protection that vitrification provides, scientists have begun testing the extreme ecological limits of these larvae – the point at which they can still survive, rehydrate and continue their life cycle. Upper and lower temperature limits (-270°C to above 100°C) were tested; duration in the drying phase (at least 3 years); radiation tolerance (rehydration observed at doses of up to 3500 Gy). As the largest animals going through the process of anhydrobiosis, they are excellent candidates for long-term space travel and space colonization. According to research by the Japan National Institute of Agrobiological Sciences, Department of Anhydrobiosis Research, they can serve as part of hydroponic ecosystems, a source of food for humans or a source of food for fish, and are excellent study material. That is why the space vacuum tolerance was tested – Polypedilum vanderplanki survived 13 months in the vacuum of space in an experiment carried out on the International Space Station, where 80% of the larvae that were exposed successfully rehydrated.
Picture 2 The journey to space and back – P. vanderplanki (source: Anhydrobiosis Research Group, National Institute of Agrobiological Sciences, 2009).
This discovery is fascinating when it comes to their role in the space exploration for the decades to come, but we must not forget their role so far on planet Earth. They are unique and, together with other organisms, they are important parts of the global and local biodiversity and a link in nature. Evolutionarily speaking, the variations in the natural stresses (in the case of Polypedilum vanderplanki those are the dry periods) were fairly constant and allowed the organisms to adapt to the changing environmental conditions. However, nowadays the human activities have introduced totally new phenomena. In addition, these stresses usually have a synergistic effect, either with natural systems or with each other, resulting in a reduced ability (or level of tolerance) of organisms to adapt to them.
Whether we continue at this pace of development or give organisms and nature a chance depends on us. The protection of the environment Now and Here will enable prosperity of the organisms without which our life cannot survive Now and Here and in the Future in Space.
About the author:
Martina Blinkova Doncevska has a master degree in ecology, in the field of hydrobiology. She works as a researcher at the PNU Institute for Environmental Research, Construction and Energy – IEGE and is an environmental consultant at the Civil Engineering Institute Macedonia – GIM. She is actively working on environmental protection and establishing sustainable development in municipalities, cities, companies, for which she has been named the leader of the United Nations Sustainable Development Goals in the company for 2019.