One of the greatest challenges limiting our ability to send astronauts on long-haul missions is the need for constant resupply of essential materials from Earth. Any manned spaceflight needs to carry enough food, water and oxygen to sustain a crew for the duration of the mission, but there is only so much weight a single spacecraft can carry. Current life support on the International Space Station still relies on regular supplies from Earth, as only 40 per cent and 78 per cent of oxygen and water are recycled and all waste must be packaged and transported back to the surface for disposal. Current life support technologies rely on chemical processes such as electrolysis, and can therefore only produce a limited variety of compounds and cannot do so indefinitely.
It is now widely accepted that the ideal way to achieve a continuous supply of life support materials like oxygen, food and water is with a system that artificially recreates the biosphere on Earth. A biological system like this would need to utilise a network of producers, consumers and decomposers to convert all waste compounds into essential nutrients, water and air. In other words, it needs to be closed-loop and completely regenerative, so that no material leaves the system and no material needs to be added into the system once it is running.
Bacteria are a massively diverse class of organisms that can produce almost any compound and degrade almost any waste product, making them ideal for use in such a closed-loop system. It is therefore unsurprising that a great deal of research has been conducted over the last few decades into how systems of bioreactors containing metabolically active bacteria can be used in space. The Micro-Ecological Life Support System Alternative (MELiSSA) model is the most advanced and well-researched closed-loop life support system to date, having been in development in cooperation with the European Space Agency for the past 30 years.
MELiSSA is made up of five compartments. The first three of these compartments are a system of advanced bioreactors that process waste products. The fourth compartment contains higher plants, which are mostly edible crop plants, and cyanobacteria for production of breathable air and filtration of potable water, and the fifth and final compartment houses the crew.
Compartment I is descriptively known as the liquefying compartment. It takes in material from the crew and plant compartments where thermophilic anaerobic bacteria digest solid waste into carbon dioxide, volatile fatty acids and ammonium. One of the most extensively studied species of bacteria to occupy this compartment is Fibrobacter succinogenes, which is naturally found in the gut of cattle where it helps to digest cellulose. This species is well suited to the liquefying compartment because it can break down fibrous waste without the use of oxygen (a precious resource on a spacecraft) and can do so at a very high temperature, which can also aid in the breakdown of waste by helping to break the bonds between macromolecules.
Compartment II exists to eliminate the terminal metabolites that are produced as byproducts in compartment I. The species Rhodospirillum rubrum has been selected to occupy this compartment, where it converts the amino acids from compartment I into free ammonium and feeds on the volatile fatty acids. This compartment is also known as the photoheterotrophic compartment, because R. rubrum needs light in order to grow and is a heterotroph, meaning it needs to grow on sources of organic carbon. Interestingly, this compartment was originally divided into a photoheterotrophic and a photoautotrophic compartment, the latter of which can manufacture its own organic molecules from carbon dioxide under light conditions. However, experimental data showed that this had no significant benefit to the overall function and therefore they were merged into a single photoheterotrophic compartment.
Compartment III, the nitrifying compartment, is there to cycle free ammonium from compartment II into nitrates, which are used later on by the higher plants. This compartment contains a mixture of Nitrosomonas and Nitrobacter species which oxidise ammonium into nitrite, then to nitrate, similarly to how the nitrogen cycle works on Earth. Both of these bacteria use carbon dioxide as their sole carbon source to grow, so this compartment acts as an additional carbon sink as well as a nitrifying bioreactor. It is also interesting to note that both nitrogen-fixing species are very efficient at removing micropollutants from the bioreactor effluent. This is relevant because pharmaceuticals and endocrine-disrupting compounds like triclosan (a component in toothpaste) could be leaked into the system via waste from the crew and if they are not removed, could disrupt the growth of other organisms in the system. Compartment III could therefore have a secondary function as a micropollutant-eliminator.
Finally, there is compartment IV. This is split into two sub-compartments, one containing the higher plants and one containing cyanobacteria. Both sub-compartments fix carbon dioxide into breathable oxygen and produce edible biomass to sustain the crew, and the plant compartment also uses up the nitrate from the waste processing compartments and generates potable water. Cyanobacteria in particular are incredibly efficient at removing carbon dioxide from the atmosphere and producing oxygen as an end product of photosynthesis. They are also a surprisingly good source of nutrition and can therefore supplement the diet of the crew where the higher plants might be lacking. A large variety of crop plants can be grown in the higher plant compartment, which are intended to form the bulk of the crew’s diet. Vegetarianism might therefore become very popular amongst astronauts out of necessity.
If we want to become one step closer to exploring the outer reaches of our galaxy and beyond, we are going to need a better life support system than we currently use on board the International Space Station. A closed-loop, fully regenerative system like the MELiSSA model would solve many of the problems space travel currently faces, by processing all waste and producing everything that a crew needs to survive. Although there is a long way to go, MELiSSA provides a solid foundation on which the next generation of scientists can build, to perfect the very first biological life support system technology.
Written by Elle Bethune and edited by Ailie McWhinnie.
Elle’s thoughts… One of the biggest things holding us back from exploring space is the fact that we can’t produce essentials like food and air indefinitely. Until we can figure out a way to do so, we are tethered to the Earth, so to speak, so I think establishing a regenerative life support system like this should be top priority for anyone invested in developing long haul space travel technologies.
Elle is an Astrobiology PhD student studying how anaerobic microorganisms grow on extra-terrestrial organic compounds from meteorites. Find her on LinkedIn @Elle Bethune.
