[Version française sur le site de La Recherche]
I mentioned that one of my projects here was to figure out how to live on Mars off the land using biology and what is already there. Part of this is involves plants, as you might have guessed.
But the core of my research involves cyanobacteria. Cyanobacteria are green bacteria which, as plants, can do photosynthesis. They are not very well known by the layman, at least not in this context, but they could become key elements in manned outposts on Mars. I will try, in this post, to explain their potential in a way which is accessible by readers outside the field. Please let me know if what I am writing is hard to understand – a common pitfall when vulgarizing your own research. If, on the contrary, what I am writing here is too obvious and you are craving for technical details, you can find the original paper here.
Sending humans to Mars within a few decades is now a realistic goal. However, even though leaving a footprint and planting a flag could be achieved with not much more than the current state-of-the-art of engineering, a definite pay-back is still in doubt. On the other hand, if a Mars mission can allow extensive human scientific activity and yield meaningful scientific data, the effort is justified. In such a case, scientists will need to spend a considerable period on site, and multiple short-term missions are not a viable option. Given the time, costs and challenges associated with the journey, long-term human bases will likely be needed. But while the vision of long-term human presence on Mars is compelling, providing consumables to sustain crews is a challenge: launch costs do not allow for a continuous resupply of colonies beyond the Moon. Sending from Earth all the needed resources does not seem financially sustainable. Should Mars colonization be consequently deemed too expensive to be realistic? Maybe not… if we can send low amounts of supplies and produce everything else we need there, from local resources.
Biological systems, and microorganisms in particular, will be extremely useful. Humans have been consuming and otherwise using microorganism-produced resources on Earth throughout their history: oxygen produced by cyanobacteria and eukaryotic microalgae, food and drinks as edible microorganisms and fermented products such as wine and yoghurt, drugs, various chemicals, biomaterials, biofuels, leached metals and so on. We also rely on them for many critical processes such as, for instance, waste recycling. Besides, they can be multiplied quickly from very low amounts; we could thus send a few milligrams of them to Mars, and multiply them there, in an enclosed culture system. The issue is: how do we feed them there? If we have to send the substrates they need from Earth, the mass problem is moved, not solved.
The good news is: all elements needed to support life have been detected in the Martian soils, rocks and atmospshere. Metal nutrients are present in rocks. There is gaseous carbon (as CO2) and nitrogen in the atmosphere, and additional carbon atoms can be found in the CO2 ice caps, in the surface and subsurface regolith (the loose soil that can be seen on photographs of Martian landscapes) due to exchange with the atmosphere, possibly in reservoirs formed when the atmosphere was thicker. Thus, Martian soil and atmosphere seem to contain all the basic elements needed to support life. Water is also there: it has been detected in large amounts as ice at the north polar ice cap, under the south carbon ice cap and in the subsurface at more temperate latitudes, as mineral hydration, and as vapor in the atmosphere, even though at low concentrations. It will also be a by-product of human metabolism and industrial activity. Solar energy is also present, as Mars’s average radiation flux is 43% that of Earth’s. The bad news? While all needed elements are naturally present and some additional sources will come from human activity, they are in a form that most organisms cannot use. In particular, many organisms – qualified as heterotrophic and including animals such as us humans, as well as most microorganisms – need organic compounds as carbon and energy sources, and their state and availability on Mars remain poorly known but is likely low. Fixed nitrogen, such as nitrate (NO3-), ammonia (NH3) and amino-acid chains (but not atmospheric nitrogen which is in the form of dinitrogen, N2), and dioxygen (O2), are also needed for most organisms. Finally, most metal elements are trapped inside rocks and inaccessible to most organisms. The main limitation is consequently not the lack of life-supporting elements, but the abilities of microorganisms to use them under the form they are encountered on Mars’s surface.
But not all microorganisms need organic compounds to grow; autotrophs such as cyanobacteria, don’t. Just like plants, cyanobacteria can photosynthesize: produce their own organic compounds from CO2, water and sunlight. In a nutrient desert such as Mars, this would give them a strong advantage over heterotrophic organisms. In addition, some can fix N2 which, as CO2, is present in the Martian atmosphere. On top of this, some have the ability to extract and use nutrients from analogues of Martian rocks. Most – if not all – nutrients needed to cover their needs could be directly provided from Mars’s resources.
As cyanobacteria produce organic compounds, fix nitrogen and release nutrients from rocks, why not use them for feeding heterotrophic organisms?
Cyanobacteria could also be used directly for various applications, including the production of food, fuel and oxygen.
What about plants? Even though basalt is the dominant rock type in Martian regolith and weathered basalt can yield extremely productive soils on Earth, regolith would probably need a physicochemical and/or biological treatment before it can be used as growing substrate for plants. Reasons for this include its poor water-holding properties (due to its low organic carbon contents), and that regolith nutrients are hardly available to plants. Besides carbon, the soil will need to be enriched in other elements, including nitrogen as most plants cannot fix atmospheric nitrogen (even though symbiotic nitrogen fixation occurs in some plants, mainly legumes, due to harboring specific bacteria in their tissues). Besides, plants are much less efficient than cyanobacteria (which could also perform photosynthesis-related tasks) regarding surface, CO2 and mineral use, are much more sensitive to environmental conditions, require more manpower, are harder to genetically engineer, take more time to regrow in case of accidental loss, are less manageable and contain inedible and hard-to-recycle parts. The most critical role of plants in human bases would be oxygen and food production, which can also be performed by cyanobacteria. However, even though some edible cyanobacterial species have excellent nutraceutical properties (ever heard of Spirulina?), they can currently not be used as a staple food due to their unpleasant and unvaried taste, lack of vitamin C and possibly essential oils, and low carbohydrate/protein ratios. These limitations could be addressed by genetic engineering, but plants have some advantages over cyanobacteria: they could provide tasty and carbohydrate-rich comfort food, and have beneficial psychological impacts on crewmembers. Small-scale cultures, relying on nutrients produced by cyanobacteria and waste from the crew, could be established.
To sum up: thanks to their photosynthetic, rock leaching and nitrogen fixing abilities, cyanobacteria could be used for processing inorganic compounds found on Mars into a form which is available to other microorganisms and to plants. Additional nutrients could come from the recycling of human waste. Finally, if some micronutrients (e.g., some metal ions) could not be mined or biologically synthesized on site, bringing them from Earth would only add negligible mass to the initial payload, as they are needed in trace amounts only.
Mars pioneers could thus bring only little tubes containing cyanobacteria and, once landed, feed them using substrates available there. Cyanobacterial cultures could then be used to grow other microorganisms and plants, and recreate a small ecosystem able to turn local resources into useful compounds. This is what I am working on.