I'm aware that cyanobacteria are responsible for the Great Oxidation Event and are very effective at turning CO2 into oxygen so it stands to reason that with the proper nutrients for the bacteria, you could make a more effective life support system. Are there any reasons why this would/wouldn't work and why it hasn't been implemented?
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12$\begingroup$ Please define "more effective". Current life support systems keep the crew alive, that seems effective enough. $\endgroup$– Organic MarbleCommented Apr 26, 2021 at 15:50
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8$\begingroup$ Having visited a mineral processing plant that used bacteria to treat ground up minerals from a mine, two things struck me about plants that rely on bacteria to do useful work. The first is the bacteria usually need a narrow temperature range to optimally do what is required of them. The second, is sometimes bacteria can produce very unpleasant odors in the natural course of what they do. Any such odors that may be produced by cyanobacteria would need to be dealt with so the crew were unaffected. $\endgroup$– FredCommented Apr 27, 2021 at 9:14
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$\begingroup$ Bacigalupi, "Wind-up Girl" $\endgroup$– Carl WitthoftCommented Apr 27, 2021 at 12:22
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1$\begingroup$ Another reason to be careful about using cyanobacteria, from Doctors investigate mystery brain disease in Canada. " ... beta-methylamino-L-alanine (BMAA) - which has been implicated as an environmental risk in the development of diseases like Alzheimer's and Parkinson's. BMAA is produced by cyanobacteria, commonly known as blue-green algae. $\endgroup$– FredCommented May 5, 2021 at 2:46
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5 Answers
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Why this wouldn't work? It works for the Earth; the reason why it is not implemented in space is purely in the engineering limitations.
- Cyanobacteria live in water, humans live in air.
Gravity is good at separating water from air, leaving a surface for the gas exchange. Microgravity is very good at mixing everything, so we should think about another approach for obtaining waterless air for humans and (mostly) airless water for cyanobacteria.
Light. Cyanobacteria are moderately efficient at producing oxygen. One will need few sq. meters per astronaut of transparent (but otherwise gastight) windows in order to allow enough light in. Not a trivial task in space. You will have to orient the windows in a favorable direction or make an extra amount of them. Also, good luck cooling the whole thing against these 1.5kW per square meter of heat input.
Closed carbon/nitrogen/phosphorus/etc cycle. Cyanobacteria don't only produce oxygen. They also reproduce at a rather high rate. One will want to recycle some of this biomass (it is barely edible and you have to extract it from the water in the first place).
In short, possible, but the transport to/from the Earth is WAY cheaper (for now).
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2$\begingroup$ Even worse - a spacecraft significantly more than 1 or 2 AU from the sun will receive rather less solar input per sq. m $\endgroup$ Commented Apr 27, 2021 at 12:23
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1$\begingroup$ I don't think running the cyanobacteria "oxygen cells" on sunlight is going to be viable in any case. I'd much rather imagine LED lighting (a'la computer screen backlight) and liquid cooling applied to tanks within pressurized volume of the ship. $\endgroup$– SF.Commented Apr 27, 2021 at 13:48
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4$\begingroup$ Minor quibble, but the main thing that separates water from air is not gravity, but surface tension. The dipole moment of water molecules causes them to stick together quite well, which is why water is relatively nonvolatile compared to most non-polar solvents, despite them having higher molar masses. In microgravity, water forms closed surface volumes that cling and, if not stuck to anything, equilibrate into spheres. They will lose some mass as vapor over time (just like water would under gravity), but the water globules won't just immediately mix with the air. $\endgroup$ Commented Apr 27, 2021 at 15:03
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1$\begingroup$ @DanBryant small scale, yes, it is a surface tension. But big shaking droplets are of no use in a space station either. $\endgroup$– fraxinusCommented Apr 27, 2021 at 18:32
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2$\begingroup$ @uhoh yes, I know that the surface tension rules in low gravity environment - and wet fabric can be a thing even in normal gravity. Still not the simple "water at the bottom, air above it" we have on Earth. Just imagine pumping the water in question ! $\endgroup$– fraxinusCommented Apr 28, 2021 at 14:39
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Another engineering aspect is controllability. Most electrical/mechanical/chemical life support systems can be throttled to anywhere between 0% and 100% of production capacity within minutes, if not seconds. In particular you can shut the system down for repairs and also carry a fully inert system as a cold spare. And since the atmosphere inside a spacecraft is far from a natural equilibrium, it is really important to keep it at a stable survivable level all the time. You definitely do not want a "Great Oxidation Event" to occur aboard and oxygen levels dropping to low are of equal danger.
With biological systems, the same level of control is much harder. Changing oxygen production is much more indirect, via nutrients and light influx and even though bacteria reproduce fast, significant changes to the population will take hours or even days. And if by some accident you manage kill your bacteria by contaminating the system, getting it back to running will take a long time.
I am not saying, that it is not possible to build such a system, but it is a challenge and probably too dangerous without having a conventional system as a backup along with a few weeks supply. So for all the missions that are done currently, using the conventional system directly might be the wiser choice.
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Environmental Control, and Life Support Systems need to be tailored to their application. There are many tradeoffs such as how much they weigh, how complex they are, how reliable they are and how efficient they are.
As a general rule the longer the duration of the mission, the greater the gains from life support closure. So missions of a few days can get by with little recycling whereas missions of many years need very high degrees of closure.
Any cyanobacteria oxygen recycling system would be too heavy for the short duration missions. But although it might be considered for long duration missions like a Mars base there are probably better materials to use. For example edible crops which can help further close the recycling loop. What happens to the dead/excess cyanobacteria?
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In light of a very recent news item I thought I'd turn my comments into answer so the information isn't lost should comments be deleted.
Having visited a mineral processing plant that used bacteria to treat ground up minerals from a mine, two things struck me about plants that rely on bacteria to do useful work. The first is the bacteria usually need a narrow temperature range to optimally do what is required of them. The second, is sometimes bacteria can produce very unpleasant odors in the natural course of what they do. Any such odors that may be produced by cyanobacteria would need to be dealt with so the crew were unaffected.
Another reason to be careful about using cyanobacteria, from Doctors investigate mystery brain disease in Canada.
... beta-methylamino-L-alanine (BMAA) - which has been implicated as an environmental risk in the development of diseases like Alzheimer's and Parkinson's.
BMAA is produced by cyanobacteria, commonly known as blue-green algae.
Not exposing a crew to BMAA and its possible neuro-degenerative effects is another reason not to have life support system based on cyanobacteria.
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Answer: O2 generation by algae has been implemented. It does work, but it is very challenging to scale the process. And it has a significant mass and power consumption penalty over stored oxygen.
I was recently involved in a feasibility study for using Chlorella vulgaris (green microalgae) for generating medical oxygen using solar energy in off-grid medical facilities. This is the same species of algae used by NASA in the photobioreactor aboard the ISS:
Light is toxic to the photosynthetic enzymes in algae. Sunlight needs to either be attenuated or intermittent. Light intensity decreases as it passes through the medium. If you are attempting to minimize the mass of the medium, you will need to use LED light source (and the attendant solar panels) rather than direct sunlight.
The algae themselves attenuate light so either algae density needs to be low (lots of medium) or LEDs must be close together (lots of LEDs).
Algae are not very efficient at converting light to O2. That’s not what evolution selected them for since O2 is their waste product. Overall, algae are 12% energy efficient at separating CO2 into O2 and carbohydrates. The bioreactor process is power intensive, so it becomes a major power load (and therefor a big cooling load as well) on a spacecraft.
The bioreactor needs to be mixed, but very gently since the algae are fragile. On Earth this is usually done with bubbles rising through the medium, but the bubble size needs to be controlled to prevent shear forces on the algae. In microgravity, bubble mixing is not available. Mixing the cubic meter of medium in small passages is a non-trivial problem.
Separating O2 bubbles from the medium is easy if gravity is present. Not so in microgravity. High O2 productivity is dependent on keeping the algae on a particular point of their growth curve by regulating density, light intensity and nutrient levels. The bioreactor will need attention, maintenance and problem solving. The astronaut becomes a farmer.
The bioreactor produces a large amount of green goo and consumes significant amounts of electrolyte nutrients. In theory a closed loop could be engineered where astronauts could poop in the bioreactor and eat the goo. You might want to run that past them first.
The mass of nutrient is one metric ton per person. If the system is run off solar panels, the total mass is several tons per person. A space voyage would need to be several years long for the system to have a mass advantage over liquid oxygen.
