Bioregenerative Life Support Systems provide for the maintenance of life in an isolated living chamber through complete reutilization of the material available, in particular, by means of a cycle wherein exhaled carbon dioxide, urine, and other waste matter are converted chemically or by photosynthesis into oxygen, water, and food.

As we'll see further down, algae is particularly well suited to converting carbon dioxide into oxygen through photosynthesis when compared to higher level plants.  The efforts to utlilze this function of algae in space environments began well before the moon landing and continue today.  An early gathering of scientists and engineers focused on the overall concept of life support took place in 1962 and provides a link between the use of algae in space enviroments and its use in global climate change remediation.  

In May, 1962, a "symposium-workshop" took place in Dayton, Ohio, called the Biologistics for Space Systems Symposium and the proceedings of the event, published as an Aerospace Medical Research Laboratories Technical Documentary Report, were described by the organizers as "a first effort to organize the information on biologistics for space systems".

The first of five technical sessions that comprised the symposium was centered on algal gas exchange systems and five papers were presented on the topic.  One of these papers was cited four times in the late 1960s and early 1970s, and then again another five times in the 21st century  - after it's paper-level indexing, digitization and inclusion in our Contrails database.  The twentieth century citations all had to do with potential bioregenerative life support systems and the twenty-first century citations all have to do with global warming mitigation.

The theme of the symposium was overcoming the human focused logistical challenges of long-term space travel, and the session in question centered on the use of the function of algae in gas exchange to help meet these challenges.  The article above, by Tew, Sane & Geckler, focused on algal gas exchange.  While current literature still suggests that this space-travel centered research is ongoing, there is simultaneously research centered on using the same function of algae in gas exchange to help remediate the effects of global climate change.

To understand why this paper remains relevant six decades after its presentation at the symposium, it would be helpful to fully understand the function that the algae perform - gas exchange - and the technologies in which the researchers hope to employ the algae, bioregenerative life support and carbon sequestration.

Gas exchange is "the physical process by which gases move passively by diffusion across a surface".    Examples of such surfaces would be the surface interface of a body of water and biological membrane that forms the boundary between an organism and its extracellular environment. In the process, gas molecules move from a region in which they are at high concentration to one it which they are at low concentration ("Gas Exchange, 2025").

Two gas exchange processes with which we are all familiar are respiration, which involves the uptake of oxygen (O₂) and release of carbon dioxide (CO₂), and photosynthesis, which conversely involves the uptake of carbon dioxide and the release of oxygen and water vapor.  It is these two processes that are aspects of the logistic concerns of bioregenerative life support.

Bioregenerative life support systems are "artificial ecosystems consisting of many complex symbiotic relationships among higher plants, animals and microorganisms" ("Bioregenerative life support systems", 2023).  The goal of these systems is to create a habitation environment similar to Earth that will support space missions with extended duration.  Systems are designed to address the production of oxygen, removal of carbon dioxide, purification of water and production of food.  It is the aspect of removal of carbon dioxide that bioregenerative life support systems have in common with carbon sequestration.

Carbon sequestration is the process of storing carbon in a carbon pool.  Carbon sequestration can be broken down into two general categories, geologic and biologic.  Geological sequestration refers to the storage of CO₂ underground in depleted oil and gas reservoirs, saline formations, or deep coal beds unsuitable for mining. ("Carbon sequestration", 2025).  Biological carbon sequestration is a naturally occurring process as part of the carbon cycle ("Carbon cycle", 2025).

While biological carbon sequestration is a naturally ocurring process, it plays a critical role in limiting climate change by reducing the amount of carbon dioxide in the atmosphere.  As such, scientists are engaged in research to find ways to enhance and accelerate this natural process.

Bioregenerative Life Support remains an "unrealized technology" (Aronowsky, 2017) despite being a "very attractive" theoretical system and the "subject of extensive study" (Averner, Karel & Radmer, 1984). The hope for Bioregenerative Life Support was that it would supplant the "picnic" approach of supplying space missions.

If humans are to live permanently in space, regenerative life support systems are an enabling technology and must replace the picnic approach of taking all supplies required for each mission (Knott, 1998).

Space travel in the 1960s required bringing along as cargo all of the needed supplies (oxygen tanks, meals, etc.).  This was seen as logistically unrealistic and expensive for longer missions.  Longer space missions would benefit if they could grow their own food, recycle their own waste and recycle their own carbon dioxide into oxygen.

The systems presently used for human life support in space require that food, water, and oxygen be stored, that excess atmospheric carbon dioxide be removed, and that other human wastes be collected and stored. Lengthy missions or large crews dictate that large masses of consumable supplies be taken along or resupplied. Ultimately, a point will be reached where the regeneration of consumables will be economically competitive with the cost of their initial transport or resupply. (Averner, Karel & Radmer, 1984)

However, a dependable bioregenerative life support system put to practical use would be a rather awesome achievement:

Closed ecological life-support systems are one of the most difficult scientific and engineering tasks in the space program .... In the spacecraft, a human being is confined in a restricted environment where it is necessary to miniaturize a completely balanced microcosm or closed ecological system.  This is an enormous biological and bioengineering problem. (Jenkins, 1968)

As it turns out, the complexities of managing an algal culture as a gas exchanger contribute greatly to the enormity of the design problem for complete bioregenerative life support systems.

It was almost self-evident that the recycling of carbon dioxide trumped all other considerations in the bioregenerative life support framework:

Man can live without food for a reasonably long period, without water, the period becomes quite a bit shorter; but without air, the result - almost instantaneous - is death.  This would make the atmosphere the most important consideration. (Kammermeyer, 1966).

With regards to the role of algae in a bioregenerative life support system, research was concentrated in a decade long period (emphasis added):

Several programs in applied algal culture have also been undertaken in the last 30 years, notably at the Carnegie Institute, and in programs sponsored by the United States Air Force and NASA on bioregenerative life support during the period of the mid-1950's to mid-1960's. (Averner,  Karel & Radmer, 1984)

An Aeronautical Systems Division report contemporary to the Biologistics for Space Systems Symposium, which took place at the tail end of that decade of research, remarked that algal gas exchange systems as options for bioregenerative life support could at that point be discarded outright:

...closed-cycle carbon dioxide management systems using algae or plants can be discarded for reasons of weight and development status. (Rousseau, 1963)

However, researchers noted that the assumption of a closed system ("Closed system", 2025) was a mistake:

Use or misuse of ecological terminology in describing bioregenerative research has led to a general misconception in the scientific community that success of the bioregenerative approach depends on development of a biologically and chemically closed ecology with complete material balance. (Miller & Ward, 1966)

Though a closed system with complete material balance could be the "ultimate goal", it was considered unlikely and unnecessary for an algal system's success, and that (emphasis added):

Algae may be used for partial regeneration of man's requirements for life in a closed environment (Miller & Ward, 1966)

Also contemporaneous to  the Biologistics for Space Systems Symposium was an assessment that, if bioregenerative life support systems were to succeed (emphasis added):

research specifically directed toward problems in bioregeneration and the mass culture of green plants (would be) greatly needed.  Indeed, contributions from researchers in many disciplines will be required before biological components can be successfully integrated into a balanced, reliable ecology for space life support. (Ward, Wilks, Craft & Wilks, 1963)

Indeed, one of the main concerns, as we'll see moving forward, was the reliability of a bioregenerative life support system - a system that needed to function as designed to keep space travelers alive.  Of great concern regarding a potential system's reliability was the complexities regarding the use of algae.

The photosynthetic efficiency ("Photosynthetic efficiency", 2025) of an algal-based system would suffer without maintenance.  Algae's ability to synthesize oxygen declines significantly as it grows in density, hence:

...the culture solution would require near-constant dilution with fresh growth medium and regular culling to remove mature algae cells. (Aronowsky, 2017)

Basically, the growth of the algae ends up "shading" subsequent growth unless it is removed, diminishing the efficiency of the photosynthesis process due to diminished ultraviolet light.  Photosynthesis is also diminished when the algae is stagnant for the same reason, a diminished amount of ultraviolet light reaching the algae.

Therefore, in addition to regular culling, the algae need to  be maintained in a:

state of continuous circulation within the growth tank to afford each cell equal exposure time to the light source. (Aronowsky, 2017)

Optimizing the photosynthetic efficiency of and in general maintaining the algal culture would require considerable attention.

Some of the daily tasks that would be necessary for upkeep of an algae culture used for carbon dioxide recycling:

Indeed, the cultivation of algae required such an effort and attention to detail that the

astronaut’s long-term survival, then, was tethered to the purity of his life-support system, a purity that, in turn, hinged on the astronaut’s sustained attention to both the hardware and wetware of the system. (Aronowsky, 2017)

Bioregenerative life support remains an unrealized technology in part due to the host of problems associated with algae cultures.  In fact, a two-day workshop was held with almost two dozen researchers attending to discuss the challenges:

Two of the concerns mentioned at the workshop are of particular interest:

At present ground-based algal growth reactors use processes that are dependent on gravity, such as gas-bubble sparging and mixing, and overflow harvesting. These processes will have to be replaced with functionally analogous processes that can operate in the space environment.... Other considerations include reactor size and weight (Averner, M. M., Marcus Karel & Richard Radmer, 1984)

The concerns about gravity dependent processes and weight and size of the reactor are common themes with regards to the utilization of algae as a gas exchanger.  We will revisit these factors when we discuss the use of algae in global warming remediation.

The complexities regarding algae in a regenerative life support system framework contributed to NASA's reticence on pushing forward:

Scaling up to the human environment, though, meant contending with the devices' fundamental design flaw and control-obsessed NASA's greatest fear: instability.  With so many interlocking components necessary to keep the algae in its most productive, pure state, the threat of system malfunction or failure loomed large.  (Aronowsky, 2017)

NASA's commitment to algal gas exchangers softened after that initial decade long period:

confidence in the viability of the algae-based system quietly waned as concerns about the system's overall reliability continued to mount; by the mid-1970s, NASA's support for algae research had winnowed to just a few lingering contracts that dealt with basic questions of photosynthesis. (Aronowsky, 2017)

Algal gas exchangers as carbon-dioxide recyclers for space travel had effectively been sidelined.

In spite of the hurdles littering the path towards the reality of an effective bioregenerative life support system, some researchers are still pushing forward with an optimism that efforts will eventually prove successful.  This optimism in support of the argument that the elusive bioregenerative system remains a possibility is justified by supporters based on successes on earth and in orbit.

Supporters of the feasibility of bioregenerative systems point to a terrestrial success story, the Soviet earth-based facility called Bios-3, which hosted experiments that

demonstrated the feasibility of sustaining human life inside a small, essentially closed ecological system. (Salisbury, Gitelson and Lisovsky, 1997)

Another success story with regards to bioregenerative life support is that of water recycling:

On the International Space Station (ISS), all water (graywater, sweat, moisture in breath, and urine) is collected in a closed-loop recycling system where impurities and contaminants are filtered out of the water (Hader, 2020).

 However, present day life support systems remove carbon dioxide chemically rather than recycling it into oxygen:

Manned spaceflight to other planets or extraterrestrial moons require a high level of sophistication....(o)xygen sustaining the astronauts during such long-term missions cannot be carried in sufficient quantity... (l)ikewise, the exhaled carbon dioxide needs to be removed from the air in addition to nitric oxide and other trace gases.  NASA is currently using solid amine sorbent and zeolite 5A molecular sieve material packed into beds for removing carbon dioxide during extended space flights. (Häder, Braun & Hemmersbach, 2018)

But Häder, etal. also argue that bioregenerative life support systems based on photosynthetic algae or plants are feasible and can:

absorb carbon dioxide, produce oxygen and remove wastes such as ammonia (Häder, Braun & Hemmersbach, 2018)

One of the National Academies suggested proof-of-concept research campaigns is the Bioregenerative Life-Support System (BLiSS) research campaign, and the justifications for pursuing a bioregenerative life-support system provided echo those of the previous seven decades.  Once again, the closed system question is addressed, and expectations are managed (emphasis added):

While it is not expected that a full, closed-system capability will be achieved by the end of the decade horizon described in this study (by 2032), the BLiSS campaign as recommended can provide appreciable offsets to resupply and quality-of-life benefits to exploration crews in that timeframe. (National Academies, 2023)

Though bioregenerative research pushes forward, hopes are couched - appreciable offsets rather than a fully closed system.

The report goes on to mention algae bioreactors, among other types, suggesting that the recycling of carbon dioxide by algae in long-term space exploration remains to this day an intriguing possibility worth further research.
 

Below are some of the technical challenges to the realization of effective bioregenerative air revitalization technologies for use in space applications, and in some cases, potential strategies to address them:

The complexities outlined above with regards to algae maintenance would be managed outside of the vehicle/base:

all important parameters (temperature, gas exchange rates, light intensity, concentration of nutrients and biomass production) have to be monitored online, allowing remote bioreactor control from Earth, to reduce the dependence on the crew (Fahrion, J., Mastroleo, F., Dussap, C-G., & Leys, N., 2021)

While this arrangement would certainly alleviate the burden on the crew, it also adds to concerns about reliability.

Reliability concerns for photobioreactors in space would also need to be addressed, in part by redundancy:

A high degree of redundancy has to be achieved.  Physiochemical emergency back-up systems, plant compartments and different PBRs could be put in parallel that can be uncoupled from each other. (Fahrion, J., Mastroleo, F., Dussap, C-G., & Leys, N., 2021)

and in part by accuracy:

reliable mathematical models for the bioreactors are essential to keep all processes predictable. (Fahrion, J., Mastroleo, F., Dussap, C-G., & Leys, N., 2021)

because the stakes are so high:

All possible scenarios have to be calculated and evaluated beforehand to avoid failure, because failure can be fatal for the crew. (Fahrion, J., Mastroleo, F., Dussap, C-G., & Leys, N., 2021)

The photobioreactor would be only one element of a bioregenerative life support system and would have to be connected to the rest of the system.  Yet:

Only a few experiments involve bioreactors that are connected to other life support compartments like the crew or a waste recycling compartment.  Consequently, many challenges remain in this research area and the connection between the different systems has to be elucidated more. (Fahrion, J., Mastroleo, F., Dussap, C-G., & Leys, N., 2021)

The concerns regarding the weight and volume required of an algal photobioreactor in bioregenerative life support applications seem to have been alleviated, if the function of the bioreactor does not extend past air revitalization and thermal control (Matula, E. E. & Nabity, J. A., 2016).

An optimal bioregenerative life support system would be an essentially closed system.  The recycling of waste materials, including carbon dioxide, would be 100%, necessitating no additional "resupply".  The life support system would not provide supply offsets but would be in a balanced self-sufficiency.

With regards to air revitalization, currently the International Space Station uses physico-chemical technologies rather than bioregenerative and is only able to recover 42% of the oxygen from the carbon dioxide the astronauts produce (Detrell, G. 2021)

In summary, the challenges to an effective bioregenerative air revitalization system for space applications include logistical issues with algae maintenance, as well as reliability, connectivity, weight/volume and the degree to which a closed system can be achieved. As we examine the use of algae in biological carbon sequestration, the challenges in that realm bear little comparison to those in bioregenerative air revitalization.

The story of Algae Across the Decades continues with Algae in Biologic Carbon Sequestration.

A team of researchers from Illinois Tech's Armour Research Foundation contributed the paper "Ezymatic Digestion of Algal Cells" to the Biologistics for Space Systems Symposium.  The paper addresses the subject of treating harvested algae as a potenttial astronaut food source.  The problem it seems with algae as a food source is an incomplete digestion by humans.  The team was interested in determining which enzymes would aid in algae digestion and therefore improve the chances of using algae as an astronaut food source.

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