Why anyone bothers growing algae off-world

Right now, seven people are alive on the International Space Station because of an accounting exercise: how much oxygen, water, and food can be carried up, and how much can be made or recycled once it's there. Almost none of that accounting involves biology.

The ISS's life support is built around physicochemical engineering. The Oxygen Generator System (OGS) electrolyzes water into oxygen and hydrogen; a 113-kilogram unit produces about 5.9 kilograms of oxygen a day. NASA engineer Harry Jones has worked through the economics of that system in detail: at roughly $70 million to build and around ten percent of that per year to run, the OGS produces oxygen at an effective cost of about $3.26 per kilogram of system mass — dramatically cheaper than launching the equivalent oxygen from Earth, which runs anywhere from $1,500 to $59,500 per kilogram depending on the rocket. Run the same comparison for water recycling and the breakeven point for "make it" beating "ship it" comes out around 1,287 days; for oxygen generation, about 402 days. Both are well within a normal ISS mission, which is exactly why NASA built mechanical recycling systems instead of biological ones for low Earth orbit.

That logic breaks down the moment a mission stops being able to resupply. A round-trip crewed Mars mission that doesn't land still needs an estimated 30 tonnes of supplies with zero recycling, plus another quarter-tonne per crew member for every month spent on the surface if it does land — against a payload capacity of about 95 tonnes for the most powerful operational rocket in the world, NASA's SLS Block 1. Mechanical systems can scrub CO2 and split water indefinitely, but none of that chemistry produces a single calorie. Electrolysis and zeolite scrubbing don't grow food. That is the actual reason Bioregenerative Life Support Systems (BLSS) exist: not because photosynthesis is more elegant than a fuel cell, but because it is the only category of technology that can close the food loop at all, while simultaneously making oxygen and processing some of the crew's own waste as a nutrient input.

Every BLSS design, from a 1960s Soviet bunker to next year's ISS hardware, is sized against the same unit: the "equivalent man" (eq-man) — the daily oxygen consumption, CO2 output, and food intake of one average adult. Keep that unit in mind, because it's how every number in this module gets its meaning.

What a closed biological loop actually looks like

The most thoroughly engineered version of this idea is ESA's MELiSSA — Micro-Ecological Life Support System Alternative — initiated in 1988 and still under active development at the MELiSSA Pilot Plant, run by the Universitat Autònoma de Barcelona.

MELiSSA's loop has five interconnected compartments, and the design logic is worth following end to end because it's the clearest illustration of why this is hard. Compartment I uses thermophilic anaerobic bacteria to liquefy crew waste — feces, inedible plant matter — into volatile fatty acids. Compartments II and III run photoheterotrophic and nitrifying bacteria (Nitrosomonas and Nitrobacter) that convert the resulting ammonia into nitrate, which is fertiliser for the next stage. Compartment IVa is the one this module is actually about: a photoautotrophic reactor culturing a cyanobacterium, historically called Spirulina platensis and now reclassified as Limnospira indica strain PCC8005 — sold commercially under the same "spirulina" name you'd find on a supplement label. Compartment IVb runs higher plants for dietary variety. Compartment V is the crew compartment, which closes the loop by breathing the oxygen and eating the biomass.

The number that matters most here is an uncomfortable one. The MELiSSA Consortium's own design target for a full-scale system is to produce one full eq-man of oxygen and twenty percent of one eq-man's daily diet. What has actually been built and run continuously, an 80-litre external-loop gas-lift photobioreactor, supplies between five and ten percent of one person's oxygen requirement. After more than three decades of engineering, the demonstrated hardware covers a tenth of the target it was designed against — which tells you more about the real difficulty of this problem than any marketing slide does.

Microgravity adds a second-order complication that doesn't show up in any ground-based PBR design covered earlier in this curriculum: gas-lift reactors depend on buoyancy to drive bubbles upward through the liquid, and buoyancy doesn't exist without gravity. Early breadboard work got around this with a rotating annular photobioreactor that substituted centrifugal force for gravity; the flight-qualified hardware that eventually reached the ISS instead used a membrane-based design. Every engineering choice that's trivial on Earth — how gas gets in and out of a liquid column — has to be re-solved for space.

The ground truth, before MELiSSA

MELiSSA isn't the first attempt at this, and it isn't the only one running today. Three other programs — one from the 1970s, one currently flying, one a deliberately different design philosophy — show how differently this problem can be solved, and how each solution traded something away.

The Soviet Union's BIOS-3, built at the Institute of Biophysics in Krasnoyarsk and operational from 1972, is still the most complete crewed test of algae-based air revitalisation ever run. Its design rule was that one person needed about 8 square metres of exposed Chlorella vulgaris to balance their oxygen and CO2 output; the facility is generally reported to have reached roughly 95 percent atmospheric closure, with Chlorella supplying the majority of oxygen regeneration and higher plants supplying the rest. Ten crewed closure experiments ran between the 1970s and 1984, the longest lasting 180 days with a three-person crew — still the longest continuously algae-supported human habitation on record, decades before MELiSSA's pilot plant existed.

The closest thing to BIOS-3's successor flying today is PBR@LSR, a joint experiment between the German Aerospace Center (DLR), the Institute of Space Systems at the University of Stuttgart, and Airbus Defence and Space. Initiated in 2014 and launched to the ISS in 2018–19, it cultured Chlorella vulgaris strain SAG 211-12 in a membrane photobioreactor for up to 180 days, coupled to the European Life Support Rack's physicochemical CO2 processing. Its explicit goal was narrower than MELiSSA's: prove that microalgae can survive long-duration, non-axenic (non-sterile) cultivation in microgravity at all, as a hybrid add-on to existing mechanical systems rather than a replacement for them.

China's Lunar Palace 1, or Yuegong-1, at Beihang University takes a different approach entirely, and it's worth understanding why. Designed by Liu Hong, the 160-square-metre facility ran a 105-day crewed mission in 2014 and a 370-day mission (branded Yuegong-365) in 2017–18, the longest continuous human stay in a BLSS on record. But its primary oxygen and food source wasn't algae at all — it was wheat, alongside fifteen vegetable crops and mealworms for protein. Liu Hong reported that roughly 80 percent of the food consumed circulated internally. Lunar Palace proves a BLSS can sustain people for a year; it does so by leaning on crops humans already know how to eat, not on the organism with the better productivity numbers on paper. That gap between theoretical efficiency and practical adoption is something this module returns to in the quiz below.

Six reasons nobody has flown a full bioregenerative loop yet

The India angle, and why it's a research thread, not a revenue line

India's space programme has, in the last two years, started talking about algae in exactly this context — and it's worth knowing the shape of that activity, even though nothing here belongs in a near-term SustaBloom business plan.

In late 2024, ISRO and the Department of Biotechnology signed a Memorandum of Understanding to integrate biotechnology with India's human spaceflight programme, explicitly naming bioregenerative life support among its focus areas and sitting under the broader BioE3 (Biotechnology for Economy, Environment and Employment) policy, which targets a $300 billion Indian bioeconomy by 2030. The first physical expression of that MoU is the BAS-1 module of the planned Bharatiya Antariksh Station — an roughly 10-tonne module carrying an indigenous Environmental Control and Life Support System, with development and launch costs estimated near ₹1,763 crore (about $180 million), targeted for launch by 2028. Separately, Indian astronaut Shubhanshu Shukla's 2025 flight to the ISS aboard the Axiom-4 mission included experiments on cyanobacteria and tardigrade radiation resilience, feeding directly into planning for Gaganyaan's first crewed flight, expected in 2027.

None of this is a market yet. It's a set of government-funded research programmes — ISRO, DBT, and the institutions they fund — exploring whether a biological layer belongs in an Indian-built life support system, on a timeline measured in space-agency years, not startup quarters.