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From Sludge to Soil: Nutrient Acquisition Using Cyanobacteria

Updated: Nov 26, 2020

Recently Galactic Farms won the agricultural award at the Mars City Design competition for our submission inspired by Cyprien Verseux, a PhD student from the University of Rome. Under the guidance of Daniela Billi, Verseux published a review paper in the International Journal of Astrobiology that describes how it may be possible to grow cyanobacteria on Mars, using Martian regolith (i.e. undeveloped soil) as a substrate. For our submission to the competition, we supposed that it may be possible to use the cyanobacteria biomass as a nitrogen source, much like manure is used in compost for generating a soil-like substrate.

Flow chart from our submission to the Mars City Design Competition

Although technically bacteria, cyanobacteria are effectively similar to plants in their ecological role: they are what ecologists call producers. This means that rather than needing to find something living to eat, like all animals do, producers are able to eat non-living minerals and gases and convert them into a biomass that can be eaten by other organisms.

Close up of cyanobacteria chains

They transform non-living matter into complex molecules that are a part of a living organism that exists within a food chain, and that within an ecosystem. Producers use energy obtained from the sun to power their nutrient extraction and bioaccumulation from non-living matter in their environment.

Cultures from our friends at SeaCrest Group

Species of cyanobacteria, like Spirulina, are special because they not only sequester carbon dioxide from the atmosphere through photosynthesis (like all plants do), but they are also able to sequester atmospheric nitrogen as well. When nitrogen is converted from the atmospheric form into the form found in biomass, it is called fixed nitrogen. In nature, there are very few organisms that are able to take nitrogen from the atmosphere and fix it into a living biomass. There are bacteria that live in the little root nodules of legumes (e.g. beans) that are able to fix nitrogen, but they can not be cultured as easily as cyanobacteria.

Fixed nitrogen is very important because this is the form that plants can uptake and convert into protein. The biomass generated by this method could potentially be used as a food source directly, for example the cyanobacteria species Spirulina is used worldwide as a complete protein source in survival situations

But rather than eating the cyanobacteria directly, we hypothesized that the nitrogen rich cyanobacteria biomass could be used instead to develop a soil-like substrate for growing higher plants. By capturing these materials in a cyanobacteria biomass, nutrients in the Martian regolith that were less available to plants could be microbially converted into a form that can be made available to a wide variety of crops.

Using traditional composting practices that require the addition of a carbon source, the cyanobacteria biomass could be digested microbially, releasing minerals into a bioavailable form. These nutrients would be converted into a soil-like substrate can be used to grow higher plants for human consumption.

Alternatively, this soil-like substrate could then be leached of those plant nutrients using pH adjusted water to create a hydroponic nutrient solution that could be used to grow food crops for Martian colonists. At Galactic Farms, we have been able to grow leafy greens and culinary herbs using a method very similar to the one described in our competition submission.

Leafy greens and culinary herbs grown using DIY hydroponic solution made from composted materials

The missing piece of the puzzle, however, is finding a species of cyanobacteria that not only does well under reduced gravity conditions like on Mars, but also accumulates minerals in ratios that are conducive to plant growth. For example, Spirulina may be a good candidate as a nutrient source for humans, but it would make a poor substrate for higher plants. This is because the species Spirulina accumulates the sodium chloride salt at levels that inhibit the growth of higher plants.

How quickly the sodium chloride levels accumulate to inhibitory levels depends on the mineral composition of the regolith and whether astronaut urine is used as a supplemental mineral source. Currently, there are no known methods for removing sodium chloride salt without also removing other important minerals that are necessary for plant growth. To accomplish our goal, we instead must find a species of nitrogen fixing cyanobacteria that does not accumulate sodium chloride at inhibitory levels.

Cyanobacteria thrive under warm conditions, but this species seems fairly well adjusted to the harsh Montana winter!

At Galactic Farms, we have been inspired by research in the field of agricultural engineering for applications in space exploration. For our submission to the Mars City Design competition, we took what the scientific community knows about culturing cyanobacteria on Martian-like substrate and ran with it. We imagined a way to incorporate known methods with a technique we have used to produce leafy greens and culinary herbs indoors using controlled environment agriculture methods. We are currently designing an experiment to test our hypothesis put forth during the Mars City Design Competition. Keep checking in for updates as we explore this topic.

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