Building an algae photobioreactor

Certain species of algae have such a high lipid content that they can be efficiently processed into biofuels as an alternative to liquid fossil fuels. But while this alone sounds promising, algae is not limited to being a fuel source. Another “green” application is to reduce carbon emissions by pumping exhaust, such as from a power plant, through an algae photobioreactor so that the CO2 is captured via photosynthesis. Some species have found use as fertilizers or even dietary supplements.

Given the opportunity to have a project fully funded by MIT projX, I decided to fulfill my long-time dream of building a photobioreactor. What’s a photobioreactor, you may ask? It’s much simpler than it sounds: a system supporting the growth of some photosynthetic organism, the algae. In commercial algae farming enterprises, this could be an open system (a pond) or a closed system (where the culture is contained in clear tubes). I went with the latter option. Forgive me for not wanting an algae pond in my dorm room.

Materials

  • (6) cylindrical glass vases (10” height and 3.5” diameter)
  • (6) rubber stoppers with holes
  • (1) Wine bottle rack from Amazon (holds 6 bottles)
  • (1) Oak plank from my parents’ basement
  • (2) handles for easy lifting

Air system

  • (1) air pump, which conveniently came with
  • (1) six-way adjustable outlet
  • (6) air stones
  • Some generic aquarium tubing and suction cups to hold the tubing in place

My design is essentially a giant test tube rack, complete with rubber stoppers and all. Many similar designs use cast acrylic tubing to contain the algae. I considered this before realizing that not only would glass vases look beautiful and be less likely to leak, but also they would not eat up my entire budget. Win-win. It’s easy to find different cylinder dimensions if you’re looking to build a small system, so unless you have access to a cheap supply of clear acrylic tubing, this is an option I’d recommend. For those on a tight budget, you could also just get some 2 liter soda bottles from the supermarket and use those.

The assembly was pretty straightforward. Here’s the finished “test tube rack,” which I tested with water to make sure I wouldn’t somehow end up with algae all over my room.

As it turns out, the heavy-duty air pump I bought was a bit overkill for my purposes.

Next was the question of which species of algae to grow in this photobioreactor. I originally tried to culture six different species, one in each tube. This proved to be pretty difficult since they require different nutrients, temperatures, and lighting conditions. Those cultures died.

For my second attempt, I got a spirulina starter culture on eBay. Arthrospira plantesis, commonly called spirulina or blue-green algae, is a helical cyanobacteria that grows quickly and is easy to culture, because its highly alkaline environment prevents contamination by other organisms. Spirulina is an excellent dietary supplement, popular among health fanatics for its protein and vitamin content. Some organisations are exploring the possibility of using spirulina to combat malnutrition in places where conventional farming is insufficient. (As tempting as it was, I did not try eating my home-grown spirulina.)

There are a lot of different recipes for spirulina nutrient media on the internet, but I used this relatively simple one:

Spirulina culture medium

Per 1 liter of dechlorinated water, add

  • 16g sodium bicarbonate (NaHCO3 i.e. baking soda)
  • 2g potassium nitrate (KNO3)
  • 1g sodium chloride (NaCl i.e. table salt)
  • 0.1g potassium dihydrogenate phosphate (KH2PO4)
  • 0.1g iron sulfate (FeSO4)
  • 0.1g magnesium sulfate (MgSO4 i.e. Epsom salt)

Right after mixing all this together, the pH was around 8, slowly increasing to spirulina’s preferred range of 10-11 over the course of a few days after adding the culture. After a few weeks by a mostly-sunny window with the air pump running constantly, I ended up with a beautiful, dense culture in the characteristic spirulina blue-green.

The original eBay spirulina culture, diluted across the six containers.

Flourishing two weeks later!

I also took some pictures with my microscope showcasing spirulina’s unique helical shape.

A bit like pasta.

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