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Pioreactor dev blog #10 - happy little algae

Pioreactor dev blog #10 - happy little algae

Nov 04, 2021 Posted by: Cameron Davidson-Pilon
🦠 The past few weeks, we've been testing out the Pioreactor's ability to grow micro-aglae. Specifically, this involves using the Pioreactor as a photo-bioreactor. Outcome: success!

Algae have a very different set of living conditions compared to other organisms that we have tested in the Pioreactor. Algae require both light and CO₂ to grow. We had the foresight to think of algae's unique growing requirements during development, hence the Pioreactor has additional pockets for LEDs in the main body. When these pockets are occupied by white-light LEDs, the Pioreactor turns from a bioreactor into a photo-bioreactor. 

We are lucky to be working nearby an algae research centre: the Canadian Phycological Culture Centre (CPCC). They supplied us with a high quality culture, Chlorella Vulgaris, and BBM media for our experiments.

In phototrophic cultures (cultures that use light for energy), it's common to use a cyclic light source: often 16h with light, followed by 8h in dark. To implement this in the Pioreactor, we created a new LED automation. Just like how a user can write Python code to automate when and how dosing occurs, users can write Python code to automate how the LEDs should behave during an experiment. In our case, the light cycle was very simple: on for 16h, and then off for 8h. However, there are a few ways to make this automation smarter, each of these is possible with the Python API: 

  • gradual dimming and brightening of LEDs (instead of abrupt on/off)
  • as the cell density increase, increasing the luminosity of the LEDs to try to keep light-per-cell approximately constant. 
  • changing the light cycle's period as the culture acclimatizes. Some cultures can be exposed to light constantly, but they need to be gradually introduced to these conditions.  

Anyways, our experiment was going to use the simple 16h on / 8h off cycle only, with constant luminosity. The Python code we used to script the LED automation is below:

The other requirement for algae is CO₂. For our experiment, we decided to skip have a bubbler and relied just on the continuous stirring to incorporate CO₂. Another way to increase the amount of CO₂ available is to leave more headroom in the vial - so our experiments only used about 10ml of media (instead of the usual 15ml).

Results

Okay, so how did things go? Great! Here's a shot of the normalized optical density: 

Figure 1. The culture ~8x in a week. You can clearly see the 16h cycles of growth and stagnating.

 

We replicated the experiment in another Pioreactor, this time with 20h light / 4h dark cycles.

Figure 2. Another culture with 20h light / 4h dark grew ~2x in a day. This culture is called "Heidi" - as that's my dogs name and I was amused by giving it that name.
 
Figure 3. Hourly growth rate of same culture as above.

 

So the Pioreactor can grow algae! However, there are some caveats:

  1. We observed that the LEDs we were using did emit some IR light that was being picked up by our optical density photodiodes. This meant that there was a "background" signal in the OD that was an artifact. A way to eliminate this is to add optical low-pass filters in front of the LEDs. However, this is costly, so we chose the simpler solution: turn off all LEDs while the OD signal is being read. The OD signal takes less than a second to read, and the frequency of OD reading can be reduced to make the effect less noticeable. 
  2. Algae wants to be stirred at a fairly low RPM - between 60 and 100 is sufficient. Our current stirring can only go as low as ~200 RPM (we hope we can improve the range of our stirring when we revisit stirring during upcoming redesign). I don't know what the effect of higher stirring is on algae growth. 

In conclusion, we are pretty happy with how our cultures have grown in the Pioreactor. Stay tuned for more!