CO2 and pH

Carbon is one of the most essential factors for microalgae growth because it is a key ingredient in photosynthesis. Microalgae take up CO2 from the seawater around them and then use light energy to convert CO2 to organic molecules like carbohydrates. This is why algae must be provided with lots of carbon so that their growth and photosynthesis are not limited.

However, this gets tricky when we consider the second factor: pH.  When CO2 dissolves in water, it dissociates to form HCO3– and H+. The H+ produced makes the water more acidic, lowering its pH.   

This chemistry may be familiar in the context of ocean acidification.  Extra CO2 in the atmosphere from human activities dissolves in the ocean, reducing its pH. For most of the 20th century, seawater pH was around 8.2, but many locations are now regularly seeing levels below 8. 

When microalgae grow, the opposite of acidification occurs: they take up CO2 from the surrounding water, increasing the pH. This happens in both fresh and salt water. The pH of fresh water is more variable than that of seawater, but it is generally more acidic, with a range of 6–8 (Fondriest Environmental 2013). Exceeding the natural pH range can reduce microalgae growth or even cause the culture to crash.

Carbon and pH control are both critical for microalgae culture, so that on the one hand, carbon doesn’t limit photosynthesis, and on the other hand, pH is in an optimal range for growth.  Because they are so interrelated, it makes sense to control them together.  Some options for pH control and carbon addition are:

• Swirling or bubbling with air: this increases gas exchange between the culture and the air, adding more CO2 (because air contains 0.04% CO2, Engineering Toolbox).  However, this works better with smaller, more dilute cultures (e.g., in flasks or carboys).

• CO2 bleed into the airflow: this adds a continuous amount of CO2, which supplements the CO2 in the air.  In combination with pH monitoring, this can work well; however, it is essential to consider changes in CO2 demand when using this approach.

• CO2 additions based on pH feedback: determine an appropriate pH setpoint and add CO2 as required to maintain this setpoint as algae grows.  This is more complicated than the previous two strategies, but it is the best for maintaining sufficient CO2 and a stable pH for algae growth.  An example of what this looks like in the PBR 1250L is below (Fig. 1).

This article is the fifth of our Microalgae Growth Series. Check out our other posts in the series, including:

• Nutrients
• Light Intensity
• Light Spectrum
• Overview of Key Parameters.

For more information on pH and carbonate chemistry in water, check out this article by Fondriest Environmental:

Fondriest Environmental, Inc. “pH of Water.” Fundamentals of Environmental Measurements. 19 Nov. 2013. Website.

Industrial Plankton manufactures and supplies PBRs in 2500L, 1250L, and 100L. Explore our algae production equipment here.