Supplementation of microalgae cultures with CO2 – is it worth it?
Our second article focuses on another frequent question: Is the supplementation of marine microalgae cultures with CO2 really useful?
On the one hand, CO2 is free in the atmosphere and unfortunately we see the increase in its concentration every year, something that worries us all. However, the price of CO2 gas for industry is quite high (usually over € 2 / Kg) compared to other consumables, so we must ensure that we meet the necessary cultivation conditions that justify such investment. We have not rarely visited facilities where CO2 injection lines had been installed by time, in continuous flow mixed in the air, by pulses and others, to only waste money and resources.
The microalgae cultures’ supplementation with CO2 is only justified when the remaining cultivation parameters are optimized: systems without light limitation (this would not be the case of large-volume tanks, pools or bags with a diameter> 450 mm); temperature around the ideal and culture media without limitation in nitrate and in trace elements such as the f / 2 medium.
The supplement with CO2 has the double advantage of, on the one hand, “feeding” the microalgae with more carbon source available in the water, and on the other, slightly acidifying the medium, thus controlling the tendency of increase of the pH resulting from the metabolism of microalgae during photosynthesis that generate OH– anions.
Mechanism of action
By injecting CO2 into water, we can observe the following reactions – CO2 (g) + H20 (l) ⇔ H2CO3 (aq) – which, due to its instability, quickly decomposes into HCO3– + H +, which neutralizes OH–, causing H2O .
When to use supplementation
Only when we have all the factors improved at the same time, it is justified to supplement the crops with CO2. If the cultures are limited by light, the metabolic rate of the microalgae is not enough to cause so many OH anions – and if they are limited in nutrients there will also be no substrate for the microalgae to create amino acids from N and P. These limiting factors could slow down the metabolic rate of the microalgae and, therefore, by injecting CO2 without need, we would even be negatively affecting the microalgae since the aqueous medium is acidified to pH <7, moving away from the ideal pH of almost all marine microalgae (between 7.5 and 8.5).
To start CO2 supplementation, crops must meet these conditions:
- There should be no limitation by light – the diameters of bags should be <300 mm, the tanks of greater surface and less depth should be <500 mm; flasks and bottles should be lit by intensities close to 12,000 – 18,000 Lux;
- Culture media should be more complete (nitrate concentration between 1.3 and 2.0 mm, for example: our GoldMedium medium, ALGAL medium);
- A photoperiod of 16H light should be implemented: 8H dark, having CO2 injections only during the “daytime” phase and making sure that CO2 is injected so that the pH of the cultures remains in the range 7.5 and 8.5.
Following this recipe, the crops increase their productivity from 4 to 10 times more than the classic systems, reaching biomass concentrations of up to 1 g / L (ex: equivalent to 30 × 106 cel./ml for Isochrysis).
Following this logic, we can say that now it would compensate to buy CO2 to supplement microalgae crops.
We are faithful to our mission: keep improving and eliminate waste!
Our next article will talk about a very commented topic among our clients dedicated to the research area. Many of them are increasingly concerned with simulating in the laboratory what really happens on an industrial scale. We may have the solution, stay tuned!