# Phytoplankton as CO\({}_{2}\) Sinks: Redirecting the Carbon Cycle
## 3 Phytoplankton Culture
### Key Parameters for the Growth
#### 3.3.2 Other Abiotic Parameters
* When the culture medium is aerated with air (0.04% CO\({}_{2}\)), the pH value will reach an equilibrium that mainly depends on the water alkalinity. The maximum pH value under these conditions is usually approximately 8.3, beyond which carbonate starts to precipitate. When the photosynthetic demand for CO\({}_{2}\) is less than the atmospheric CO\({}_{2}\) solubilisation rate, the pH stays stable and functions as a buffer system. However, if the CO\({}_{2}\) demand exceeds the atmospheric delivery rate due to the increment in cellular concentration and/or light intensity, the initial scenario of proton depletion will be gradually reached. * In some culture systems, the medium is aerated with CO\({}_{2}\)-enriched air. The final pH value, after reaching equilibrium, decreases linearly concerning the concentration of this gas in the air. At a CO\({}_{2}\) concentration between 0.5% and 2.0%, the pH remains strongly buffered between 7.0 and 8.5 (depending on the alkalinity of the medium), which is suitable for the growth of most species. At the photosynthetic level, this strategy ensures an ample supply of CO\({}_{2}\), even in high cell concentrations or light intensities. Hence, CO\({}_{2}\) is never depleted, so the pH remains constantly buffered, and the photosynthesis rate is never limited. In conclusion, the pH of photosynthetic cultures can be buffered by simply bubbling with air. In this condition, the pH value read at the equilibrium finally depends on the medium alkalinity and is usually suitable for most photosynthetic species. However, when PBR develops high photosynthetic rates, the CO\({}_{2}\) partial pressure in the air may not be sufficient to buffer the system and sustain the Calvin cycle. The artificial rise of CO\({}_{2}\) concentration in the air amends this depletion, protecting the buffering system and avoiding the depletion of precursors for the carboxylation reactions. * Major ions and trace metals:  
The mineral composition of the medium plays a critical role in the proper development of microalgae metabolism, being a determinant in structural, physiological (osmoregulation), catalytic, and regulatory functions [52]. From this point, many suitable growth media have been developed trying to replicate and optimise the chemical features of natural environments [53]. On the one hand, freshwater media shows a low ionic strength, only with the necessary ions to maintain the osmotic equilibrium and to provide the essential cofactors and elements for metabolism (Mg\({}^{2+}\), Ca\({}^{2+}\), HCO\({}_{3}\)\({}^{-}\), etc.). Alternatively, media based on seawater (natural or artificial) are suitable for most marine microorganisms. Each recipe's metals and vitamins essentially contain the same significant elements, but their concentrations are adapted to a particular group of microorganisms. In addition to these two main clusters, specialised media are available for species with unique environmental origins (halophiles, alkaliphiles, etc.) or specific nutritional requirements (cyanobacteria, diatoms, etc. ).