# Phytoplankton as CO\({}_{2}\) Sinks: Redirecting the Carbon Cycle
## 3 Phytoplankton Culture
### Key Parameters for the Growth
#### 3.3.2 Other Abiotic Parameters  
Different phytoplankton species have specific ranges for various abiotic factors, such as salinity, pH, temperature, etc. Implementing the proper tools to monitor and maintain these values within the PBR [48]. However, the control systems mechanisms are usually expensive devices with high energy consumption. Finding this equilibrium between control and expenditure is critical for the project's viability. Temperature  
PBRs are designed to maximise the captured light; however, light and heat are two sides of the same coin. Approximately 5% and 45% of solar radiation comprise UV light (250-400 nm) and infrared radiation (700-2500 nm). As a result, outdoor closed PBRs with a high biomass concentration and surface-to-volume ratio become very efficient heat collectors whose temperature control is economically infeasible [49]. For photosynthetic microorganisms, the standard temperature range is established between 5 \({}^{\circ}\)C and 35 \({}^{\circ}\)C. Low temperatures (until 2-3 \({}^{\circ}\)C) have a lesser impact, causing only a decrease in cellular metabolism and a slowdown in growth. However, a slight increase in temperature above the optimal range, if sustained for a prolonged period, can dramatically reduce cell viability. Large natural bodies of water typically have a low surface-to-volume ratio, facilitating efficient heat dilution and dissipation through evaporation. This leads to high thermal inertia, resulting in less drastic temperature fluctuations and providing aquatic organisms more time to adjust their metabolism. From here, two different strategies are proposed to manage the temperature of a culture system (Figure 5). The first preventive strategy involves adjusting the type and geometry of the PBR and the surrounding infrastructure (orientation, spacing, shading, etc.) to a specific latitude with a well-known climate. The second strategy involves corrective measures such as using cooling/heat exchangers, water sprays to counteract and mitigate harmful thermal fluctuations, and manual or automatic shading to avoid long exposures to direct radiation [50]. In any case, these corrective methods often involve significant operating costs that significantly decrease the economic viability of the culture system, especially if insufficient attention is paid to the preventative aspect. Medium pH  
The chemical composition of marine and freshwater bodies of water usually includes varying concentrations of dissolved CO2, bicarbonate, and carbonate. Carbonate strongly tends to precipitate due to its reaction with dissolved cations, particularly divalent calcium, so it is not typically present in high concentrations. For this reason, the pH value in aquatic environments is primarily regulated by the relative concentration between CO2, carbonic acid (H2 CO3) and bicarbonate ion (HCO3-), providing a broad pH range between  
Figure 5: Comparative scheme about the different strategies proposed for the control of temperature in photobioreactors. 6.5 and 8.5. During photosynthesis, the carbonic anhydases (CAH) of photosynthetic organisms use HCO\({}_{3}\)\({}^{-}\) ions and H\({}^{+}\) to provide an adequate concentration of CO\({}_{2}\) to RuBisCO, thus sustaining the Calvin-Benson cycle (Figure 6). This proton depletion increases the pH during the light phase of the photoperiod [51]. The magnitude of this increase is directly related to the amount of incident light and the cellular concentration of the PBR. This chemical system can result in three different scenarios when applied to microalgae culture systems. * In the absence of any gas exchange, protons are gradually consumed as they are incorporated into new organic compounds through the Calvin cycle, increasing pH. This increase can also cause the precipitation of carbonates, negatively impacting photosynthesis since e pH values are outside the optimal range and a lack of precursors accumulates.