# Phytoplankton as CO\({}_{2}\) Sinks: Redirecting the Carbon Cycle
## 3 Phytoplankton Culture
### Key Parameters for the Growth
#### 3.3.1 Light  
The available light for each cell at a specific moment is the main factor that defines the potential growth rate of a specific system. Regarding utilization, light can be considered from two complementary points of view. On the one hand, regarding the values of power or energy (W/m\({}^{2}\), uE/m\({}^{2}\)-s, Wh/m\({}^{2}\)-day, etc.), and on the other hand, considering the quality (referring to the wavelengths spectrum of the light source). Combining both features defines a specific organism's suitability [43; 44]. It is commonly believed that the light intensity reaching the surface of a PBR is proportional to the expected growth. However, two main factors complicate this relation, as shown in Figure 4.  
Figure 3: Diagram of the main factors that delimit microalgae growth in a photobioreactor.  
(i) The light that impacts the surface of a PBR does not display isotropic transmission across the culture volume. Instead, it exhibits a negative, directional gradient from the surface to the interior of the volume. This decrease follows a logarithmic relationship described by the Beer-Lambert law (I = I\({}_{0}\cdot e^{-([c]\varepsilon\,\varepsilon\,L)}\)), which states that the light intensity at any point within the optical path is determined by the concentration of the culture at the time of measurement, the optical path itself (from the lighted surface to the point of measurement), and the extinction coefficient specific to each microorganism. This feature contrasts with the heterotrophic cultures where the energy source (generally one/several soluble organic molecules) is homogeneously distributed across the entire culture volume and, therefore, available for every cell with the same probability. Thus, light is not available equally to every cell of the culture. However, it depends on the short position within the PBR and the probability of reaching the lit volume in the future. Moreover, the lighted volume of the PBR decreases simultaneously as the concentration of the culture increases. Consequently, the PBR permanently contains cells that are over-illuminated, correctly illuminated, and possibly in complete darkness. The sum of the light regimes within the PBR defines its rapid growth rate.  
(ii) The kinetics of photochemical transformation follows a saturation pattern concerning the light intensity, analogous to that described by [PERSON] and [PERSON] [45] for the enzyme-substrate interaction. This relationship is termed the PI (photosynthesis-irradiance) curve. Therefore, the chloroplasts can linearly increase their oxygen production rate within a limited range of increasing light intensities. However, as the light intensity continues to increase, the photosystem eventually reaches saturation and attains a maximum value of photochemical conversion rate. At this point, the photosystems cannot process all the energy received per unit of time, and they waste it in the form of fluorescence and heat. If the light intensity increases even further, a state of photoinhibition is reached where the O\({}_{2}\) production values drop below the maximum and the photosynthetic apparatus is partially damaged [46]. In light's absence, O\({}_{2}\) production is not zero but has negative values due to phytoplankton consuming this molecule as the final electron acceptor. Additionally, it is worth mentioning that the PI curve is highly adaptable and continuously adjusts through photoacclimation by changes in chloroplast pigment concentrations and ratios. This modulation is triggered by factors such as light intensity, temperature changes, or the nutritional status of the culture, and it is specific to each photosynthetic organism [47].  
Figure 4: Approach summary diagram visualising the relationships between light intensity reaching the photobioreactor, the photosynthesis/irradiance curve, and the system’s key parameters.