# Phytoplankton as CO\({}_{2}\) Sinks: Redirecting the Carbon Cycle
## 4 Mitigating Fossil Energy Dependence by Enhancing Photosynthesis
### A Matter about Figures and Politics  
This study attempts to show a numerical and conceptual evaluation of the feasibility of using biological capture, specifically by cultivating large quantities of phytoplankton, as a solution for excess CO\({}_{2}\). From Figure 1, a net flux of approximately 5.1 Gtc/year is proposed, which is equivalent to 18.75 Gt CO\({}_{2}\). Assuming that 1 g of produced microalgae biomass (55% carbon) can neutralise 2 g of CO\({}_{2}\)[81], it would be necessary to produce 9.25 Gt/year of biomass to offset the excess CO\({}_{2}\). Figure 8 shows the hypothetical evolution of atmospheric CO\({}_{2}\) in a pessimistic scenario where the emission of this gas continues uncontrolled as nowadays, reaching over 700 ppm in a few decades. In comparison, other scenarios are simulated, including the restorative effect of different multiples of the initial phytoplankton production for zero net emissions. Hence, a hypothetical biomass production of approximately 13 Gt/year (7.5 GtC) would be able to block the current rise in atmospheric CO\({}_{2}\) concentration even if any restriction is applied to the current rate of emissions. To approach the impact of this production, and despite the significant risk of bias due to the multifactorial nature of the parameter, it is necessary to establish a biomass production figure per unit area of cultivation. In this sense, we can use the figure of 17 g/m\({}^{2}\)-day proposed in Section 3.5 as the average production value for horizontal outdoor cultures. This is equivalent to saying that each square metre of cultivated surface can capture 34 g of CO\({}_{2}\) per day. From here, it can be estimated that it will take a culture area of approximately 2.1 \(\times\) 10\({}^{6}\) km\({}^{2}\) to reach this target. To put this value in context, this surface is comparable to Greenland (2.16 \(\times\) 10\({}^{6}\) km\({}^{2}\)) and represents 0.41% of the planet's total surface and 1.41% of the land surface. Note that the current land area expended in agriculture is approximately 38% of the planet's emerged land [82], which is 30 times higher than the proposed percentage for phytoplankton cultivation surface. Table 1 shows a hypothetical distribution of the proposed cultivation surface based on the emission rates of the main CO\({}_{2}\)-producing countries. Additionally, the percentage that this cultivation area would represent with respect to the total area of each country is reflected. \begin{table}
\begin{tabular}{l c c c c} \hline \hline \multirow{2}{*}{**Country**} & **Emissions (Mt** & **Total Surface** & **Proportional** & \\  & **CO\({}_{2}\)/Year** & **(\(\times\)10\({}^{6}\) km\({}^{2}\))** & **Culture Surface** & **\% Total Surface** \\  & & & **(\(\times\)10\({}^{3}\) km\({}^{2}\))** & \\ \hline China & 10,065 & 9.60 & 494.9 & 5.2 \\ \hline USA & 5416 & 9.83 & 266.3 & 2.7 \\ \hline India & 2654 & 3.29 & 130.5 & 4.0 \\ \hline Russia & 1711 & 17.10 & 84.1 & 0.5 \\ \hline Japan & 1162 & 0.38 & 57.1 & 15.1 \\ \hline Germany & 759 & 0.36 Although implementing these phytoplankton sinks is justified on their own merit, the obtained biomass is not a waste. Instead, it can be used as feedstock in different sectors (feed, energy, nutraceuticalals, etc.). For example, 13 Gt/year of biomass equals approximately 6.5 \(\times\) 10\({}^{4}\) TWh/year, about 40% of the current energetic global demand. The proper harvesting and chemical transformation (gasification, anaerobic digestion, hydrothermal liquefaction, etc.) of this biomass could become an additional energy source either to defray the energetic demand of the own technology or to supply energy to other services [84]. From these gross numbers, the biological capture of CO2 using phytoplankton sinks is proposed as a clever and potentially feasible strategy to redirect the current and dangerous climatic trend. The available scientific data and the correlation between anthropic activities and natural consequences become more and more evident.