# Phytoplankton as CO\({}_{2}\) Sinks: Redirecting the Carbon Cycle
## 4 Mitigating Fossil Energy Dependence by Enhancing Photosynthesis
### Technical Challenges
#### 4.4.6 Sources of Nutrients  
The adequate supply of nutrients is another key factor distinguishing phytoplankton growth in the natural environment from that in PBRs. Thus, intentional and controlled eutrophication of these artificial waters is necessary. Though variable, the average nitrogen content for the dry biomass of phytoplankton is estimated to be approximately 6% when there are no limitations on the nutrient supply. The production of 13 Gt of biomass annually would therefore require the addition of 780 Mt of nitrogen per year. However, the annual consumption of nitrogen as a fertiliser is approximately 200 Mt annually [94]. Using this type of fertilization for the ponds would cause a market collapse and a severe conflict with agriculture. The same situation occurs with phosphorus, with an estimated 80 Mt/year required for phytoplankton synthesis, while the current phosphorus consumption in agriculture is 30 Mt/year. As a result, it appears unfeasible to constantly supply new nutrients, presenting another potential obstacle to the successful implementation of this technology. To address this technological challenge, three lines of progress could be pursued. The first approach involves recycling the nutrient content in the synthesised biomass by regularly processing it. This means that the produced biomass should be continuously harvested at a rate of 17 g/m\({}^{2}\)-day and subjected to a continuous degradation process such as composting, hydrothermal liquefaction, or anaerobic digestion [95]. This would generate nutrient-rich leachates that can be returned to the culture ponds. Moreover, this process would also increase the carbon content of the biomass, making it a more attractive energy source. If a steady concentration of 300 g/m\({}^{2}\) is assumed for the whole proposed surface, the locked amount of biomass at any given time would be 630 Mt, equivalent to 38 Mt of nitrogen and 3.8 Mt of phosphorus. Consequently, only an initial and gradual dosage of nutrients is necessary, which should be maintained and corrected. The second approach uses nitrogen-fixing species that thrive in marine waters, specifically cyanobacteria from the Nostocales order. These organisms have specialised cells called heterocysts that can reduce atmospheric nitrogen to ammonia, providing a constant source of nitrogen enrichment in the ponds [96]. Finally, a third strategy involves developing a robust infrastructure for recovering nitrogen and phosphorus from urban, agricultural, and livestock waste. Approximately 380 km\({}^{3}\) of wastewater is produced each year globally, containing approximately 16.6 Mt of nitrogen and 3 Mt of phosphorus [97]. Additionally, 2.2 Gt of solid waste is produced annually worldwide, equivalent to 22 Mt of nitrogen and 11 Mt of phosphorus [98]. From the potential development of these three strategies, it appears more optimistic that, after an initial addition, the nutrient concentration in the ponds can be maintained to avoid limiting the growth rate at any time. Alternatively, managing other micronutrients, such as iron and other trace metals, should not pose a high technological challenge as they are needed in lower concentrations. Even atmospheric exposure (wind, dust, rain, etc.) may replenish some of them.