# Phytoplankton as CO\({}_{2}\) Sinks: Redirecting the Carbon Cycle
## 4 Mitigating Fossil Energy Dependence by Enhancing Photosynthesis
### Technical Challenges
#### 4.4.7 Biomass Management  
Many applications have been proposed for different phytoplankton biomasses to develop their potential markets [99; 100]. It should be noted that without stabilization methodologies (drying, hydrolysis, etc.), biomass in paste form is rapidly degraded; hence, fresh biomass should be promptly processed. The following are the main uses and associated technologies for this biomass:  
1. Carbon lock: This option involves closing the biomass by burying it or sealing it in airtight bunkers to prevent its spontaneous decomposition from releasing CO\({}_{2}\) and CH\({}_{4}\) into the atmosphere [101]. This way, the captured carbon is permanently removed from the carbon cycle and detoxifies the atmosphere. However, the economic viability of this option is limited as it does not generate added value to the final product of the technology and becomes waste instead. 2. Energy source: As discussed, 13 Gt of biomass per year can store approximately 6.2 \(\times\) 10\({}^{4}\) TWh, which can be economically reused through processes such as hydrothermal liquefaction [102; 103] or anaerobic digestion [104] to obtain high-density energy carbon (hydrocarbons and methane, respectively) and a liquor or effluent rich in macronutrients. This effluent can be incorporated into PBRs [95; 105], allowing for a closed cycle to reuse these elements in growth. It is necessary to highlight that this use would return part of the fixed CO\({}_{2}\) to the atmosphere. 3. Animal Feed: Over 6 Gt/year of forage and grain are consumed to sustain livestock worldwide, producing 340 Mt of meat and 800 Mt of milk, among other products [106]. To achieve these numbers, about 40% of the world's cultivated land is used [107] and is not available for cultivating human food. Phytoplankton is a suitable food source for complementing and sometimes substituting livestock feed for ruminants and monogastric [108]. Microalgae and cyanobacteria are rich in vegetable proteins and high in omega-3 fatty acids, antioxidants, and other compounds, making them not only a source of macronutrients but also a source of compounds that improve the health and vitality of farmed animals [109; 110; 111]. 4. Human Feed: Similarly, the obtained biomass could be an alternative source of nutrients and healthy compounds for human consumption. Phytoplankton is rich in essential amino acids and has a high content of polyunsaturated fatty acids and other beneficial compounds for human health [112]. Currently, 230 Mt of dry protein is consumed annually in our society [113]. To reach this amount, heavy dependence on animal protein is necessary, with environmental and animal welfare consequences. Using phytoplankton as a source of protein could reduce the dependence on animal protein and contribute to a more sustainable and healthy food system.  
In addition to the proposed options, there are combined strategies [114; 115] where animal feeding, agriculture, and energy production are intercalated, enhancing the product value and increasing the system's economic viability.