Microalgae (phytoplankton), unlike macroalgae (seaweed), are single-cell microscopic organisms that live in freshwater or marine ecosystems. Through their growth, they produce biomass, which is organic material that is used to generate energy. Compared to terrestrial plants and animals, microalgae have the highest net biomass productivity which makes them a rapidly-generating renewable resource.

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Cultivating microalgae is a simple process as the organisms only require sunlight, nutrients, carbon dioxide and water to grow. This process is called algaculture, and the microorganisms grow quickly in favorable conditions.

Related: World’s largest algae growth pond promotes carbon capture

There are over 25,000 species of microalgae and their lack of roots, stems, and leaves allows them to photosynthesize and sequester carbon more efficiently than terrestrial plants. Additionally, phytoplankton are very resilient and do not need soil or clean water for growth. In fact, they can even thrive in saltwater and wastewater.

Because of microalgae’s incredible versatility, they can be used to support other sustainable cultivation systems such as aquaculture, sequester carbon in the atmosphere and as a biofuel.

Fish at a market

Microalgae biomass for aquaculture feed

Aquaculture is the process of cultivating aquatic creatures such as fish, crustaceans and aquatic plants in a controlled environment. The cultivation process is sustainable as it meets global consumption demands without pressuring wild populations.

One of the ways phytoplankton can be used in aquaculture are as fishmeal alternatives. Through photosynthesis, microalgae produce nutrients such as proteins, vitamins, minerals and carbohydrates. These are beneficial to marine and freshwater species because of antioxidants and immune-stimulant properties that boost health. By mixing various species of microalgae, the feed can be higher in nutrients and growth-promoting compounds.

Advantages of phytoplankton feed

Because of its high protein content, algae can be used as a supplement for meat- and soy-based feeds. Clearcutting for soya crop cultivation is becoming more prominent, particularly in South America. Also, transportation of soya contributes greatly to carbon emissions. Furthermore, excess soy in agricultural areas leads to high nitrate concentrations, which can contaminate littoral ecosystems and groundwater. Unlike soy-based feeds, microalgae production requires little space, is low maintenance and is more eco-friendly.

Challenges of microalgae-based fishmeal

However, there are drawbacks to using phytoplankton-based fishmeal. Some types of microalgae have thick cell walls and proteins that are hard for fish to digest. Additionally, phytoplankton are susceptible to the absorption of heavy metals during cultivation. These include metals like arsenic, lead and mercury, among others. If microalgae with high heavy metal concentrations are used for fishmeal, these compounds can travel up the food chain and cause severe health problems. Currently, microalgal pellet production is also a challenging process. The processes used to harvest and dry biomass are time-consuming, costly and require significant amounts of energy.

Though using microalgae for fishmeal currently presents challenges, over time harvesting systems and pellet production are likely to become more refined. Thus, microalgae will be able to serve as a sustainable and healthy means of aquafeed, unlike most currently on the market.

Tower with polluting smoke

Sequestering carbon with microalgae

Increasing carbon emissions from fossil fuel combustion are leading to global warming and climate change. These phenomena are exacerbated by reduced carbon absorption as a result of deforestation and poor soil quality in degraded land. Decreasing dependency on fossil fuels is one of two ways to lower carbon emissions. The second method is carbon sequestration.

Marine phytoplankton sequester approximately 50 gigatons of CO2 each year. In fact, depending on the species, carbon capture efficiency rates range between 40-93%, making them 10-15 times more efficient than terrestrial plants.

The use of microalgae for biofuel

One way to reduce greenhouse gases is by switching to renewable clean energy sources. Alternatively, biomass products can be used. Biomass products can be converted into liquid fuels that are compatible with pre-existing petroleum-based infrastructure.

Interest in microalgae-based fuels is growing because of their numerous advantages. High-energy-density oil can be extracted from microalgae daily. During this process, 30% of the algae and 90% of the water can be reused to continue biofuel production. The remaining 70% of processed microalgae can be used for aquafeed or fertilizer.

Sheep eating at a trough

Lowering microalgae biofuel production costs

Unfortunately, the production of microalgae for biofuel alone can be costly. Since phytoplankton has a plethora of uses, it can be co-cultivated for other products. For example, by combining algaculture for biofuel and aquaculture in the same controlled environment, aquatic health improves and the phytoplankton acts as a bio-pump to provide oxygen and limit CO2 levels. However, there can be drawbacks, such as if the microalgae release toxins during growth that impact fish health.

Because of phytoplankton’s rich nutritional value, it can also be cultivated for simultaneous use in other products. The algae can be used for its nutritional properties in the healthcare industry or for human and animal feed. Nevertheless, the extraction of the algal oil may impact the quality of the algae used in value-added products, but this still requires further investigation.

Microalgae biofuel can also be produced while treating wastewater. This includes cultivating the algae in municipal and industry-produced wastewater. Though the phytoplankton is excellent at absorbing toxins, these compounds can affect algal yield, which consequently impacts how much biofuel is produced.

In conclusion, the potential use of adopting microalgae-based solutions in our industries is too good to pass up. Through further research and refinement of these cultivation and production processes, we can use this abundant resource sustainably and to our advantage.

Via National Center for Biotechnology Information, MDPI, BioScience, Frontiers in Energy Research, The Conversation

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