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Algae are aquatic, plant-like, organisms that are an essential part of our climate system. Because algae are fast-growing and cover more surface area, they are much more efficient than trees at removing CO2 from our atmosphere. Algae can be farmed for commercial uses or "sunk" into the deep ocean, trapping the carbon for thousands of years.
<aside> 💡 Research is still needed around the environmental impacts of enhancing the natural CO2 drawdown of algae, and regulation will need to catch up with the growth of the industry.
Algae are aquatic organisms from two groups: macroalgae (aka seaweed) and microalgae (aka phytoplankton). Scientists estimate there might be more than 1 million different species of algae out there and each one possesses different, often valuable, qualities. Algae has also been called a "secret weapon" against climate change.
Source: What are Phytoplankton?
80% of all algae are microalgae: single-celled organisms found in the upper layers of fresh and saltwater environments because, like plants, they need sunlight, nutrients, and CO2 to live and grow. During the day, microalgae draw down CO2 and give off oxygen (actually 50 to 80% of our oxygen comes from algae).
Nutrient pollution, from things like agricultural runoff, can speed up microalgae growth and cause harmful algal blooms. As the algae die, oxygen is removed from the water, creating a "dead zone" that wipes out all plant and animal life in the area. Certain species will even give off toxins that make the water unsafe, possibly deadly, for people and land animals.
Microalgae can be grown in areas where agriculture isn't possible and without using fresh water but requires significant amounts of fertilizer. There are two types of microalgae cultivation: open and closed.
Source: Cinar et al., 2020
Microalgae drive the "biological carbon pump" that is responsible for sequestering about 10 gigatons of carbon in the deep ocean every year. It has been proposed that adding nutrients (like iron and rock dust) to the ocean is an effective way to increase microalgae production and draw down even more CO2.
"If you give me half a tanker of iron, I can return you an ice age."
There are some questions about how useful macroalgae (seaweed) is at capturing carbon because many studies have not looked at how much CO2 is released by the organisms supported by seaweed. This “net ecosystem production” might mean that seaweed ecosystems are a source, rather than a sink for CO2, essentially because the increase in seaweed enhances the food web—bringing in more fish that breath out CO2. This does not mean we give up on seaweed, because it is likely still better than the alternatives, but this is something we should keep in mind when planning aquaculture projects.