What is Ocean Acidification?
Ocean acidification refers to the ongoing decrease in the pH of the Earth’s oceans, caused by the uptake of carbon dioxide (CO2) from the atmosphere. CO2 dissolves in seawater and forms carbonic acid, lowering ocean pH. Since the beginning of the Industrial Revolution, ocean pH has dropped by ~0.1 units, representing a 30% increase in acidity. If CO2 emissions continue increasing, predictions estimate ocean pH could fall by another 0.3-0.5 pH units by 2100. This rapid acidification threatens ocean life and ecosystems.
Why is Ocean Acidification Harmful?
Ocean acidification harms organisms that build shells from calcium carbonate like corals, oysters, clams, and plankton. Lower pH levels make it more difficult for these organisms to form shells, as carbonate ions needed for shell-building become less abundant. Acidification also affects the behavior of fish and disrupts marine food webs. Loss of shell-building organisms and shifts in ecosystems can impact food supply and economies that depend on seafood. This makes reversing ocean acidification critical.
How Can Algae Help Reverse Acidification?
Algae and other autotrophs can reverse ocean acidification through photosynthesis. Photosynthesis converts CO2 into organic carbon to build algal biomass. Algal growth therefore acts as a natural carbon sink, removing CO2 from seawater and raising pH levels. Cultivating and growing large amounts of algae, especially fast-growing macroalgae species, could potentially counter acidification on local or global scales.
Mechanism of Algal CO2 Uptake
During photosynthesis, algae take up CO2 and convert it into organic carbon:
CO2 + H2O ➝ (CH2O) + O2
This reaction uses CO2 and releases oxygen (O2), decreasing CO2 availability in surrounding waters. With less CO2 to form carbonic acid, pH rises. Algae are extremely efficient at converting inorganic carbon into biomass, providing an effective method of extracting excess CO2 from seawater.
Algal Growth Rates and CO2 Uptake Capacity
Macroalgae like kelp and sargassum exhibit rapid growth rates, fixing large volumes of CO2. Studies show kelp forests can absorb 20 times more CO2 per acre than land-based forests. Fast-growing macroalgae could therefore counter ocean acidification through sufficient upscaling of cultivation. However, more research is needed to quantify potential CO2 removal capacity on global scales.
Algae Cultivation Strategies
To leverage algae’s CO2 removal capacity, methods and technologies for large-scale algae cultivation must be developed. Some potential strategies include:
Open Ocean Algae Farms
- Farming macroalgae like giant kelp in offshore aquaculture systems. This direct ocean cultivation allows maximum space and proximity for algae to counter acidification.
Onsite Cultivation at CO2 Source
- Growing algae near CO2-emitting industries like power plants. Algae ponds could utilize flue gas emissions as a CO2 source to accelerate growth.
- Using closed photobioreactors to grow microalgae under controlled conditions, optimized for high productivity. This allows year-round algae production unaffected by seasons.
- Combining algae cultivation for CO2 capture, with production of biofuels and bioproducts. Integrating carbon removal with generation of value-added products improves financial viability.
Challenges and Limitations
While algae cultivation has significant potential for carbon removal, there are several challenges:
- Scaling up algae farming to industrial levels is technically and economically challenging.
- It is energy-intensive to circulate and harvest large algae volumes.
- Nutrient supplies could limit algae growth and CO2 uptake capacity.
- Local pH increases may not affect global ocean acidification without massive cultivation scales.
- Removal of algal carbon from oceans could impact marine food webs.
More R&D is needed to address these barriers and quantify realistic carbon removal potential.
Algae are highly efficient CO2 assimilators that could help reverse ocean acidification through large-scale cultivation. By leveraging algae’s rapid growth and carbon fixation rates, ocean pH could potentially be increased on local or global levels. However, massive cultivation facilities would be required. Continued research and technology development are needed to make algal carbon removal methods feasible and scalable. With sufficient investment and innovation, algae farming could become an impactful tool for restoring ocean health in the fight against climate change.