Coral reefs are not only one of the most productive marine ecosystems but also among the most vulnerable.

The global decline of coral reefs, primarily attributed to climate change and human activities, has significantly impacted coral reef calcification and carbon cycling processes.

This has led to a longstanding debate regarding the source-sink nature of coral reef carbon dioxide (CO2). Despite the release of CO2 during coral reef calcification, their function as a carbon sink should not be underestimated.

The intricate biogeochemical processes within coral reef ecosystems, coupled with the unique trophic properties of reef-building corals, contribute to their significance as carbon sinks. Coral reefs stand out as the most biologically diverse marine ecosystems, estimated to fix approximately 900 million tons of carbon annually on a global scale. The primary productivity of coral reefs surpasses that of non-reef systems by a significant margin, ranging from 300 to 5,000 g C-m-2-a-1 compared to 50 to 600 g C-m-2-a-1, respectively.

Despite the recognition of their potential carbon sink function, coral reefs have historically been categorized as carbon sources due to CO2 release during calcification processes. The ongoing controversy surrounding the carbon source/sink attributes of coral reefs has yet to be resolved, and these ecosystems have not been included in the blue carbon balance of coastal zones, which typically focuses on coastal wetland ecosystems like mangrove forests, salt marshes, and seagrass beds.

Consequently, there is an urgent need to elucidate the source-sink mechanism of coral reef ecosystems and explore ecological control methods to transition coral reefs from carbon sources to carbon sinks. This endeavor aligns with the imperative of ecological restoration for coral reefs, underscoring their role in achieving national carbon-neutral goals and fostering green development strategies.

Often referred to as "tropical rainforests in the ocean," coral reefs boast the highest marine ecosystem productivity, playing a pivotal role in the global carbon cycle. Their productivity hinges on symbiotic photosynthetic microorganisms known as Zooxanthellae, primarily belonging to the family Symbiodiniaceae. Zooxanthellae contribute up to 95% of photosynthesis products, such as sugars, amino acids, and oxygen, to coral hosts for growth and calcification. In return, corals provide Zooxanthellae with CO2, nitrogen, phosphorus, and other metabolic waste products as nutrients.

However, coral reefs are exceedingly vulnerable to environmental changes. Since the Industrial Revolution, the excessive emission of greenhouse gases, rapid coastal economic development, and human encroachment have precipitated ecological challenges like climate warming, ocean acidification, and rising sea levels. These factors have pushed nearly one-third of reef-building corals to the brink of extinction, resulting in the ongoing degradation of coral reef ecosystems and an alarming increase in coral bleaching events.

Coral bleaching, a stress response triggered by external environmental factors, leads to the expulsion of symbiotic zooxanthellae from coral polyps, causing them to lose color and become pale or transparent. If left unchecked, this phenomenon can culminate in widespread coral mortality and even extinction. Coral reefs, while critical for carbon sequestration and supporting marine biodiversity, face unprecedented threats from climate change and human activities. Addressing these challenges is imperative to safeguarding these invaluable ecosystems and their crucial role in the global carbon cycle.