• Jennifer Huang

Corals and Seagrass: The Two Besties of the Sea

Updated: Oct 22

Corals as marine nurturing habitats

Have you ever seen the beautiful reef full of color on the ocean’s floor? You may wonder that it may be some rocks or any abiotic components of the ecosystem. However, it is actually the coral reef! Again, you may think that corals are kinds of plants, dancing and swinging with the rhymes of sea waves. Uniquely, the coral are actually non-motile animals whose classification resides between the jellyfish and the sea anemones. Albeit coral reefs cover less than one percent of the whole sea floor, it is a home for one fourth population of the marine organisms. Coral reefs provide protection especially for the small fishes as well as acting as the ocean buffer, minimizing the harsh energy of the sea waves by 95%, which, otherwise, can cause tsunami, erosion, and flood, leading to the loss of property (or even life). Many people living in the equator are also dependent on coral reefs' sea ecosystem, especially for fishing and tourism.

Corals have existed million years away before the dinosaur, colonizing the sea floor by forming the reef by secreting the calcium carbonate and forming the hard skeleton. The skeletons are produced on top of the old dead corals, thus, making it into multiple layers. These built up skeletons can range up to thousands of kilometers in length; making the Great Barrier Reef in Australia to be the largest coral reef (2,300 km).



Image source: https://www.britannica.com/place/Great-Barrier-Reef


Furthermore, a single coral consists of multiple polyp that can live by forming colonies or solitary. Although each polyp’s body is white translucent, the phenotype colors that we see when diving are produced due to the natural symbiosis between the polyp and zooxanthellae microalgae. These microalgae are growing inside the polyp’s tissue without harming the host. In fact, more than 90% of the nutrients needed for the polyp to grow come from the photosynthesis activity by the microalgae, which also emits various colors because of the pigments present within its singular cell body.



Image source: https://scubadiverlife.com/coral-biology-part/


The dying tropical rainforest of the sea

Under severe stress conditions mainly due to water pollutants and ocean acidification, oxidative stress inside the microalgae cells induces the formation of multiple free radicals, leading to the energy imbalance and production of toxic substances. As the defense mechanism, the coral polyp will expel the microalgae colony out of its tissue, turning it into white colors; this condition is called coral bleaching. During this stage, the corals have not died, however, it is barely alive! Although the microalgae can re-colonize the polyp, however, the continuous production of greenhouse gases in the atmosphere increases the temperature of the sea water, making it unfavorable for the microalgae to live. Prolonged coral bleaching can cause the loss of nutrients, which, eventually, leads to the dead corals.


Ocean acidification in a nutshell

Oceans absorb a large amount of carbon dioxide (CO2) from the atmosphere, thus making it to have a pivotal role in mitigating climate change as a whole. To date, 24 million tons of carbon dioxide has been absorbed by the ocean and its accumulation rate is 100 times faster than the preindustrial era. In total, atmospheric carbon dioxide has increased by 40% since the beginning of the industrial revolution. Increased CO2 concentrations will lower the pH level of the water, turning it into more acidic. In fact, our ocean is already 34% acidic and its acidification rate is 10 times faster than any time in the last 55 million years. The acidification will cause multiple threats to the marine environment, particularly impacts the marine organisms which form calcium carbonate skeletons and shells, including the coral reefs.

To develop the solution towards this condition, we need to know how it happens at the beginning: when carbon dioxide compounds are absorbed by the ocean, it will have a chemical reaction with water, forming the carbonic acid. The byproduct of this reaction is the hydrogen ions that are then released into the surrounding water, causing the accumulation of positive ions and lowering the pH level of the water, thus, our ocean will become acidic. In principle, this chemical reaction used to be in the equilibrium state, meaning that the addition and removal of carbon dioxide are in the equal amounts. However, due to massive emission of carbon dioxide in the atmosphere nowadays, this equilibrium has been altered, leaving the acidic environment of the ocean.


Grooving together with seagrass

To date, we have lost at least a half of the coral ecosystem in the Australia Great Barrier Reef alone due to global acidification events. Besides, the 29 World Heritages Reef System sites given by the United Nations are predicted to be lost in 2100 if the carbon and greenhouse gases emission cannot be reduced. So, what can we do to prevent coral bleaching? Worry not, the hope is still there! We can support coral reef conservation organizations as well as by trying to reduce the carbon footprint emission. Sound as pretty as the relationship between the polyp and microalgae, other marine plants as well as the mangrove tree can be super beneficial to capture the carbon dioxide in the atmosphere, thus, preventing the harsh effects of ocean acidification.


Moreover, earlier in June, scientists discovered a promising solution derived from seagrass towards ocean acidification. The waving underwater seagrass beds in Susquehanna Flats displays an unpredicted photosynthesis activity, the so-called magnificent chemical trick: it seems that they produce the carbon-based material acting similarly as the “natural antacid” tablet. The seagrass and other vegetations pull the particular carbon molecules out of the ocean water, elevating the surrounding pH level. The “engulfed” carbons are used to form tiny crystals of calcium carbonate for its tissue. The crystals are big enough to feel, approximately sized as a fine grit. These crystals can alter the pH level of the bay by increasing the pH level by 0.6, longed more than 90 kilometers away. To note that the pH value is logarithmic, thus, small changes in pH value equals a significant change in its acidity.


Image source: Vox - https://www.youtube.com/watch?v=BO44JlAElXM

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