The Mutual Affair

Have you ever noticed and wondered why Coral Reefs out skirting the southern islands of Singapore appear to be in various colours?

 This is due to the presence of Mutualistic Relationship between the Reef-building hard corals (Order Scleractinia) and the Zooxanthellae, Symbiodinium spp. These microscopic single cell algae live within the corals’ polyp tissues and are responsible for its colourations (WildSingapore, 2008).


Picture 1: Zooxanthellae living within coral found in Pulau Hantu in 2005

Picture 1: Zooxanthellae living within coral found in Pulau Hantu in 2005


These marine species are metabolically interdependent and both benefits from the symbiosis.

Corals have the inability to generate sufficient foods for survival (Megan, 2009).  By having zooxanthellae living in its cell tissues, these photosynthetic algae help to make foods for the corals polyps. The carbohydrates made are used by the polyp as nutrient for growth (e.g. to build their calcium carbonate skeletons) and the oxygen for respiration. Carbon dioxide in return cycled back to the zooxanthellae. In the process, carbon dioxide are removed. This is important for polyp calcification under optimum conditions, thus accounting for the many coral reefs observed in Singapore.

Also, nitrogen and phosphorus are cycled. Zooxanthellae take in ammonia as nutrient given off as waste by the polyp, and return amino acids back for its growth (Megan,2009). This thus provides a nutrient rich environment for excellent growth for both.


However, this mutualism can be upset by environmental stresses.

Unusually warm or cool water temperatures, a change in salinity or excessive exposure to sunlight or shading or human activities (e.g. sedimentation or land reclamation) can lead to expulsion of the zooxanthellae by the corals (WildSingapore, 2008). This is known as Coral Bleaching.


Coral bleaching found on the Pulau Hantu in July 2007

Picture 2: Coral bleaching found on the Pulau Hantu in July 2007 (Photo reference: 070701hntd2943 )


When this happens, this poses serious threat to Singapore reefs as this may lead to coral death unless another algal mutualism can be re-established.  This thus in turns affect the marine ecosystem as coral reefs support the survival of many marine organisms . One example is the 1998 Coral bleaching event, affecting 50-90% of the reef organisms (Reef Ecology Study Team, NUS, n.d). URL:


It is thus strongly believed that more should be done to protect these Coral Reefs in Singapore, both fiscal and public. Corals have already many a time experience threats, losing their “beautiful” colouration and some even die locally. Are you ready to let this happen again and bear the consequences of the possible complete extinction?




Second source link: Reef Corals: Mutualistic Symbioses Adapted to Nutrient-Poor Environments, by L. Muscatine and James W. Porter © 1977 American Institute of Biological Sciences. URL: (Accessed on 10th April 2010)

“Hard Corals and coral reefs” by Wildfactsheets. WildSingapore, October 2008. URL: (Accessed on 10th April 2010)

“Symbiotic Relationship between Coral and Algae-corals and zooxanthellae need each other to survive” by Megan. J, 20th May 2009. URL: (Accessed on 10th April 2010)

“Coral Reefs of Singapore” by Reef Ecology Study Team, National University of Singapore, n.d. URL: (Accessed on 10th April 2010)

“Pulau Hantu-A celebration of marine life: Silts starving Hantu’s reefs” by Debby. Hantu Blog by Habitatnews, 24 March 2005. URL: (Accessed on 10th April 2010)

“Coral bleaching” by Ria Tan, WildSingapore. Flickr from yahoo channel, 12 July 2009. Photo reference: 070701hntd2943. URL: (Accessed on10th April 2010)


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