(First of Two Parts)
In 2018, the International Year of the Reef, a Shizuoka University research team has become the first in the world to identify important mechanisms involved in coral bleaching, a deadly “disease” threatening coral reefs.
The team now intends to launch a full-fledged project to treat stressed corals and help threatened reefs to recover their pristine characteristics.
Yoshimi Suzuki, Professor Emeritus of Shizuoka University and the research team leader, explained:
How do coral live? How do they react to high water temperatures and ultraviolet rays? No one knew the answers to these very basic questions.
To date, most conclusions made about coral bleaching have been based on field observations of the phenomenon and ecological approaches and not on choral physiological studies. Until now, it was thought that a type of phytoplankton called zooxanthellae, which lives in an endosymbiotic relationship within the coral, was fleeing to the surrounding water when water temperatures rose over a certain threshold (30 degrees Celsius) resulting in bleaching, since the colors of corals are mainly due to the brownish-green color of their zooxanthellae.
In fact, this point of view is still mainstream. The results of our research using the branching stony coral (Montipora digitata) showed that actually, when the coral is under thermal stress, very few zooxanthellae are being released from the coral. In spite of that, the zooxanthellae abundance (or concentration) remarkably decrease within the coral tissues.
A research team that includes Professor Suzuki and Professor Beatriz Estela Casareto of Shizuoka University has worked to elucidate the mechanisms of coral bleaching in their ongoing research. In doing so, the team challenged the views of experts.
With financial support and employee volunteers from the Mitsubishi Corporation, Professor Suzuki’s team has engaged in scientific experiments since 2005 to identify the mechanisms behind coral bleaching. The results have turned up a series of new facts, upending conventionally-accepted views.
Why Do Corals Die?
The coral bleaching phenomenon, up to now, has been seen to be the result of zooxanthellae leaving from the coral due to high sea water temperature in combination with high solar irradiance, exposing the white coral's skeleton throughout the transparent coral soft tissues. While corals do have some color, most of their beautiful colors arise from reflection of the pigment of the zooxanthellae.
Massive numbers of zooxanthellae — between 1.0 million and 1.5 million — live on each square centimeter of the coral’s surface.
Professor Suzuki’s team studied the pigment of the zooxanthellae (chlorophyll and peridinin) and the zooxanthellae cell population, both within the body of the coral and in surrounding waters, in a range of sea water temperature conditions varying from 28 degrees to 32 degrees Celsius. In both conditions, only small portions of zooxanthellae, between 0.05 to 0.1%, left the coral.
Professor Casareto explained: “Under normal conditions, zooxanthellae divide at least once a day inside the coral. We also learned that coral have the ability to expel zooxanthellae from their bodies when they become full in order to maintain balance, and that they attempt to select damaged ones to expel.”
When high sea water temperatures persist, the situation changes. Both the coral and the coexisting zooxanthellae undergo a great deal of stress.
Professor Casareto explained:
Under strong illumination and stagnant seawater conditions, sea water temperatures also rise. This isn’t a problem for a short while, for example, around six hours during low tide conditions, but when 30-32 degrees temperature persist for two to three weeks or a month, the photosynthetic capacity of the zooxanthellae declines. Furthermore, numbers of damaged abnormally-shaped zooxanthellae rise and those of normal zooxanthellae fall, resulting in bleaching. The issue is the intensity of the stress and the duration of high temperatures.
In other words, the coral obtains nutrition (organic matter as food) from the zooxanthellae. When zooxanthellae are in decline and bleaching continues for more than one month, the coral dies due to nutrition deficiency.
Coral Bleaching Is Not Always Deadly
However, bleached coral has not necessarily died. While bleaching has been largely considered a “deadly disease” from which coral cannot recover, research all over the world is starting to show that coral can indeed recover.
Coral bleaching has not been extensively observed this year in Okinawan waters, and some of the bleached corals in Okinawa had partially or totally recovered.
Why? The giant typhoons that caused so much damage, mainly to western Japan, also stirred up sea water. As a result, water temperatures have not exceeded 30 degrees and have remained suitable for the coral. The normal zooxanthellae remaining in those partially bleached corals were able to increase through cell division and occupy coral tissues after water temperatures returned to normal.
According to Professor Casareto:
Our research showed that when a 32-degree sea water temperature persists for some time, 70-75% of the zooxanthellae disappear from the coral, making them look bleached. Of the two million zooxanthellae per square centimeter of surface area on coral, 1.5 million of them vanish within the body of the coral. Upon examining these bleached coral, we found that about 500,000 to 600,000 normal zooxanthellae per square centimeter remain.
She expressed hope about the fate of the corals: “If temperatures return to normal and zooxanthellae engage in cell division, they can double to 1.2 million. They can then provide nutrition to the coral therefore the coral may overcome bleaching.”
(To be continued)
Author: JAPAN Forward
(Click here to read the article in Japanese.)