Article: Brown, B. E., et al. (2015). “Decadal environmental ‘memory’ in a reef coral?” Marine Biology 162(2): 479-483.
The importance of coral reefs has been well documented. These colony-forming invertebrates create some of the most complex and diverse ecosystems in the world, supporting large numbers of marine species. Not only are corals important habitat providers to marine life , but they are also economically important, supporting fisheries and ecotourism. But corals are also sensitive to environmental changes. Ocean warming, ocean acidification, and UV radiation could all wreak havoc on coral reefs and the communities they support.
Research on corals typically focuses on their sensitivity to the environment, with many studies investigating the phenomena of coral bleaching (Fig 1). Coral bleaching is an event in which corals lose an important symbiont (an animal that lives with the coral in equal partnership). Corals have a symbiotic relationship with tiny, microscopic algae called zooxanthellae that live within their polyp tissue. Zooxanthellae are photosynthetic organisms that assist the coral host in nutrient production and carbon acquisition used for growth and energy. It’s the zooxanthellae that give the corals their famously bright colors. Corals, in return, provide a protected environment and a supply of carbon dioxide. This relationship benefits both parties, well, at least until environmental stress kicks in. During a bleaching event, corals will typically lose 60-90% of their zooxanthellae (Buccheim 2013) and thus appear bleached. But if the bleaching event is short enough in duration, corals can rebound and regain their symbionts.
Coral bleaching is typically tied to unusually warm waters and increased solar irradiance, meaning increases in temperature and UV radiation. In the case of solar irradiance, a bleaching event can be very spatially specific. For example, if you have two coral colonies in the same location but one is exposed to sun while the other is shaded, the exposed colony may undergo a bleaching event even though the other coral will not. Going through a bleaching event may imprint on the coral and as a result the coral may ramp up its defenses, such as increasing certain proteins or shifting their energy partitioning. So can past bleaching events help corals deal with them in the future? Is there a memory of this response?
In reef flats off of Thailand, researchers began to notice something interesting in the way corals were responding to environmental changes. These flats are dominated by a few coral species, but mainly Coelastrea aspera (Fig 2). In this specific flat, the western side of each C. aspera colony was receiving higher doses of solar irradiance, while the eastern side was shaded. In 1995, there was a mass-bleaching event at this site caused by higher water temperatures. Researchers noticed a striking pattern in the bleaching: the eastern sides of the colonies were white and almost totally bleached, while the western sides still retained most of their pigment. The higher tolerances of the western sides were attributed to their history of high solar irradiance and increased stress proteins, antioxidants, and nutrient cycling. These specific corals weren’t very old and the irradiance they experienced was relatively recent; so researchers began to wonder: how long would the imprint of stress last?.
In the same study area in 2000, researchers performed a field manipulation. They took 24 C. aspera colonies, detached them from the reef, rotated them 180° and reattached them. So now, the western facing sides (that had experienced high irradiance) were facing east, and the eastern sides (which were more protected) were facing west. Another 24 colonies were detached from the reef and ultimately placed in the same orientation; these were the controls (Fig 3).
In 2010 there was another severe bleaching event. At this time, researchers started to sample their manipulated colonies. They took tissue samples from the eastern side of the manipulated coral colonies to assess them for symbiont density. They found that samples from the rotated colonies had significantly higher symbiont densities than the non-rotated colonies, almost 4 times more (Fig 4)! So colonies that had once experienced high irradiance were able to maintain higher densities of symbionts than the colonies that hadn’t experienced that stress.
These findings are quite interesting, particularly because these coral colonies were manipulated in 2000 and then reassessed in 2010 (that’s a decade!). That means that the sides of the corals that were originally exposed to high irradiance got a 10-year break from that stress, and that even after 10 years, these corals still “remembered” life under high irradiance and had the symbiont densities to prove it. Other studies have shown that past stress inducing events have led to higher tolerance in the future, but no study has shown how long these events stick in the “memory” of these organisms. This study showed that this memory lasts for at least 10 years.
As always, there is still work to be done, including honing in on the mechanisms that make all of this happen. But in an age where climate is highly variable and ecosystems are on the brink, it is somewhat of a comfort to know that corals won’t be forgetting harm done to them in the past, it’s the old adage: what doesn’t kill you makes you stronger.