Thermal Adaptation - Coral Symbiosis

The Great Barrier Reef (GBR) is an important environmental and economic asset to North Queensland. As a result of warming ocean temperatures, six coral bleaching events have occurred in the GBR since 1980 (Berkelmans 2001). Progression of coral bleaching in this ecosystem will affect countless marine species with negative economic impacts to tourism (Strasser et al 1999). This blog post will discuss the potential of thermally-resilient symbiotic algae in order to reduce coral bleaching in the GBR.

Coral is the foundation of the GBR, providing shelter, feeding and breeding grounds for many marine organisms. However due to increasing ocean temperatures, coral bleaching is occurring (Fig.1). A 2001 JCU study found that upper thermal limits of coral in the GBR were only 2-4°C higher than average summer temperature, and within 1°C of daily average temperatures during summer periods (Berkelmans 2001). The white colour is caused by the death of zooxanthellae (Symbiodinium), occurrent within the gastrodermic cells of the coral as a symbiotic relationship (Ban & Graham 2012). The physiology of bleaching is not fully understood but involves the production of oxygen radicals, which are highly corrosive and damage chloroplasts (Berkelmans 2001).

(Fig. 1 - Coral Bleaching events between 1994 and 1998 - dark spots represent reports of bleached coral, light spots represent reports of healthy coral)
(Berkelmans 2001)

Thermal adaptation of coral could be achieved by transplanting heat-resilient zooxanthellae. A 2004 study found that in Guam, corals of the genus Pocillopora associate with both Symbiodinium C and thermally resilient species ‘D’ (Rowan 2004). In a 2006 research project, coral nubbins containing Symbiodinium D were transplanted to Magnetic Island, North Keppel Island and Davies Reef. Magnetic and North Keppel islands experienced significant increases in zooxanthellae density and reductions in bleaching/death of coral. Corals exposed to 32°C in the Keppel control had no zooxanthellae, with more than 60% of corals bleached and the rest dead. After treatment, over 50% of corals in this area at 32°C were surviving with reduced bleaching and significant reduction in mortality (Fig. 2) (Berkelmans & Van Oppen 2006).

(Fig. 2 - Zooxanthellae density in corals before and after transplanting at North Keppel Island sites (bar graph), pie graphs represent coral status; (grey) healthy, (white), bleached and (black) dead) (Berkelmans & Van Oppen 2006)

Coral bleaching is a significant problem for the GBR. In 1998, 87% of 2900 reef systems were affected by at least moderate bleaching (Berkelmans 2001). Coral transplanting has increased zooxanthellae density, and reduced coral bleaching/mortality but only in 30 - 32°C conditions. The increased tolerance of Keppel corals with Symbiodinium D is only about 1 - 1.5°C (Berkelmans & Van Oppen 2006). Transplanting of corals with Symbiodinium D has resulted in increased thermal tolerance of corals, however application of this research as a thermal adaptation would only be effective temporarily as temperatures continue to rise. 

References:

Ban, S.S. Graham, N.A.J. 2012, ‘Relationships between temperature, bleaching and white syndrome on the Great Barrier Reef’, Coral Reefs, Vol. 32, pages 1–12

Berkelmans, R. 2001, ‘Bleaching, upper thermal limits and temperature adaptation in reef corals’, PhD thesis, James Cook University

Berkelmans, R. Van Oppen, M.J.H. 2006, ‘The role of zooxanthellae in the thermal tolerance of corals: a ‘nugget of hope’ for coral reefs in an era of climate change’, Proceedings of The Royal Society of Biology, Volume 273, pages 2305-2312. 

Rowan, R. 2004, ‘Thermal adaptation in reef coral symbionts’, Nature, Vol. 430, page 742

Strasser, R.J. Tsimilli-Michael, M. Pecheux, M. 1999, ‘Perpetual adaptation in a perpetually changing environment as a survival strategy of plants: a case study in foraminifers concerning coral reef bleaching’, Photosynthetica, Vol. 37, pages 71-85

Figures:

Figure 1: Berkelmans, R. 2001, ‘Bleaching, upper thermal limits and temperature adaptation in reef corals’, "Records of bleaching on the reef from 1994 to 1998", PhD thesis, James Cook University

Figure 2: Berkelmans, R. Van Oppen, M.J.H. 2006, ‘The role of zooxanthellae in the thermal tolerance of corals: a ‘nugget of hope’ for coral reefs in an era of climate change’, "Zooxanthella densityGs.e. (bars) and coral condition (pies) of coral nubbins", Proceedings of The Royal Society of Biology, Volume 273, page 2311