Corals, Zooxanthellae & Bleaching

Zooxanthellae not only provide corals with important nutrients, but they are also the reason why corals display a variety of different colors (Morais et al. 2018). Corals that lose their zooxanthellae, also lose their pigments and appear stark white (Jokiel 2004). This process is known as coral bleaching and occurs when the coral must expel its zooxanthellae from its tissues because of a combination of thermal stress and high solar irradiance (Brown 1997; Williams and Bunkley Williams 2000; Fitt et al. 2001; Jokiel 2004; Hughes et al. 2017). Specifically, corals bleach when water temperatures exceed the longterm mean maximum summer sea surface temperatures by 1-2 or 2-3 degrees celsius for a specific period of time (the bleaching threshold) (Brown 1997; Jokiel 2004; Lesser 2006).

In the Caribbean, coral bleaching occurs when sea surface temperatures exceed 30 degrees Celsius for two consecutive weeks (Rogers et al. 2009). During these periods of high temperatures, coral zooxanthellae produce high levels of oxygen reactive species (ROS) that damage coral cells and tissues (Lesser 1997; Lesser 2006; Suwa and Hidaka 2006; Rodriguez-Troncoso et al. 2013). Theses high concentrations becomes toxic to the coral and the coral must expel its zooxanthellae in order to avoid further cellular damage and death (Lesser 2006; Suwa and Hidaka 2006; Rodriguez-Troncoso et al. 2013).

However, without their zooxanthellae, the coral cannot obtain the organic compounds needed for survival and ultimately begin to starve ( Hoegh-Guldberg 1999; Spalding et al. 2001; Morais et al. 2018). If sea surface temperatures decrease, corals may be able to regain their zooxanthellae and recover from bleaching (Wooldridge 2010). However, bleaching severely damages the coral’s tissue, skeletal growth and immune system; this weakened immune system makes the coral susceptible to disease (Miller et al 2009a; Rogers et al. 2009; Morais et al. 2018).

Depending upon their location, corals are adapted to constant sea surface temperatures, therefore, when sea surface temperatures fluctuate, corals suffer. Studies suggest that a 1-2 degrees Celsius increase in temperature for a few weeks can cause widespread, regional bleaching events called mass bleaching (Spalding et al. 2001; Jokiel 2004; McWilliams et al. 2005). Since the 1980s, the frequency and magnitude of these events have increased on coral reefs around the world (Jokiel 2004; McWilliams et al. 2005).

In the Caribbean region alone, six mass bleaching events have been observed and recorded since 1980. The first mass bleaching event to be recorded in the Caribbean region occured in 1987 and lasted for an entire year. During this event, more than 80 species of coral reef symbionts surveyed, including important reef-building species, bleached throughout the Caribbean region and high rates of coral mortality (33-40%) were observed on coral reefs in the Bahamas, Culebra, Puerto Rico, the British Virgin Islands and Jamaica (Williams and BunkleyWilliams 1988).

Two years later, a more devastating mass bleaching event occurred on coral reefs in the Western North Atlantic region. Bleached corals were observed on reefs in Bermuda, Texas, Florida, the entire Caribbean region and in parts of Brazil (Williams and Bunkley- Williams 2000). During this event, species of stony and fire coral suffered massive mortalities in many areas throughout the region (Williams and Bunkley-Williams 2000). In the fall of 1995, another severe mass bleaching event occurred in the Western Atlantic Region. During this event, mass bleaching was recorded for the first time on reefs in Belize and other previously unaffected areas throughout the region (McGrath and Smith 1998).

High levels of mortality were recorded in species of Orbicella annularis and O. faveolata, Agaricia, Diploria strigosa, Millepora alcicornis and Porites astreoides throughout the region (Woodley et al. 1997). Another Caribbean and North Atlantic mass bleaching event occurred in 1998 causing coral cover to decline by 5-10 % throughout the region. Approximately 60 to 80% of coral colonies in affected areas were recorded as bleached (Goreau et al. 2000). In 2005, the Caribbean region suffered the most severe massive bleaching event ever recorded (Miller et al. 2009a).

Bleaching was observed in most Caribbean coral species at depths up to 40 meters and in 22 countries. In certain locations, coral mortality exceeded 50% and in the US Virgin Islands, the combined effects of bleaching and disease caused the average coral cover to decline by 51.3% (Eakin et al. 2010; Miller et al. 2009a; Rogers et al. 2009). Since climate change is projected to increase global sea surface temperatures in the future, the magnitude and frequency of massive bleaching events will increase over time (Spalding et al. 2001; Eakin et al. 2010).