Coral bleaching is a major cause of the decline of coral reefs worldwide. Bleaching or whitening of the coral results from the loss of the symbiotic dinoflagellate algae ( zooxanthellae) from the host coral. The environmental causes of bleaching are linked to climate change and disease, however understanding how bleaching occurs in the host coral is just beginning to be investigated.

Scientists from Oregon State University investigated the bleaching process in the symbiotic sea anemone Aiptasia pallida. Specifically, they investigated two different pathways that could cause symbiont release from the anemone. They hypothesized that if one or both of the pathways were active in bleaching, then getting rid of these pathways would change the amount of bleaching observed. 

A little background…

Aiptasia anemones, just like corals, have formed a unique partnership with algae. These zooxanthellae provide the anemone with food, and in turn, the algae are protected and receive nutrients from the anemone host. This partnership is stable under normal conditions, but when there is stress from increased seawater temperatures, the partnership breaks down. The anemone host cell can release the zooxanthellae through several different mechanisms and the symbiont can be degraded, resulting in bleaching. The reason why scientists make use of these anemones is that they can be kept alive easily (even without zooxanthellae), can be manipulated in the laboratory (such as induced bleaching) and grow fast.  Scientists can even disrupt gene function in this animal, allowing them to discover how genes regulate all life aspects of Aiptasia.

Figure 1: Aiptasia pallida, a well-known pest anemone amongst marine aquarists, has become a celebrated model-system voor biologists. These animals are easy to keep and experiment on, and they are similar to corals. This allows scientists to relate their results to coral health, bleaching and genetics (photograph: Dr. Santiago Perez).

Two pathways involved in bleaching

Apoptosis (pronounced: ap-oph-toe-sis) is the process of a cell being ordered to die because it is not working properly, is infected, or has been exposed to toxins. Initiation of this cell death process has been observed during bleaching and could occur because: (i) cell death lessens tissue damage by removing algal symbionts that no longer function, and (ii) apoptosis acts as part of a natural immune response where algae, instead of being recognized as partners, are recognized as pathogens and removed. Once the symbionts are removed, they may be broken down within the host cell, resulting in bleaching.

Autophagy (pronounced: aw-toe-phage-e) is another pathway that could also be active in bleaching. The word ‘autophagy’ is derived from Greek roots: auto, meaning ‘self’, and phagy, ‘to eat’. Basically, autophagy is a process where the cell breaks down itself. It can be caused by nutrient deficiency and infection from invading organisms. This pathway begins when the target (i.e., zooxanthellae) is identified, followed by the formation small sacs around the target. The surrounded target is then digested. The digestion of symbionts also contributes to coral bleaching. Apoptosis and autophagy can be closely linked processes, operating together as a complete cell death pathway to get rid of zooxanthellae that are no longer working properly.

 Figure 2: Aiptasia sp. can be bleached easily in the laboratory, allowing researchers to study this process (photograph: Dr. Carlo Caruso).

The experiment

Aiptasia pallida were placed in individual wells in seawater and exposed to one of two temperature treatments. The used control temperature was about 26ºC (79°F), and the heat stressed temperature was approximately 33-34ºC (91-93°F). The heat stressed conditions resulted in bleaching of the anemones.  Separate treatments were used to determine the roles of apoptosis and autophagy during bleaching. Control and heat-stressed animals were given a chemical which stopped apoptosis, called wortmannin. The researchers hypothesized that the control and heat-stressed animals would not bleach due to the added chemical. Under the control condition, there would be no bleaching because the animals were not under any stress, and so the zooxanthellae would still be functioning properly (i.e., there is no reason to kill the cell because it is working properly). Under the heat stressed condition, the animals would normally bleach because the zooxanthellae would not be working properly and would be killed through apoptosis. But, having removed the ability of the anemone to recognize the symbionts as non-functioning (by stopping apoptosis), the anemones should not bleach during or after stress exposure.

Interestingly, their results were not what they expected. Instead of seeing a decrease in bleaching in the heat-stressed anemones where cell death had been stopped, the amount of bleaching was the same as the heat stressed anemones in which apoptosis functioned properly. How could they explain this observation? Previous studies had shown an increase in apoptosis during bleaching. They figured that some other pathway must be recognizing the symbionts as non-functional and causing the anemones to bleach.

The researchers decided to stop both apoptosis and autophagy in anemones to see if they would still bleach under heat stress. The anemones were simultaneously treated with chemicals to stop both pathways of cell death and exposed to either a control temperature or heat-stress temperature. They saw that when the anemones were heat-stressed and treated with chemicals to stop both pathways of cell death, there was very low bleaching (figure 3). This result marked a pivotal point in their research!  In the untreated animals (apoptosis and autophagy active), significant amounts of zooxanthellae  were released during exposure to heat-stress. However, in the treated (i.e., inhibitors where apoptosis and autophagy are inactive) anemones, very few zooxanthellae were released during  heat-stressed exposure. This proved that both cell death pathways were responsible for releasing the zooxanthellae from the anemones!

Figure 3, above right: Blocking both apoptosis and autophagy in Aiptasia pallida greatly reduces temperature-induced bleaching. White bars are control temperatures (26ºC or 79°F) and grey bars are heat-stress temperatures (33-34ºC or 91-93°F) (Dunn et al, Journal of molecular evolution, 2006).

These results suggested that no single process or pathway is active in bleaching and that these two pathways, apoptosis and autophagy, are linked in a see-saw manner. This means that when one pathway is stopped or not working properly, the other is initiated as a back-up and vice versa. Although the expulsion of useful algae doesn’t seem to be beneficial to an anemone or a coral, this process probably prevents the tissues of these animals from being badly damaged by the production of poisonous oxygen by improperly functioning zooxanthellae.

Bleaching insights; taking it a step further

This research has provided new crucial insights into bleaching of Cnidarians such as anemones. Given the fact that these animals highly related to corals, this newly discovered bleaching process is probably active in corals as well. Past research indicated that the clade of zooxanthellae determines how sensitive corals are to bleaching, and how the first step of bleaching is initiated by inducing zooxanthellae membrane damage. This research takes it further by showing which cellular processes are activated upon recognized zooxanthellae damage. The process as a whole is quite simple when translated to a set of principles; high temperature induces zooxanthellae damage and their recognition by coral tissue, after which apoptosis and autophagy of zooxanthellae are activated. This causes bleaching, which is followed by either restoration of the zooxanthellae population or coral death by starvation.

Figure 4: Bleaching of coral reefs during summer periods may be mediated by both apoptosis and autophagy (photograph: Hans Leijnse).

References:

Dunn, S.R., Schnitzler, C.E., and Weis, V.M. (2007) Apoptosis and autophagy as mechanisms of dinoflagellate symbiont release during cnidarian bleaching: every which way you lose. Proc R Soc Lond B 274: 3079-3085.