RELATIONSHIP BETWEEN ZOOXANTHELLAE AND CORALS

• Zooxanthellae, residing within coral polyp tissues, play a crucial role in supporting coral by engaging in photosynthesis to produce essential nutrients. 
• These activities not only supply the coral with fixed carbon compounds for energy but also bolster calcification processes and regulate the flow of elemental nutrients.
• In this mutual relationship, the coral polyp offers a protected haven for the zooxanthellae to thrive and a continuous provision of carbon dioxide necessary for their photosynthetic activities. 
• In return, the zooxanthellae aid the coral in various ways, fostering a symbiotic alliance.
• This interdependency allows slower-growing corals to compete with faster-growing multicellular algae.
•  During the day, corals can sustain themselves through photosynthesis and, at night, through predation.
• Interestingly, the vibrant colours of coral reefs do not stem from the coral tissues themselves, which are actually transparent. 
• Instead, the hues originate from the zooxanthellae inhabiting the coral, providing the mesmerising and diverse pigmentation seen in these ecosystems.

CORAL BLEACHING

Coral reefs face a multitude of disturbances stemming from both human activities and natural phenomena. 

The recent and rapid decline of these reefs primarily links to human-induced impacts such as overexploitation, overfishing, heightened sedimentation, and an overload of nutrients. Yet, natural events also play a role, causing damage through violent storms, flooding, temperature fluctuations, El Nino Southern Oscillation (ENSO) occurrences, sub-aerial exposures, predatory outbreaks, and epizootics.

• One of the common stress responses observed in corals, triggered by various disturbances, is coral reef bleaching. 
• This phenomenon manifests when there is a reduction in the density of zooxanthellae or a decrease in the concentration of photosynthetic pigments within these symbiotic organisms. 
• Consequently, the coral expels or "bleaches" these zooxanthellae when they are no longer beneficial.

During bleaching, corals typically lose a substantial portion of their zooxanthellae, often ranging from 60% to 90%, and each zooxanthellae cell can experience a significant reduction of 50% to 80% in its photosynthetic pigments.

• In less severe instances where the stress causing bleaching diminishes over time, affected corals can usually recover. 
• They tend to regain their symbiotic algae within several weeks or a few months.
• However, if the stress persists and the zooxanthellae populations remain depleted without recovery, the coral host faces the risk of eventual demise. 
• Prolonged loss of zooxanthellae leads to the demise of the coral.

ECOLOGICAL CAUSE OF CORAL BLEACHING

TEMPERATURE

• Coral species thrive within a relatively narrow temperature range, and extreme deviations, either low or high sea temperatures, can trigger coral bleaching. 
• For instance, the absence of corals on the west coast of tropical temperate continents is attributed to cold currents. 
• Bleaching events occur during sudden temperature fluctuations such as intense upwelling episodes (such as during El-Nino events) and seasonal cold-air outbreaks.
• Although most reefs have shown signs of recovery with relatively low levels of coral mortality, certain areas have suffered severe damage due to coral bleaching. 
• This fragility is a testament to the susceptibility of coral reefs to increased water temperatures, often coupled with ocean acidification. 
• While elevated temperatures have heightened the frequency and intensity of bleaching, acidification has hindered corals' ability to calcify.
• Even a slight and gradual rise in temperature over several weeks or a rapid increase of 3-4°C over a few days can disrupt coral function and potentially induce bleaching. 
• Coral bleaching events tend to occur during summer seasons or towards the conclusion of prolonged warming periods. 
• These incidents are reported to take place during periods of low wind, clear skies, calm seas, and low turbidity, which favour localised heating and increased exposure to high ultraviolet (UV) radiation. 
• However, rising temperatures can reduce the concentration of UV-absorbing compounds in corals, diminishing their ability to block potentially harmful UV radiation.

SUB AERIAL EXPOSURE

• Sub-aerial exposure, such as sudden exposure of reef flat corals to the atmosphere during extreme low tides, ENSO-related sea level drops, or tectonic uplift, has the potential to induce bleaching. 
• This exposure subjects corals to extreme temperatures, intensified solar radiation, desiccation, and dilution of seawater due to heavy rains, all of which contribute to zooxanthellae loss and could lead to coral mortality.

SEDIMENTATION

• Moreover, rapid dilution of reef waters from storm-generated precipitation and runoff has been shown to cause coral reef bleaching, although such events are generally rare and confined to relatively small nearshore areas.
• An increase in ambient elemental nutrient concentrations does not directly cause coral reef bleaching; instead, it actually increases zooxanthellae densities. 
• However, elevated nutrient levels, known as eutrophication, might weaken coral resistance and heighten susceptibility to diseases.
• Coral bleaching can also occur when corals are exposed to high concentrations of chemical contaminants like copper, herbicides, and oil.

EPIZOOTICS

Additionally, pathogen-induced bleaching presents differently from other forms, often leading to patchy or whole colony death and the shedding of soft tissues, resulting in a white skeleton, which should not be confused with bleached corals.

SPATIAL AND TEMPORAL ASPECT OF CORAL BLEACHING

• During the 1980s, nearly all major coral reef regions worldwide, including the Caribbean/western Atlantic, eastern Pacific, central and western Pacific, Indian Ocean, Arabian Gulf, and Red Sea, witnessed some level of coral bleaching and mortality. 
• Prior to this period, most significant coral deaths were linked to non-thermal disturbances such as storms, exposure during extreme low tides, and outbreaks of the Acanthaster starfish. 
• Although coral bleaching was noted before the 1980s during phases of heightened seawater temperatures, these incidents were geographically isolated and confined to specific reef zones. 
• In contrast, the 1980s saw a significant shift where numerous coral bleaching events occurred across expansive geographic regions and at various depths, signifying a more widespread and profound impact.

The third global coral bleaching event, occurring from 2015 to 2016, follows previous incidents in 1998 and 2010. 

• This ongoing event stands out as the most prolonged and severe to date.
• The primary factor behind this extensive bleaching is attributed to the longest and most severe El Niño ever recorded.

CONSERVATION OF CORALS

• The Environmental Protection Act of 1986 in India explicitly prohibits the utilisation of corals and sands obtained from beaches and coastal waters for construction and other purposes. 
• Additionally, India is actively involved in the Green Coast project, aiming to enhance coastal protection measures.
• Under the Coastal Regulation Zone (CRZ) rules, the collection, destruction of corals, dredging, and underwater blasting around coral formations are strictly banned. 
• The establishment of marine protected areas and marine national parks plays a vital role in the conservation of coral reefs. 
• For instance, the Gulf of Mannar's Marine National Park in Gujarat, the Great Nicobar Biosphere Reserve, and the Mahatma Gandhi Marine National Park in the Andaman Islands are pivotal in coral conservation efforts.
• India's commitment to global environmental initiatives is evident through its ratification of the Paris Agreement, joining as the 62nd country in the world to do so. 
• This is a significant step in the country's efforts toward mitigating the effects of climate change. 
• Moreover, India has ratified the second commitment period of the Kyoto Protocol (2013-2020), indicating its dedication to international efforts aimed at curbing greenhouse gas emissions and global climate change.

CONSEQUENCE OF GLOBAL CORAL BLEACHING

• Coral reef ecosystems, occupying less than 0.1% of the ocean's total area, play a disproportionately vital role in supporting marine life. 
• Despite their small spatial coverage, these ecosystems serve as homes and sources of sustenance for approximately 25% of all marine species. 
• They also underpin the fish stocks that approximately 500 million people globally rely on for their livelihood and sustenance.
• However, when mass bleaching occurs, a rapid shift can take place, transforming a coral-dominated reef into one dominated by algae in just a few months. 
• Reversing this process can take decades, or even longer, highlighting the severe impact of such events on the delicate balance of these ecosystems.

FAQs regarding coral bleaching

1. What is the role of zooxanthellae in coral reefs?

Ans. Zooxanthellae are essential symbiotic algae living within coral tissues. They engage in photosynthesis, producing nutrients that supply corals with fixed carbon compounds for energy, aid in calcification processes, and regulate elemental nutrient flow.

2. How do corals and zooxanthellae benefit from their mutual relationship?

Ans. Corals provide a protected environment for zooxanthellae to thrive and supply carbon dioxide for their photosynthesis. In return, zooxanthellae aid corals by providing nutrients and fostering a symbiotic alliance.

3. What is coral bleaching, and what triggers it?

Ans. Coral bleaching occurs when corals expel zooxanthellae due to a reduction in their density or a decrease in the concentration of photosynthetic pigments, often triggered by stress factors like temperature fluctuations or chemical contaminants.

4. What environmental factors contribute to coral bleaching?

Ans. Temperature fluctuations, especially extreme highs or lows, coupled with factors like increased UV radiation, nutrient overloading, sub-aerial exposures, and pollutant exposure, contribute to coral bleaching.

5. How does coral bleaching impact the coral reef ecosystem?

Ans. Mass bleaching events can swiftly transform coral-dominated reefs into algae-dominated ones. Reversing this process may take decades or longer, significantly disrupting the delicate ecological balance of these ecosystems.

6. Why have there been widespread coral bleaching events in recent decades?

Ans. The frequency and severity of coral bleaching have increased, primarily due to rising water temperatures, ocean acidification, and various disturbances caused by human activities and natural phenomena.

7. How do conservation efforts protect coral reefs?

Ans. Laws and regulations, such as the Environmental Protection Act and Coastal Regulation Zone rules, prohibit destructive activities like coral collection, dredging, and blasting. The establishment of marine protected areas and national parks, such as the Gulf of Mannar Marine National Park, plays a vital role in conserving coral reefs.

8. What is the significance of global initiatives like the Paris Agreement in protecting coral reefs?

Ans. Global agreements like the Paris Agreement and the Kyoto Protocol, which India has ratified, emphasize commitment to mitigating climate change and reducing greenhouse gas emissions, thereby indirectly supporting coral reef conservation.

9. Why are coral reefs crucial to marine life and human populations?

Ans. Coral reefs, though covering less than 0.1% of the ocean's area, are vital. They support a quarter of all marine species and provide sustenance for around 500 million people globally who depend on fish stocks associated with these ecosystems.

10. What are the long-term consequences of global coral bleaching?

Ans. Rapid shifts from coral-dominated reefs to algae-dominated ones due to mass bleaching events can disrupt the balance of ecosystems. Reversing this process may take decades or longer, impacting the delicate balance of marine life and human dependence on these ecosystems.