Inspiration: Where the Story Began ​

A clear sunny day, a novice diver on our iGEM team eagerly set out to explore the underwater world, anticipating a magical encounter with vibrant corals and playful fish. Little did she know, her underwater adventure was about to take an unexpected turn...

In reality, she was met with silence—lifeless, pale corals, seemingly fragile enough to crumble with a touch, a sight far from her vibrant imagination. This stark reality left her wondering: What caused this? And how can I help?

Motivated by her experience, our team, Coral Cola, took on the challenge of tackling coral bleaching. Through extensive research, we’ve developed a synthetic biology solution that we’re eager to share with you. But first, let’s dive into the current state of coral reefs.

Picture 1._Photo taken by Hongqi GUO, one of our friends._ Showing the coral reefs that are tangled with fishing net.

The Urgent Threat to Coral Reefs ​

Coral reefs are facing increasingly frequent and severe marine heatwaves, leading to coral bleaching and death. Coarse-grained climate models indicate that only a few coral reefs are likely to endure a 3°C rise in sea surface temperatures over the next century (Van Woesik & Kratochwill, 2024).

Figure 1.This figure illustrates the global trend of coral bleaching. The upper panel shows the distribution of coral bleaching in 2017 (Sully et al., 2019), while the lower panel displays the distribution in 2020 (Van Woesik et al., 2022). Comparing the two panels, it is evident that coral bleaching has worsened globally over time.

From figure 1 we can know that corals are suffering from globally bleaching events.

This alarming increase in coral bleaching not only signifies a loss of marine biodiversity but also threatens one of the most crucial ecosystems on our planet. To understand why coral reefs are so important and why their loss has far-reaching consequences, we must first appreciate their role in the ocean's health and the benefits they provide to both marine life and humans.

Coral Reefs are Vital ​

Covering only 1% of the world's oceans, coral reefs provide a habitat for at least 25% of all marine life, supporting over 4,000 species of fish and other organisms. The health of coral reefs is crucial for both coastal communities and global biodiversity.

Figure 2._The functional importance of high-seas coral reefs_ (Source: https://ar.inspiredpencil.com/pictures-2023/coral-reef-diagram-labeled)

Coral reefs are essential for food security and economic stability, as they support more than half of the U.S. fisheries species at some stage of their life cycle. The commercial value of fisheries linked to coral reefs in the U.S. exceeds $100 million annually. Moreover, they boost local economies through tourism-related activities, generating billions of dollars from diving, snorkelling, and recreational fishing.

Beyond their economic importance, coral reefs serve as natural barriers, protecting coastlines from storms and erosion, thus safeguarding lives and property. They also play a key role in building beaches and filtering ocean water, contributing to the health of marine ecosystems.

Importantly, coral reefs hold untapped potential for medical research, with compounds from reef organisms being explored for the treatments of various diseases. Their cultural significance is profound and deeply intertwined with the traditions and heritage of coastal communities worldwide.

Despite their vital importance, coral reefs are increasingly vulnerable to environmental stressors. One of the most devastating outcomes of these stressors is coral bleaching, a phenomenon that has dramatically increased in frequency and severity in recent decades.

What is Coral Bleaching ​

Relationship Between Algae and Coral ​

Corals engage in a symbiotic relationship with various species of algae, predominantly residing within the gastrodermal cells of the coral host. This mutualistic interaction allows the algae to furnish the corals with essential nutrients synthesized via photosynthesis. In return, the algae benefit from the protective environment offered by the coral structures (Wooldridge, 2010).

Figure 3._The location of corals and Zooxanthellae_ (Wooldridge, 2010)

However, scientists have also found that certain types of algae exist outside the coral structures. These algae can regulate the population density of algal cells residing within the coral, ensuring ecological balance, and can even form symbiotic relationships with the coral (Hirose et al., 2008).

Under stress conditions (e.g., high temperature, light), hyperphotosynthesis in zooxanthellae leads to increased levels of reactive oxygen species (ROS), resulting in cell necrosis, apoptosis, or detachment. In such circumstances, corals often expel zooxanthellae to ensure their own survival. However, this can result in coral bleaching, as the coral's resilience decreases significantly when its primary source of carbon disappears.

The Meaning of Coral Bleaching ​

Coral bleaching is the term used to describe the loss of all or some of a coral's symbiotic algae and photosynthetic pigments, resulting in the white calcium carbonate skeleton becoming visible through the now translucent tissue layer (Plass-Johnson et al., 2014).

Figure 4._Macro photographs showing unbleached and bleached polyp tissue of the scleractinian coral Stylophora pistillata_. Panel (**a)**: healthy corals (**b)**: the bleached corals (Plass-Johnson et al., 2014)

What Causes Coral Bleaching ​

Through extensive literature research, we gained an overview of the reasons for coral bleaching. Here, we list several influential environmental triggers and their mechanisms.

Factors ​

Heat ​

Elevated sea temperatures are one of the most important factor contributing to coral bleaching. This rise in temperature affects the metabolism of coral holobionts, with current research focusing on their respiration and photosynthesis. Researchers have found that under extreme heat, the speed of photosynthesis increases, resulting in the overproduction of ROS, which is caused by the increased levels of antioxidant enzyme activity. The rise of ROS level also affects the photosynthetic electron transport capacity, leading to coral bleaching through autophagy-dependent host cell death or symbiophagy (Lesser, 2010; Helgoe et al., 2024).

Light ​

The main light-related factor causing coral bleaching is visible radation and ultraviolet radiation (UVR), which are intensified by heat. Light stress damages photosynthetic components and increases reactive oxygen species (ROS) levels.

As light intensity rises, zooxanthellae demand more CO₂. When this demand surpasses the coral's supply, ATP and NADPH accumulate, leading to excess electron production. These electrons damage photosystem II (PSII) and generate ROS. High ROS levels overwhelm coral defenses, causing cell damage and expulsion of zooxanthellae, resulting in coral bleaching (Wooldridge, 2009).

Figure 5.Mechanisms of coral bleaching caused by light. (Wooldridge & Done, 2009)

Other ​

Other factors like bacterial infections, and low pH are also contribute to coral bleaching by increasing ROS levels and facilitating the symbiolysosomal digestion process. However, since they are not the main factors in coral bleaching, we focused more on heat and light in our research.

Interaction between Factors ​

In reality, factors that affect coral bleaching do not function independently; they influence each other significantly. We illustrate this by examining heat and light, the two major factors.

As seen in the figure below, even though two groups of corals were exposed to the same high light levels for the same duration, the level of bleaching differed due to temperature variations

Figure 6.High light-induced bleaching of the coral Acropora digitifera.(Takahashi et al., 2004).

In some studies, the corals didn't bleach if the weather was cloudy even when temperatures were rising. A paper on the ENSO event in 1998 showed that, although sea surface temperatures rose as in previous coral bleaching years, Society Island did not experience bleaching, while in many parts of French Polynesia, 99% of corals died (Mumby et al., 2001).

From their results, we can find that it's of great necessity to consider both the heat and light factors while preventing coral bleaching.

Variance Between Species and Geographical Areas ​

The following figure clearly shows that, under the same circumstances, the degree of bleaching varies significantly among different species. Therefore, flexible strategies are needed to address coral bleaching.

Figure 7._Taxa-specific bleaching indices sorted from most to least susceptible._ (McClanahan et al., 2005)

Given the critical role coral reefs play and the rapid acceleration of coral bleaching, the urgency to find effective solutions has never been greater. Over the years, scientists and conservationists have explored various approaches to combat this issue.

Existing Solutions ​

Coral bleaching has been a significant environmental concern since the late 1970s, leading to the development of various strategies aimed at protecting coral reefs and mitigating their global decline.

Figure 8._Examples of actions to restore/rehabilitate reefs and mitigate their global loss_ (Voolstra et al., 2023).

Policy Interventions ​

Policy measures have played a crucial role in addressing coral bleaching by aiming to protect coral reef ecosystems at a broader level. The primary policy interventions include:

1. Reducing Pollution: Government can establish the relevant policy to prohibit the marine pollution to ensure the living conditions good enough. For instance, Land-Based Sources (LBS) Protocol is initiated by the Caribbean Environment Programme (CEP).
2. Climate Change Mitigation
3. Sustainable Fisheries Management: Unsustainable fishing practices like overfishing and destructive fising can harm coral reefs by directly damaging the coral reefs or disrupting the ecological balance. Some stratigies like setting no-take areas are aimed to relieve this.
4. Promoting Eco-Friendly Tourism

Overall, policy interventions address the root causes of coral bleaching and aim to create a more sustainable environment for coral ecosystems. However, they require long-term commitment and collaboration across multiple sectors, and their effects may not be immediate. During this time, coral reefs may continue to suffer, underscoring the need for complementary strategies.

Coral-Based Approaches ​

Coral-based approaches directly address coral health and resilience by focusing on biological methods to restore and protect coral populations:

1. Coral Transplantation: One of the most commonly used methods is coral transplantation, where corals are cultivated under controlled conditions and then transplanted into damaged areas to increase the abundance of corals.

2. Selective Breeding, Hybridization, and Genetic Modification: Scientists have been investigating selective breeding, hybridization, and genetic modification to develop coral strains with enhanced resilience to stressors like rising temperatures (Caruso et al., 2021).

In summary, coral-based approaches can be highly effective and adaptable to specific conditions, offering targeted solutions for coral restoration. However, they also present challenges such as limited scalability, high costs, and potential impacts on biodiversity, making it essential to integrate these methods with other conservation strategies.

Microbial-Based Approaches ​

Probiotics offer a promising microbial-based solution to coral bleaching. In a study by Rosado et al. (2018), corals treated with beneficial bacteria showed a slower rate of bleaching and improved health under high temperatures or pathogenic stress compared to untreated corals.

As these microorganisms are already part of coral ecosystems, they can integrate naturally without disrupting coral communities. However, concerns remain about their long-term impact, as they could interfere with coral physiology in healthy conditions. Additionally, their broad-spectrum nature may limit effectiveness in extreme conditions like prolonged heatwaves, raising uncertainty about their sustained effectiveness as global warming intensifies.

Conclusion ​

In conclusion, while existing solutions for coral bleaching—including policy interventions, coral-based approaches, and microbial-based methods—each offer valuable strategies for coral protection, they all have inherent limitations. Given these constraints, our project, Coral Cola, aims to leverage synthetic biology to develop an innovative, integrated solution to coral bleaching.

Our Solution ​

Circuit ​

For a comprehensive description and detailed explanation of our circuit, please refer to our Circuit Design page for more information.

Recognizing the limitations of policy regulations, which rely heavily on human factors and may not provide sustainable protection for coral ecosystems, we propose a synthetic biology approach to address coral bleaching. Our strategy involves engineering the symbiotic bacteria of corals and ensuring their colonization within the coral symbiotic system. These bacteria will reside within the coral's cells, enhancing the coral's tolerance to environmental stress over an extended period.

Coral bleaching is driven by both environmental stressors and the downstream physiological effects they trigger, particularly Reactive Oxygen Species (ROS). While reducing ROS levels might mitigate bleaching, ROS also serve critical roles in cell signaling and metabolism, making uncontrolled reduction detrimental to coral health. What's more, a sensor for ROS detection is impractical due to the low ROS levels in the coral symbiotic system, which fall below the detection threshold of current technologies. Therefore, we aim to address this issue from the perspective of environmental factors. Given the difficulty in lowering the overall temperature of coral reefs below their ambient temperatures, shading the coral reefs from surrounding light might be a more feasible approach.

After extensive research, we propose expressing a light-shielding protein in coral symbiotic bacteria to reduce light intensity reaching the zooxanthellae. However, constant expression of this protein could reduce the adaptability of both the bacteria and their coral hosts. Therefore, a regulated expression system is essential for effective shading.

Simultaneously, field observations have shown that coral bleaching events due to heat stress occur less frequently in turbid waters. For example, during thermal-stress events in Palau (van Woesik et al., 2012) and Brazil (Teixeira et al., 2019), less bleaching was observed on turbid nearshore reefs compared to other reef systems, suggesting that turbidity moderates bleaching at a local scale. This effect may result from reduced light intensity, which slows the energy flow through the photosynthetic electron transport chain, thereby alleviating oxidative stress caused by high temperatures. In this context, by expressing chromoproteins, our project can mimic the light-reducing effects of turbid water, not only counteracting bleaching induced by high light exposure but also mitigating the impacts of heat stress. This further demonstrates the vast potential of our approach.

Thus, our solution aims to express a light-shielding protein within the symbiotic system when it is under environmental stress, reducing the system's photosynthetic activity and alleviating the downstream destruction caused by light, as well as the damage caused by heat through its action on the photosynthetic electron transport chain. The circuit primarily detects environmental stress by monitoring light and heat in the surroundings. Our circuit design is illustrated in the figure below.

Figure 9.The schematic drawing of our circuit design. By our teammate. Created with BioRender.com.

For potential applications, benefits, and future developments, please refer to our Future Implementation page for more information.

Chassis ​

For a comprehensive description and detailed explanation of our choice of chassis, please refer to our Circuit Design page for more information.

We selected Endozoicomonas as our chassis for the following reasons:

1. It is widely present in various coral species, offering broad-spectrum applicability.
2. It has been extensively studied, providing valuable prior research and experience for reference.
3. Endozoicomonas naturally exists in a symbiotic relationship with corals, minimizing potential adverse effects.

Figure 10.Endozoicomonas montiporae (Hsu, 2015).

References ​

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