Symbiosis will help some corals survive in a warming world

In a world gripped by climate change, one of the most vivid illustrations of its devastating impact has been the widespread bleaching of coral reefs. With oceans warming at an unprecedented pace, the delicate symbiotic relationship between corals and their resident algae is disrupted, which often leads to mortality.

These tragic events have led scientists to assume a tradeoff scenario – for corals to withstand warmer waters, they might need to ally with more heat-tolerant algal symbionts, but at the cost of their own growth and vitality.

New hope for corals

Contradicting this widely held belief, a breakthrough study from Penn State offers a glimmer of hope. The research suggests that certain corals can, in fact, weather the storm of climate change without significant metabolic cost. The results challenge the established notion of an unavoidable tradeoff.

“Contrary to the widespread perception that reef-building corals with heat-resistant algal partners suffer from stunted growth, our research shows these heat-adapted symbioses can better withstand severe marine heatwaves. As a result, they might become more prevalent in future reef ecosystems,” said Professor Todd LaJeunesse.

Coral reefs, as Professor LaJeunesse explained, are built by colonies of tiny, individual polyps akin to sea anemones. The tissues of these polyps house numerous photosynthetic algae, or “dinoflagellates,” in a symbiotic relationship. 

However, the ability of these dinoflagellates to tolerate high temperatures varies widely, and when the ocean becomes excessively warm, many symbiont species die, leading to the collapse of their coral hosts.

Focus of the study

The team’s investigation began with the first documented instance of a variance in coral mortality based on the species of their resident symbionts. This occurred in the Eastern Pacific Ocean following the 1997-1998 El Niño Southern Oscillation event, during which water temperatures soared 2-4°C above the historical average. 

Corals playing host to the symbiont Durusdinium glynnii survived the thermal upheaval, while those with Cladocopium latusorum symbionts perished.

The durability of D. glynnii under high heat conditions prompted the question of whether their presence incurred a metabolic cost to the corals. 

“Past studies suggested that thermal tolerance comes with reduced nutrient transfer from symbiont to host, triggering negative physiological tradeoffs such as diminished growth and reproductive success,” said study co-author Professor Mark Warner of the University of Delaware. 

“Our aim was to determine whether a similar tradeoff existed within these corals and how it might affect the future of coral reef ecosystems.”

How the research was conducted

To answer these questions, the research team, led by Kira Turnham, compared the growth and reproduction of Pocillopora corals hosting both D. glynnii, the heat-tolerant symbiont, and the more temperature-sensitive C. latusorum symbiont. They are prevalent throughout the Indian and Pacific oceans, representing co-evolved and ecologically successful symbiotic relationships.

The scientists monitored several metrics, including skeletal growth, total mass increase, and calcification rates, to measure the coral’s growth. 

The team also assessed reproductive output and response to thermal stress to gauge the performance of these symbiotic pairings.

What the researchers learned 

“Our findings suggest that the association with D. glynnii doesn’t lead to noticeable tradeoffs,” said Turnham. “This pairing grows and reproduces just as efficiently as the more temperature-sensitive relationships.” 

Interestingly, the variance in performance between the two symbioses was only visible during experimental heating, underscoring the higher thermal tolerance of D. glynnii and its ability to confer heat resilience to the host corals.

Implications of the study

Even as coral reefs face an uncertain future under climate change, this study provides a valuable perspective on the resilience of these ecosystems. 

The research emphasizes the need to delve deeper into the symbiotic dynamics between corals and their algal partners, the understanding of which could potentially help predict the survival trajectory of these magnificent underwater cities.

“This study sheds light on the incredible biology of coral symbioses,” concluded Turnham. “By exploring their co-evolutionary history and employing advanced symbiont species recognition, we can make more nuanced predictions about the persistence of corals in an era of escalating climate change.”

The results are published in the journal Proceedings of the Royal Society B Biological Sciences. 

More about coral symbiosis

Coral symbiosis, an intriguing biological relationship, is a crucial element in the formation and sustainability of coral reefs. This symbiotic relationship primarily occurs between coral polyps, the animals that build coral reefs, and microscopic algae known as zooxanthellae, a type of dinoflagellate.

In a mutualistic symbiosis, both organisms benefit from their partnership. Here’s how it works:

Coral polyps provide a protected environment and compounds required by the algae for photosynthesis.

Zooxanthellae, in turn, produce oxygen, help remove wastes, and supply the coral polyps with organic products of photosynthesis. These compounds, including glucose, glycerol, and amino acids, are used by the coral for energy, growth, and reproduction.

Moreover, it’s the photosynthetic pigments in the zooxanthellae that give coral their striking array of colors. 

When corals are subjected to environmental stress, such as unusually high water temperatures, they may expel their symbiotic algae. This phenomenon results in coral bleaching, where the corals lose their vibrant colors and turn white. If the stress persists and the symbiosis isn’t reestablished, the coral can die, having lost a significant source of its food.

Coral-algal symbiosis is a very complex interaction and can vary among coral and algal species. Some symbionts are more heat-tolerant and enable the coral to withstand higher temperatures, which can be a significant advantage in a warming ocean due to climate change.

The research into different types of symbioses is ongoing and scientists are particularly interested in the potential resilience some symbiotic combinations could offer in the face of environmental change. 

The ability of corals to switch symbionts, known as symbiont shuffling, could potentially provide a mechanism for corals to adapt to changing conditions, although our understanding of this process is still limited.

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