How to protect coral reefs in today's oceans? - Lessons from corals that built reefs 330 million years ago

Produced by: Science Popularization China

Author: Yao Le and Liu Yun (Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences)

Producer: China Science Expo

Coral reefs play an important role in the global marine environment. They are the largest ecosystem in the ocean and are known as the "tropical rainforest" of the ocean (Bouchet, 2006). Coral reefs can provide shelter for marine animals. Currently, about 100,000 species are known to live in them (Reaka-Kudla, 1997; Plaisance et al., 2011). In addition, the organisms attached to coral reefs also provide food and medicine resources for humans. More than 100 countries and more than 450 million people on the earth are closely related to the lives of coral reefs (Pandolfi et al., 2011).

However, the current survival status of coral reefs in the ocean is not optimistic. How can humans protect the increasingly degraded coral reefs? Scientists have found the answer from reef-building corals 330 million years ago.

Current status of marine coral reefs

In recent years, global warming and human activities have led to increased rainfall, accelerated melting of land glaciers, enhanced chemical weathering on land, and increased input of nutrients into the ocean, which in turn has led to eutrophication of seawater, increased turbidity, and increased hypoxia and acidification, resulting in the death of marine reef-building corals and the collapse of coral reef systems.

In addition to marine environmental pollution caused by human factors, the natural factor of terrestrial debris input has also had a serious impact on marine coral reefs, which is often accompanied by the death or morphological changes of reef-building corals.

Terrigenous debris refers to the debris formed by the weathering of rocks on land, such as sand and clay. Its composition is relatively complex, and is mainly characterized by the enrichment of silicon (Si) and aluminum (Al). Under the influence of factors such as rainfall and flourishing plants, terrigenous weathering has intensified, and the input of terrigenous debris has further increased, resulting in an increase in the content of nutrients in the ocean. Although human emissions today can also cause eutrophication of seawater, this effect cannot be reflected in the geological period, while the impact of terrigenous debris input has long existed and spans ancient and modern times.

Figure 1. Relationships between terrigenous debris input and reef-building corals in Southeast Asia and Australia

(Image source: McLaughlin et al., 2003)

Evolution of Coral Reefs through Geological History

The phenomenon of increased terrigenous debris input has also occurred in geological history, but the current understanding of how reef-building corals respond to terrigenous debris input is still unclear. The so-called reef-building corals refer to corals that develop on the ocean continental shelf and can grow in situ. They can form coral framework rocks with positive uplift landforms on the seabed, that is, coral reefs.

During the Middle and Late Mississippian (Vessian-Serpukhovian) periods, terrestrial plants flourished and a significant Hercynian orogeny occurred. These two factors together led to the intensification of terrestrial chemical weathering and the increase in the input of terrigenous debris and nutrients, which in turn led to a sharp cooling of the global climate and a drop in sea level.

The Late Paleozoic Ice Age is the longest ice age in the Phanerozoic Eon, consisting of a series of multi-episode glacial-interglacial periods, characterized by the development of glacial deposits in the southern hemisphere's Gondwana continent. The latest research found that in the late Visian period (Asbian-Brigantian transition), the oxygen isotopes of brachiopod shells underwent a significant positive excursion, indicating that the ancient seawater temperature during this period was significantly lower, which may represent the beginning of the main episode of the Late Paleozoic Ice Age. At the same time, the marine coral reef system also collapsed, accompanied by a decrease in benthic biodiversity (Yao et al., 2022).

Therefore, studying the morphology and size of reef-building corals in the Middle and Late Mississippian can provide new insights into the evolutionary trends of reef-building corals under the influence of terrigenous debris input today.

Figure 2. Diversity changes and evolutionary patterns of marine reef-building corals and overall organisms during the Mississippian

(Photo source: Nanjing Institute of Paleontology)

Deep time reef-building coral research

Recently, Yao Le, associate researcher of the Late Paleozoic Research Team of Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, and Lin Wei, assistant researcher, collaborated with Markus Aretz, professor of Toulouse III University, France, David J. Bottjer, professor of University of Southern California, USA, and Wang Xiangdong, professor of Nanjing University, to conduct a systematic study on the morphology, size and terrigenous debris input of reef-building corals in the Middle and Late Mississippian.

The research team counted the individual size parameters (single skeleton diameter, transverse plate band diameter and number of septa) of the globally distributed reef-building coral fossils Aulina rotiformis and Lithostrotion decipiens in four different sedimentary phase sections of the Serpukhovian period in China, namely Yashui in Guizhou, Malanbian in Hunan, Wangjiacun in Anhui and Jianshanzi in Inner Mongolia. The team also studied the elemental content of the reef-building corals and surrounding rocks. The relevant research results were published in the Proceedings of the Royal Society B.

The study found that the coral individuals in the Yashui section were the largest, while the coral individuals in the Jianshanzi section were the smallest. The diameters of the transverse plates of Aulina rotiformis and Lithostrotion decipiens decreased by 31% and 23%, respectively.

Among them, the reason why the coral individuals from the Yashui section to the Jianshanzi section gradually become smaller may be related to the increase in terrestrial debris input - the debris covers the surface of the coral polyps, which may cause them to die directly, or require them to consume extra energy to remove the debris, thereby reducing the energy supplied to the coral growth, further causing the coral individuals to become smaller.

Figure 3. Changes in the diameter of individual carcasses, transverse plate diameters, and number of septa of the reef-building corals Lithostrotion decipiens and Aulina rotiformis in different sedimentary facies during the Late Mississippian

(Photo source: Nanjing Institute of Paleontology)

The team members also studied the burial characteristics of the coral Lithostrotion decipiens and found that the preservation of individual Lithostrotion decipiens from the Yashui section to the Wangjiacun section gradually deteriorated, and the content of terrigenous debris (mud and quartz) in the coral surrounding rocks gradually increased, accompanied by an increase in the content of silicon (Si), aluminum (Al), and phosphorus (P). The coral surrounding rocks in the Wangjiacun section contain rich microorganisms and lack metazoan development.

Microbial respiration can lead to hypoxia and environmental acidification, thus inhibiting coral growth. Combining the phenomenon of coral individuals gradually becoming smaller from the Yashui section to the Wangjiacun section and the changes in the content of coral surrounding rock debris, it can be found that terrigenous debris input is the main reason controlling the shrinkage of coral individuals.

Figure 4. Microfacies and element distribution characteristics of reef-building coral Lithostrotion decipiens and its surrounding rocks in different sedimentary facies sections of the late Mississippian

(Photo source: Nanjing Institute of Paleontology)

In addition, the research results also show that in the Serpukhovian period, from the shallow-water open carbonate phase of the South China Plate, the carbonate-clastic rock transition phase, to the shallow-water clastic rock phase, the reef-building coral individuals gradually became smaller, while the content of Si, Al, and P elements in the coral surrounding rocks increased significantly.

On a long scale, based on the individual size data of reef-building corals Lithostrotion decipiens and Siphonodendron pauciradiale from the Middle to Late Mississippian in China, Western Europe and North Africa, it can be found that the number of reef-building corals decreased significantly in the late Visian period (Asbian-Brigantian transition), which is consistent with the enhanced terrestrial weathering and increased terrigenous debris input associated with the beginning of the main episode of the Late Paleozoic Glacial Age.

Figure 5. Changes in the morphology and size of reef-building corals Lithostrotion decipiens and Siphonodendron pauciradiale during the Middle and Late Mississippian in relation to terrigenous input, sea surface temperature, low-latitude sea level, and mid- and high-latitude glacial records during this period.

(Photo source: Nanjing Institute of Paleontology)

Response of modern marine reef-building corals to terrigenous debris

The researchers collected clump-like complex corals Acropora tenuis, A. millepora and Pocillopora acuta with a diameter greater than 20 cm from the Great Barrier Reef of Australia today, and then transplanted them into an outdoor water flow system to maintain a living environment similar to that of marine coral reefs (such as water temperature of 27°C, aragonite substrate, symbiotic coralline algae, etc.), and raised coral larvae in it.

In the experiment, researchers selected three types of corals aged 3-6 months and exposed them to water bodies with four different suspended sediment contents (0, 10, 30 or 100 mg l−1) and nutrient contents, and observed the coral growth for 40 days (Figure 6).

The experimental results showed that suspended sediments greatly reduced the survival rate of A. millepora, and the number of A. millepora larvae that died was proportional to the sediment concentration. In addition, although the number of A. tenuis and P. acuta larvae did not decrease significantly, their larval growth size was reduced to less than half or stopped growing (Figure 7).

Figure 6. Survival status of modern marine corals Acropora tenuis, A. millepora and Pocillopora acuta in waters with different suspended sediment contents

(Image source: Humanes et al., 2017)

Figure 7. Correlation between the growth size of larvae of three coral experiments and sediment content

(Image source: Humanes et al., 2017)

This suggests that exposure of larvae of some coral species to suspended sediments has an energetic cost, which means that increased sediment input will have a certain impact on the growth of specific species populations and reef-building corals (Humanes et al., 2017).

When the input of terrigenous debris increases, the energy consumption of corals for activities such as breathing, removing sediments and repairing the ecological environment will also increase. In addition, organic matter will cause sediments to form large particles, further increasing the energy consumption of corals. In addition, the increase in nutrients will promote the prosperity of microorganisms, and the organic matter metabolized by them will form an oxygen-deficient and acidic environment in local areas that is not conducive to the survival of corals. Many of the above factors will lead to slow growth or death of corals.

Implications for current marine coral reef protection

The Late Paleozoic Ice Age is the only period since the prosperity of plants and animals on Earth with a concentration of carbon dioxide in the atmosphere close to that of modern times. Therefore, studying the evolution of marine life during the Late Paleozoic Ice Age can provide reference and inspiration for exploring the evolution of today's marine ecosystems.

Through a systematic study of the changes in the morphology and size of reef-building corals about 330 million years ago and their response to terrigenous debris input, scholars have revealed in time and space the evolutionary trend of the onset of the Late Paleozoic Ice Age and the increase in terrigenous debris and nutrient input leading to the smaller individuals of reef-building corals .

Faced with the input of terrigenous debris and changes in paleoenvironment during the Late Paleozoic Ice Age, some corals suffered extinction, but some others overcame the deteriorating environment and survived. Why?

The study found that these corals that have overcome environmental changes may have strong phenotypic plasticity, that is, in the face of environmental changes, they can better adapt to new environmental conditions by changing their individual size. In other words, the reef-building corals that survived 330 million years ago could adapt to the paleoenvironmental changes that accompanied the Late Paleozoic Ice Age by becoming smaller.

In addition, the study provides inspiration for the protection measures and future development trends of today's marine coral reefs from the perspective of long-term biological evolution: reef-building corals with strong phenotypic plasticity may be more adaptable to today's environmental changes such as terrigenous debris input and water hypoxia. Therefore, in the future protection of coral reefs, we should avoid "band-aid" repairs that only focus on quantity and speed, and give priority to reef-building corals with strong phenotypic plasticity for restoration, so as to achieve high-quality protection of the largest ecosystem in today's oceans - coral reefs.

(Note: This research was jointly funded by the National Natural Science Foundation of China, the Youth Innovation Promotion Association of the Chinese Academy of Sciences, and the Strategic Priority Research Program of the Chinese Academy of Sciences (Class B).

Editor: Ma Yiqun

References:

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