9.2 Reef ecosystems through time: from stromatolites to modern coral reefs

Reef ecosystems have evolved dramatically over Earth's history. From ancient stromatolites to modern coral reefs, these structures have shaped marine environments for billions of years. Understanding their development provides insights into past climates, ocean chemistry, and the evolution of marine life.

Today's coral reefs are the product of a long evolutionary journey. Scleractinian corals, which emerged in the Triassic, now dominate reef-building. Their symbiosis with algae enables them to thrive in nutrient-poor waters, creating biodiversity hotspots that support countless marine species.

Early Reef Builders

Stromatolites: Earth's Oldest Reefs

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• Stromatolites are layered structures formed by cyanobacteria (blue-green algae) that trap and bind sediment particles
• Oldest known stromatolites date back to ~3.5 billion years ago, providing evidence for the earliest life on Earth
• Stromatolites dominated reef ecosystems for billions of years until the evolution of grazing and burrowing organisms in the Cambrian Period (~541 million years ago)
• Modern stromatolites are rare but can still be found in hypersaline environments (Shark Bay, Australia) where grazing and burrowing organisms are less abundant

Archaeocyathids: The First Animal Reef Builders

• Archaeocyathids were the first animal reef builders, appearing in the Early Cambrian Period (~541-509 million years ago)
• These ancient sponge-like organisms had a conical or cup-shaped skeleton made of calcium carbonate
• Archaeocyathids formed extensive reefs during the Cambrian but went extinct by the end of the period, possibly due to competition with other reef-building organisms or changes in ocean chemistry

Rugose and Tabulate Corals: Paleozoic Reef Builders

• Rugose corals, also known as horn corals, were solitary or colonial corals that appeared in the Ordovician Period (~485-444 million years ago) and became dominant reef builders in the Devonian Period (~419-359 million years ago)
• Tabulate corals were exclusively colonial corals that also appeared in the Ordovician and formed extensive reefs alongside rugose corals
• Both rugose and tabulate corals had a skeleton made of calcium carbonate and were important reef builders throughout the Paleozoic Era
• Rugose and tabulate corals went extinct during the Permian-Triassic mass extinction (~252 million years ago), possibly due to changes in ocean chemistry or competition with other organisms

Modern Coral Reefs

Scleractinian Corals: The Architects of Modern Reefs

• Scleractinian corals, also known as stony corals, are the primary reef builders in modern oceans
• These corals have a hard skeleton made of calcium carbonate (aragonite) and can be solitary or colonial
• Scleractinian corals first appeared in the Triassic Period (~252-201 million years ago) but did not become dominant reef builders until the Jurassic Period (~201-145 million years ago)
• Modern coral reefs are built primarily by scleractinian corals and are found in shallow, warm, and clear waters in the tropics and subtropics

Coral-Algal Symbiosis: The Foundation of Modern Reefs

• Many scleractinian corals form a symbiotic relationship with single-celled algae called zooxanthellae
• Zooxanthellae live within the coral tissue and provide the coral with nutrients through photosynthesis
• In return, the coral provides the zooxanthellae with a protected environment and compounds necessary for photosynthesis
• This symbiotic relationship allows corals to grow and build reefs more efficiently in nutrient-poor tropical waters

Carbonate Platforms: The Foundation of Coral Reefs

• Coral reefs often develop on carbonate platforms, which are large, flat, submarine structures composed of calcium carbonate
• Carbonate platforms provide a stable substrate for coral growth and can be formed by a variety of processes, including the accumulation of coral skeletons and other calcareous organisms over time
• As corals grow and die, their skeletons accumulate and cement together, forming the complex three-dimensional structure of a coral reef
• Carbonate platforms and coral reefs can grow and keep pace with rising sea levels, creating thick sequences of limestone in the geologic record

Reef Characteristics and Importance

Reef Zonation: Adapting to Environmental Gradients

• Coral reefs exhibit distinct zones characterized by different coral species, growth forms, and associated organisms
• Reef zonation is influenced by environmental factors such as water depth, wave energy, light availability, and temperature
• The reef crest is the shallowest and most wave-exposed zone, dominated by robust, wave-resistant coral species (Acropora palmata)
• The reef flat is a shallow, protected zone behind the reef crest, characterized by a diverse array of coral species and growth forms
• The reef slope is the deepest zone, where light availability decreases with depth, and coral growth forms transition from branching to platy or massive

Biodiversity Hotspots: Coral Reefs as Oases of Life

• Coral reefs are among the most diverse ecosystems on Earth, supporting an estimated 25% of all marine species
• The complex three-dimensional structure of coral reefs provides a wide range of habitats and niches for various organisms
• Coral reefs support a diverse array of fish, invertebrates, and other marine life, many of which are found nowhere else on Earth
• The high biodiversity of coral reefs is attributed to factors such as high productivity, complex habitat structure, and long evolutionary history

Mass Extinctions and Reef Gaps: The Rise and Fall of Reef Ecosystems

• Throughout Earth's history, reef ecosystems have been impacted by mass extinction events, resulting in "reef gaps" in the fossil record
• The Permian-Triassic mass extinction (~252 million years ago) eliminated over 90% of marine species and resulted in a prolonged reef gap, with reefs not recovering for several million years
• The Cretaceous-Paleogene mass extinction (~66 million years ago) also had a significant impact on reef ecosystems, with the extinction of rudist bivalves and a decline in coral diversity
• Following each mass extinction, reef ecosystems eventually recovered and were rebuilt by new groups of organisms, demonstrating the resilience and adaptability of these ecosystems over geologic time
• Modern coral reefs face numerous threats, including climate change, ocean acidification, and overfishing, which could lead to a new reef gap in the future