by Anna Waldeck
figures by Kaitlyn Choi

Picture an ocean reef. If you’re like me, then the first image that comes to mind is filled with colorful corals, clapping scallops, and exotic fishes swimming in clear, sunny waters; something like the beginning of Disney’s Finding Nemo. If you’ve been following the news recently, maybe you’ve thought of the ghostly pictures of bleached corals on the Great Barrier Reef. Today, ocean reefs are changing dramatically, and scientists have labeled this as a crisis that could quickly spiral out of control.

To predict where our reefs are headed, we need to understand the relationship between the reefs and the changing environment. To do this, scientists study both modern reef ecology and the geologic history of reefs. The advantage of studying the modern is that we can get up close with it. We can measure temperature, nutrient, and sunlight inputs and watch how the reef responds. Geologists do this also, but with less detail. They reconstruct what reefs looked like based on fossils preserved in the rock record. Not every part of the reef is preserved, so we lose information when a reef is fossilized. The advantage to using the rock record is that we can view environments over geologically-long timescales. Reefs have been around for billions of years, and modern-type ones for at least the last 5 million.

A reef deconstructed

Reefs come in many shapes and sizes, but all are loosely described by a simple framework. Geologists agree that reefs are two things: first, they are structures built from soft-bodied organisms that secrete a hard shell around them; second, they contain more species diversity than in the area of the ocean directly around them. The number of species on reefs is often high because reef-dwellers gain more from the mutually-beneficial relationships they share on the reef than they would living alone.

For much of Earth history, reefs have cycled carbon and other nutrients, something like this. Algae (like the zooxanthellae algae that live inside of corals) and bacteria trap light and/or chemical energy to produce nutrients for the rest of the reef. Primary consumers like corals and herbivorous fishes use the energy from the producers. Secondary consumers like carnivorous fishes eat the primary consumers, and so on. When life on the reef dies, it is consumed by filter feeders like oysters, worms, or sponges, to name a few (Figure 1). Reefs are incredibly efficient communities; little energy is wasted.

Once upon a reef

The first reefs that we know of date back to 3.5 billion years ago (Figure 2) and were built by bacteria. The bacteria formed films on the seafloor that trapped sediments, forming a layered, rock-like structure called a stromatolite. Stromatolites were globally successful for billions of years, during which Earth’s climate entered two prolonged periods of mostly glacial conditions followed by hothouse conditions. Incredibly, stromatolites survived throughout this period, and live to tell the tale.

When complex multicellular organisms evolved billions of years after the first stromatolite (Figure 2), some of them began to live in association with these reef structures. Each new organism on the reef filled a role: producer, consumer, filter feeder. The first sponges (filter-feeders) appeared as early as 635 million years ago, and then the first corals (primary consumers) some 542 million years ago. Modern type stony corals did not evolve until around 252 million years ago, in the aftermath of a global mass extinction that caused the death of 96% of marine species. Despite the mass extinction event, of which there have been five, reef communities in general survived. The types of species living on reefs evolved as some were wiped out by climate changes, yet the relationships (primary producers, consumers, filter feeders) between species remained loosely similar.

 The reefs respond

We see the effects of climate on reefs today in the warm, shallow waters of the tropics. Warming oceans have led to “coral bleaching”, which is a process in which coral polyps release the zooxanthellae symbionts that live inside of them, turn white, and subsequently die. Some corals have genes that enable them to withstand hotter temperatures without bleaching. This finding has led some scientists to think about the possibility of breeding corals that can survive in the warmer and more acidic oceans of the future.

Some reefs grow outside of the tropics, in what are perceived as suboptimal conditions. These are called ‘marginal reefs’, and they are less well-studied than their tropical cousins. Cold water coral reefs can be found at depths of more than 3,000 meters (Figure 3). In these dark waters, organisms cannot produce energy from sunlight, so instead corals prey on organic debris and plankton. Though they are limited by light, temperature, and nutrient levels, cold water reefs host a diversity of species and probably extend over large swaths of the ocean that we haven’t studied yet.

Another marginal reef was recently located where the Amazon River meets the Atlantic. The waters here are cloudy, which prevents organisms from getting much sunlight. Conditions are also more acidic because of the freshwater from the river, which makes it difficult to grow a coral skeleton. Nevertheless, scientists found a rich community of organisms, including algae, corals and a diverse population of bacteria (Figure 2).

A murky future

Not only are reefs shaped by their environment, but they actively shape the global environment. Reefs literally build a home around themselves. Importantly, this home protects our coastlines from large waves and erosion. Without reef ecosystems, many of the world’s shorelines would be vulnerable to the next passing storm.

Often called “the rainforests of the sea”, reefs have also shaped Earth’s environment by stabilizing the carbon cycle. Like giant trees, corals store large amounts of carbon that, if released, would have disastrous environmental consequences. A study in 2011 showed that long periods of instability in the carbon cycle followed two different mass extinction events. The instabilities, which they called “chaotic carbon episodes”, each lasted for millions of years after the extinction event. If our modern reef ecosystems collapse, we will likely face another “chaotic carbon episode”.

Will reefs still exist in 100 years from now? It’s likely that they will, at least according to the geologist’s criteria (1. a hard structure and 2. more diversity than the surrounding area). Select species of corals may continue to build reef structures, even in the face of increased CO₂ emissions. Just like the finding of heat tolerance in some corals, others are better adapted to deal with acidity, though growth will be slower and existing coral skeletons more prone to dissolve.

Reefs may also continue to host more species diversity than in the areas around them. Reefs have persisted for so long because organisms that cooperate have a better chance of survival. Whatever happens to the oceans in the future, it is likely that organisms will continue to cohabitate and develop relationships that help them deal with environmental stresses.

What will reefs in the future look like? It’s too difficult to say. We can’t draw direct comparisons from the past because climate today is changing faster than it ever has. Temperatures, ocean chemistry, and human impacts are all impossible to forecast with certainty. Our actions to preserve or exploit parts of the oceans will shape reefs in the future.

Anna Waldeck is a graduate student in the Department of Earth and Planetary Sciences at Harvard. Her research in the Johnston group focuses on past and present interactions between microbes and their environments.

For more information

To read more about the history of fossil reef study and how scientists started to recognize reefs in the rock record, see this article.

To find more coral reef facts and to learn about conservation strategies and how to get involved, see this website by NOAA (the National Oceanographic and Atmospheric Administration).

To learn more about ocean health in response to climate change, see these websites from the Smithsonian National Museum of Natural History and the Ocean Health Index.

Cover image: Drawings depicting the large diversity of extinct and extant corals from Kunstformen der Natur, by Ernst Haeckel.