Healthy hard corals in a shallow-water reef. Will this be a rare sight in the near future? Photo © K M Stiefel

A warming planet and acid oceans will radically transform marine ecosystems. How will our beloved reefs survive?

It was Friedrich Nietzsche who wrote: ‘What does not kill me makes me stronger.’ The aphorism offers good guidance during triathlon training and, often, through the tough times in your private life. Does it also apply to the ecosystems of Earth? We think it could. While plant and animal life would be better off without humanity’s climate-altering activities, our planet nonetheless shows resilience and the ability to adapt. That’s hardly an excuse for inaction in the face of the climate crisis, but it is certainly fascinating to predict how the major ecosystems of our home Earth will look in the intermediate and distant future, should atmospheric chemistry and climate change continue along the trajectories seen today. Our main interests are coral reefs, and we want to share some of our thoughts about their future based on our knowledge of unusually stressed ocean ecosystems and evolution. Corals will live on, though in very different form than the ones that thrive now; we also think that the food web will become simplified and dominated by different, much smaller fish.

How will the oceans transition to this new normal? It’s useful to think about coral reefs on a gradient, from the least to the most disturbed. On one end are the pristine, sun-flooded coral reefs well known and loved by scuba divers. This is where impressive towers of hard and soft corals cover almost every square metre of the seafloor, and countless species of colourful reef fish frolic among the intricate three-dimensional structures created by the corals. Because the multiple cracks and crevices between the coral fingers help hide smaller fish from the hungry mouths of predators such as groupers and sharks, the small fish never venture too far from their coral homes. Witnessing such a reef is an enthralling experience for the novice diver and expert marine biologist alike.

These ecosystems thrive in clear water, where nutrient levels are low. Despite these low levels, coral reefs still grow significant amounts of new biomass through two diverse nutrient pathways.

Corals are animals and, in the first pathway, they eat like any animal would. A coral is, simply put, a small sea anemone with a skeleton. Each coral takes the form of an individual polyp and lives in a colony with many other tiny polyps. Each individual coral is a tiny filter feeder that catches plankton (microscopic plants and organisms) for nutrition with its tentacles.

But corals have another means of nutrition as well: they harbour microscopic green algae within their tissue. Algae are aquatic plants, and through the process of photosynthesis, convert sunlight into biological energy, just like any green plant would. In this way, these symbiotic algae power the coral and themselves. Because of the coral’s dual ability to make a living, it’s called a holobiont, a ‘total organism’, incorporating the best of the animal and plant worlds. Especially for many hard corals, which build the most massive calcium carbonate skeletons, the energy derived from photosynthesis makes up the majority of the organism’s energy budget. Hence, clear water, transparent to sunlight, is of paramount importance for the flourishing of a classic coral reef.

Human interference has put all this at risk.[…]

Giant kelp in La Jolla, California. Will future generations still be able to witness this beauty? Photo © K M Stiefel

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