Pitcairn Islands Expedition Photos: Strange and Beautiful Algae

By Andrew Howley & Kike Ballesteros

With all our emphasis on charismatic fish and stunning coral formations, boring old algae tends to get skipped over by most observers of the underwater world. Being out here in the South Pacific with an algae expert though, it doesn’t take long to be won over by these intriguingly important life forms.

Kike Ballesteros of the Centre d’Estudis Avançats de Blanes, CSIC, is NG Explorer-in-Residence Enric Sala’s long-time mentor and collaborator. He is also the master of all things algal for the Pitcairn Islands expedition, and he agreed to enlighten me about the wonders of algae.

Algae Great and Small
First off, there are two major kinds of algae in a coral reef. One is the tiny microalgae living inside the coral itself and providing a food source (and coloration) for these ancient animals. The other is macroalgae, which includes seaweed, and can be either soft and fleshy or hard and crusty (calcareous).

Basis of the Food Chain
In shallow temperate areas huge plains of seaweed and other macroalgae are the main components of the seascape. Here in the tropics the main component is coral. But don’t be fooled—macroalgae are still hard at work making life at the coral reefs possible.

Algae, both soft and crusty, provide the only food source for plant-eating fishes (such as parrotfishes, chubs, damselfishes or surgeonfishes) and invertebrates (some sea urchins, small crustaceans, and snails). Together with the microalgae that live inside the coral colonies, these algae provide the basic food and energy source much of what lives on the reef.

Builders of the Reef
Crusty algae are also active builders of the reef structure itself.

Red algae (such as Hydrolithon and Lithophyllum in the gallery above) produce limestone which cements together the coral pieces into a solid chunk. Without this ever-rising base, the corals themselves would not be able to build up vertically.

The algae then is what allows the reef to build up towards the ocean surface and cause waves to break off shore, at once forming the lagoon and protecting the interior island’s beaches from the full power of the ocean.

Where Tropical Beach Sand Comes From
Finally, other crusty species of green algae (Halimeda in the gallery) are important sand producers. Beaches along the reef and sand flats under the water are made up of many components, including coral debris and the skeletons of several marine invertebrates (like molluscs, sea urchins, and tiny shelled foraminiferans) but far and away, the biggest contributors are broken up Halimeda.

So next time you’re sipping mai-tais on a tropical beach, raise your glass to algae, the workhorses that make paradise possible.


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Andrew Howley is a longtime contributor to the National Geographic blog, with a particular focus on archaeology and paleoanthropology generally, and ancient rock art in particular. In 2018 he became Communications Director at Adventure Scientists, founded by Nat Geo Explorer Gregg Treinish. Over 11 years at the National Geographic Society, Andrew worked in various ways to share the stories of NG explorers and grantees online. He also produced the Home Page of nationalgeographic.com for several years, and helped manage the Society's Facebook page during its breakout year of 2010. He studied Anthropology with a focus on Archaeology from the College of William & Mary in Virginia. He has covered expeditions with NG Explorers-in-Residence Mike Fay, Enric Sala, and Lee Berger. His personal interests include painting, running, and reading about history. You can follow him on Twitter @anderhowl and on Instagram @andrewjhowley.