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16 March 2023 3 min read

In August 2022, scientists on our research vessel (RV) Investigator travelled to the Great Barrier Reef to study an unusual lifeform lurking 20-50 metres below the surface. Looking like underwater impact craters from meteor strikes, scientists were seeking to unravel the mysteries of living seafloor circles affectionately known as ‘green donuts’.

This voyage, called Project HALO (Halimeda Bioherm Origins, Function and Fate), aimed to increase our understanding of how the green donuts form. In research just published, scientists share the knowledge they've gained about these enigmatic structures and their role as habitats within one of the most critical but vulnerable biodiversity hotspots on Earth.

Halimeda algae create a leafy green coating over donut-shaped sediment deposits left by their ancestors. Image: Mardi McNeil.

The recipe for green donuts

First, bioherms are the technical name for green donuts. Bioherms are mound-like formations. The accumulation of the calcareous (made of calcium carbonate) bodies of living organisms creates them. In this case, countless generations of an algae called Halimeda make up their composition. The green colour of the donuts comes from the leafy growth of living Halimeda atop the pile of previous generations. This gives the donuts a vibrant green frosting of life.

Second, the most extensive deposits of the Halimeda bioherms known on Earth exist in the Great Barrier Reef. They cover more than 6000 square kilometres of seafloor in the region. Each bioherm can be several hundred metres wide.

However, unlike the simple ingredients that go into making a normal donut, these enigmatic green namesakes are poorly understood. Scientists aren't entirely sure what controls their distribution and development.

Seafloor mapping and sub-bottom imagery of the green donuts. Image: Victorien Paumard and Carra Williams.

Studying the (w)hole box and dice

Led by scientists from the University of Sydney, the team used RV Investigator’s advanced multibeam sonar to map the donuts in higher resolution than ever before, down to less than one metre resolution. This revealed spectacular shapes and patterns not previously visible.

At the same time, other sonar systems helped create maps of the texture and composition of the seafloor. In addition, scientists used corers and dredges to sample the sediment that make up the donuts, and study the life in and on them. They also deployed drop cameras from the ship to get an up-close view of the creatures that call the donuts home.

Scientists found that the donuts were hotspots of biodiversity. During the voyage, they collected more than 1000 biological specimens from these living green meadows of the seafloor. This included life both on the surface and burrowing into the sediments of the donuts.

Some of these species are likely new to science.

Invertebrates such as urchins, sea stars and nudibranchs were found living on the green donuts. Image: Matthew Clements, Stefano Borghi, Monique Webb, and Maria Byrne.

Life sprinkled on the seafloor

Nestled among the leafy Halimeda fronds, scientists found juvenile sea urchins and brittle stars. This indicates that the habitat is important as a nursery and for recruitment of these animals. Additionally, small sea stars collected are likely to be new species.

Scientists found populations of the solitary coral Heteropsammia cochlea. These unusual ‘walking corals’ provide habitat for crabs and worms that move the coral around and keep its surface free of sediment. They also observed the bêche-de-mer sea cucumber Holothuria fuscogilva living in the green meadows. The Convention on International Trade in Endangered Species lists this species as highly endangered.

Unfortunately, the surveys also identified the crown-of-thorns starfish Acanthaster sp. This invasive species is a devastating coral predator and a threat to reef ecosystems and diversity.

Scientists have already learnt much from the voyage data. However, many more mysteries await unravelling.

Why a 130-kilometre-long section in the middle of their distribution doesn't contain the donuts is one of the big questions scientists are looking to answer. Scientists will use the data collected by RV Investigator during this voyage to help answer this question. This includes oceanographic and geoscientific data.

A corer touches down on the surface of a green donut to probe the secrets of its sediments. Image: Luke Nothdurft.

Donut impacts that won’t glaze you over

Importantly, this research contributes to the World Heritage Convention’s Outstanding Universal Value designation of the Great Barrier Reef Marine Park. It is increasing our knowledge of bioherm distribution, geomorphology and habitats in new levels of detail.

Furthermore, these observations enable a better understanding of why these fascinating features occur where they do. This provides a more holistic understanding of these inter-reef (between reef) habitats. Significantly, this knowledge will contribute to improving the future management of these habitats and the Great Barrier Reef Marine Park.

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