Dr. Charlie Cornwallis

Lund University

Dr. Stuart West

University of Oxford

Dr. Nicole Hynson

University of Hawai'i Manoa

Dr. Alex Wegmann

The Nature Conservancy

Dr. Kabir Peay

Stanford University

Dr. Caroline Daws

Stanford University

Andressa Venturini

Stanford University

Dr. Leho Tedersoo

Tartu University

In the middle of the Pacific Ocean is Palmyra Atoll – a cluster of islands that are furthest from any land mass on earth.

The atoll is home to hundreds of thousands of seabirds, untouched coral reefs, the largest crab species on earth, and a fish population that is 44% sharks – the highest density of sharks globally.

Because the atoll is extremely remote and has no permanent inhabitants, it is a prime location to sample mycorrhizal biodiversity.

The remote and fragile nature of these islands means that scientists have to be extremely careful about keeping out non-native plants and insects. The SPUN crew had to get special permits and freeze our clothes every day before stepping out onto the small islands that surround the research base.

Coral reefs and mycorrhizal networks may seem far removed from one another. But researchers are now hypothesizing that complex dependencies among marine and terrestrial organisms may be driving underlying nutrient cycles across the isolated atoll.

The Nature Conservancy and US Fish and Wildlife have become increasingly interested in how fungi cycle nutrients to the atoll’s trees and how mycorrhizae may be key in restoring these remote rainforest ecosystems.

Despite being so remote, the island ecosystem is seriously threatened by two invasive species. Invasive coconut palms, which were introduced in the 1850s, and black rats from when the islands were used as a military base in World War II. To stop their spread and restore the atoll’s native rainforest ecosystem, over 1 million non-native palms have been removed and black rats have been fully eradicated.

By eradicating palms and rats, scientists are hoping that the islands’ unique native forests will rebound. Palmyra Atoll contains one of the last remaining stands of Pisonia grandis, a rainforest tree that is able to grow up to 30 meters tall, even in poor coral soils. These striking trees provide critical nesting sites for seabirds, such as boobies, noddies and terns, who leave behind massive amount of guano that fuels the growth of plants and marine life, including fish and the surrounding coral reef.

The ability of Pisonia trees to grow in coral rubble and provide nesting habitat appears to critically depend on mycorrhizal fungi.

How these fungi can survive under Pisonia trees is a mystery, because metabolic by-products of guano decomposition are usually toxic to fungi.

The expedition allows us to begin mapping the diversity of the mycorrhizal communities across the atoll in both pristine Pisonia rainforests and in areas that have been invaded by coconut palms.

When the rats were eradicated, the crab populations began to boom, including those of the massive coconut crab, who live to be over 60 years old, hunt, and can even climb trees. SPUN sampled for fungi on roots and in crab holes, which we hypothesize help spread the fungi to new roots. The crustaceans molt underground, and so soil sampling had to be done very carefully so as not to disturb them.

The water presented challenges as well. As we hiked between islands in waist high water, baby sharks would actually bump our legs out of curiosity. We have never had to worry about sharks while collecting fungi before.

Once the fungi are sequenced, these data will allow SPUN to understand how Pisonia survives in these harsh, isolated conditions, and how these native forests can be restored in areas that have been taken over by coconut palms. Understanding how mycorrhizal networks function in isolated, remote islands like Palmyra, gives us baseline data on their importance, and the role they may play in future restoration efforts.