Palmyra Atoll in the North Pacific is one of the most remote island systems on Earth. A native rainforest tree on the island performs a critical ecological service by providing nesting sites for thousands of seabirds, whose guano fuels the surrounding coral reefs. But a new study revealed that this entire cycle depends on an invisible partner: Symbiotic mycorrhizal fungi.
Researchers mapped the fungal diversity across the atoll and discovered the native pisonia (Pisonia grandis) trees have a 100% with a specific genus of fungi called Tomentella — meaning the trees depend on the fungi to survive. These fungi capture intense pulses of nitrogen and phosphorus from bird guano that would otherwise wash into the ocean. This relationship was present in every tree the team sampled.
“Most ectomycorrhizal fungi struggle in extremely nutrient-rich soils, but the Tomentella fungi associated with Pisonia appear to be adapted to the high phosphorus levels created by seabird guano,” study co-author Alex Wegmann told Mongabay over email. “This suggests a long evolutionary partnership between the fungi, the trees, and the massive seabird colonies that shape these atoll ecosystems.”
The discovery has major implications for the ongoing effort to restore Palmyra’s native forests by removing 1.5 million invasive coconut palms. The study found that Tomentella abundance drops off sharply when there are more than 250 meters (820 feet) away from a pisonia tree. Therefore, natural regeneration might fail in large areas cleared of coconut palms, because the necessary fungi aren’t present in the soil, the study authors suggested.
Wegmann said it’s possible that conservationists will need to “inoculate” soils with fungi to ensure the success of reforestation. “Especially in areas far from existing Pisonia forests where Tomentella becomes much less common or in atoll forest systems where Pisonia has been absent for long periods due to legacy coconut palm agriculture or other impacts,” he said. “However, we still need additional field experiments to determine whether fungal inoculation significantly improves seedling survival and growth.”
The study also revealed that Palmyra is a hotbed for globally rare, and potentially new, species of fungi that have never been recorded in worldwide databases. These fungi were even found colonizing aerial roots hanging 1.5 meters (4.92 feet) in the air, suggesting they might be dispersed by wind or birds.
Furthermore, the study said the atoll’s hundreds of thousands of land crabs, some with a leg span more than three feet across, according to Wegmann, were found to be “ecosystem engineers” for this microbial world. By excavating and mixing the soil, crabs significantly increase total fungal richness within their burrows.
“Atoll forests may contain unique microbial communities found nowhere else on Earth,” Wegmann said. “Protecting this hidden biodiversity is important because these microbes can play critical roles in ecosystem health, resilience, and forest regeneration .”
Banner image: Pisonia grandis in Palmyra Atoll. Image credit to Society for the Protection of Underground Networks.
This story first appeared on Mongabay
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