Species of red algae discovered outside Papahānaumokuākea marine monument creates more questions

A species of red algae first discovered by University of Hawaiʻi at Mānoa researchers smothering coral reefs in Papahānaumokuākea Marine National Monument has now been detected outside the marine monument for the first time.

The research team detected DNA matching Chondria tumulosa more than 2,000 miles away in Majuro Atoll in the Republic of the Marshall Islands in the eastern Central Pacific Ocean.

Researchers originally called it a “nuisance alga” because it was identified as being introduced from another region.

“Our discovery in Majuro suggests that C. tumulosa may be more widespread than we previously thought,” said researcher and doctorate candidate Patrick Nichols, a graduate student in the University of Hawaiʻi at Mānoa School of Life Sciences. “This could have significant implications for coral reef ecosystems across the Pacific, potentially requiring broader monitoring efforts and a more comprehensive understanding of the alga’s distribution and ecological impact.”

C. tumulosa — named in 2020 by University of Hawaiʻi researchers — earned a reputation for smothering coral reefs in Papahānaumokuākea after being first detected in 2016.

It forms dense mats that blanket corals and other native organisms.

The red algae species was only observed spreading throughout the three northwestern-most atolls of Papahānaumokuākea until now, first at Manawai (Pearl and Hermes), then Kuaihelani (Midway Atoll) and Hōlanikū (Kure Atoll).

A new detection outside Hawaiʻi caught researchers by surprise.

“It was really unexpected,” Nichols said. “We were just kind of doing this routine surveying of all the [environmental DNA] that we collected across the Pacific, and when it lit up from Marshall Islands, it was unbelievable.”

The discovery was only possible because the algae species’ DNA was detected in water samples obtained earlier this year.

Nichols led the research team in 2022 as it developed the genetic test to detect the red algae species’ environmental DNA, or eDNA, across vast areas without the need for extensive in-water surveys.

C. tumulosa is extremely difficult to identify visually, so the two-step process Nichols and his team created combines molecular surveillance with targeted fieldwork was developed.

The bio-monitoring technique screens water samples for the alga’s genetic “fingerprint.”

Nichols and fellow School of Life Sciences graduate students Jimmy Fumo and Keolohilani Lopes Jr. armed with the new evidence traveled in November to Majuro to conduct an extensive survey, which included the site of the initial positive eDNA detection.

They were able to visually confirm the presence of C. tumulosa using microscopy and additional genetic sampling.

The algae observed at Manawai is forming thick mats that suffocate coral reef communities, displacing native flora and fauna. In Majuro, however, the species was observed in much lower abundance, forming small patches at some sites.

This growth pattern resembles what was discovered in 2023 at Hōlanikū (Kure Atoll), where C. tumulosa grew cryptically and was difficult to identify.

“Even though we have now observed C. tumulosa outside of [Papahānaumokuākea], we can’t be sure of its native status,” Fumo said. “Its relatively low abundance in Majuro could mean it is either an emerging invader in the region or a long-established species being naturally controlled by herbivorous reef fishes or other ecological factors.”

With other potential sightings throughout the region, researchers plan to extend surveys beyond the Marshall Islands to better understand the algae’s distribution and ecological role.

They hope to explore the historical connections between populations in Hawaiʻi and elsewhere to determine if the species has spread across the North Pacific or has been established in the region.

“While this discovery raises concerns about the spread of C. tumulosa, it also showcases the incredible potential of new technologies and collaboration,” Nichols said. “By working together on new tools, we can better understand and mitigate the risks posed by nuisance species.”