Crust-forming algae are suffocating corals with the potential to dominate reefs

Scientists have identified a rapidly expanding threat to global coral reefs – the spread of peyssonnelioid alga crusts (PACs). These crust-forming algae have been suffocating corals and sponges, potentially hastening the decline of coral reef habitats in the face of climate change.

According to a new study, these algae have been increasingly displacing corals from their natural environments. The research, led by Peter Edmunds from California State University, Northridge, has brought to light the severe implications of the spread of PACs.

Startling increase in PACs 

Edmunds and his team report a startling increase in PACs across tropical reefs, indicating a significant ecological shift that could transform entire marine ecosystems.

PACs are particularly insidious due to their ability to form a hard crust over corals and sponges, effectively smothering them. This not only kills the existing organisms but also prevents new growth, disrupting the ecological balance of reef systems. 

Rapid proliferation 

The study reveals that in regions such as the Caribbean, these algal crusts are spreading at an alarming rate, with the potential to accelerate the degradation of coral reefs already weakened by human activity and climate change.

“PACs are an ecological surprise arriving late to the scene of widespread ecosystem degradation of coral reefs in the Anthopocene epoch,” wrote the researchers. “Within this seascape, PACs may serve as an ecological catalyst that could hasten the global demise of corals on reefs under accelerating climate change.”

The rapid proliferation of PACs has been linked to their resilience against climate change effects, including ocean acidification and severe weather events like hurricanes, which have been increasingly prevalent.

Potential to dominate reefs

In St. John, US Virgin Islands, it was found that between 2012 and 2019, PACs had overtaken 47 to 64 percent of the shallow reefs. This stark increase is indicative of the potential for these algae to dominate tropical reefs globally.

“PAC outbreaks appear to be a rapidly developing crisis on coral reefs throughout the world, where they are exploiting the ecological legacies of decades of reef degradation,” wrote the study authors

The difficulty in combating the spread of PACs lies in their identification. With an estimated 48 species varying in color, shape, and structure, PACs are challenging to distinguish from benign algae, complicating efforts to manage outbreaks.

Addressing the crisis

To address the crisis, the researchers emphasize the necessity of early detection of PAC outbreaks and advocate for further research into the effects of these algae on marine benthic communities. 

The experts call for a concerted effort that combines ecological, phylogenetic, and multi-omic studies, which would be bolstered by adequate funding and a collaborative approach. A key starting point, as noted by the research team, is the development of methods for quick and accurate identification of PAC species that are driving their global proliferation.

“Suitable progress in these areas will only be obtained by a well-funded synergy of ecological, phylogenetic, and multi-omic studies that must start with the ability to quickly and accurately identify the taxa driving the global advance of PACs,” wrote the researchers.

As PACs continue to spread, the study warns of their capacity to restructure tropical benthic habitats rapidly, a prospect that could lead to irreversible damage to coral reef ecosystems and the biodiversity they support. 

The study is published in the journal Current Biology.

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