Algae have been gradually displacing corals from their native reefs across the globe by blocking sunlight, physically wearing down the corals, and producing harmful chemicals. However, a new type of algal threat has emerged in tropical regions, such as the Caribbean, that spreads quickly and forms a crust on top of coral and sponges, suffocating the organisms underneath and preventing them from regrowing. These crusts are called peyssonnelioid alga crusts (PACs), and they are expanding rapidly across reefs worldwide, killing off corals and transforming entire ecosystems.
A team of marine biologists led by Peter Edmunds of California State University, Northridge, reports in the journal Current Biology that PACs are an ecological catalyst that could hasten the global demise of corals on reefs under accelerating climate change. PACs are an ecological surprise that has arrived late to the scene of widespread ecosystem degradation of coral reefs in the Anthropocene epoch. Within this seascape, PACs may serve as an ecological catalyst that could hasten the global demise of corals on reefs under accelerating climate change.
One of the most challenging aspects of the emerging PAC threat is that the algae can be incredibly hard to identify. There are an estimated 48 different species of PACs, and they can be difficult to tell apart from harmless species like seaweeds because their morphology varies significantly when it comes to color, shape, and structure.
The researchers predict that PACs will eventually dominate reefs worldwide because they appear to be much more resilient than other related species to the impacts of climate change, including ocean acidification and extreme weather events like hurricanes. The recent increases in cover and distribution of PACs on tropical reefs demonstrate their capacity to accelerate the restructuring of tropical benthic habitats.
Between 2012 and 2019, PACs took over 47%-64% of the shallow reefs in St. John, US Virgin Islands. To slow the spread of PACs and protect the reefs, the authors stress the importance of detecting PAC outbreaks as early as possible. They also encourage researchers to investigate the transformative impact of PACs on benthic communities and learn more about tropical reef resilience against PAC outbreaks.
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.