Over the past few decades, algae have been slowly edging corals out of their native reefs across the globe by blocking sunlight, wearing the corals down physically, and producing harmful chemicals. But in recent years, a new type of algal threat has surfaced in tropical regions like the Caribbean — one that spreads quickly and forms a crust on top of coral and sponges, suffocating the organisms underneath and preventing them from regrowing. Marine biologists report that peyssonnelioid alga crusts, or PACs, are expanding quickly across reefs worldwide, killing off corals and transforming entire ecosystems.