Bad News for Coral Reefs: Toxic Seaweed Adds Another Threat

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Bleached coral (Photo: Peter Arnold / Getty Images)

If you’re already daydreaming about your winter getaway to the tropics as the weather gets crisper and gloomier, consider adjusting your plans to include some swimming among the world’s coral reefs – not just because they offer an unparalleled panorama of underwater life, but also because they’re disappearing in the midst of coastal development. And while some of the biggest contributors to coral disappearance are the sewage and agricultural runoff that encourage algae and seaweed to grow on the seabed and outcompete corals, it turns out biology may not be on our side either: even without the added competition, corals don’t stand a chance against seaweeds containing chemicals that effectively poison corals upon contact, according to new research.

Led by ecologist Douglas B. Rasher of the Georgia Institute of Technology, scientists placed eight common seaweed species in direct contact with corals and found some worrying results. The seaweed caused visible coral bleaching in 50% of the corals, suppressed photosynthesis in 79%, and actually caused death in 33% of the examined algal-coral interactions they looked at – and it doesn’t look like the coral bites back to get rid of the algae, since the team found the algae unharmed in all cases. The coral damage is likely a result of toxic chemicals transferred by algal-coral contact, rather than by physical damage from the seaweeds: when Rasher’s team used inert seaweed mimics in the same test, they found no evidence of damage from shading or abrasion by the seaweeds.

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Ordinarily we’d think that natural selection and evolution would sort this kind of biological hiccup out, since both algae and coral are foundational organisms in marine ecosystems and must somehow learn to coexist. But with so many local and global threats driving unnatural seaweed blooms around the world’s coral reefs, it’s hard not to be concerned. The World Resources Institute cites sewage as the most widespread pollutant in developed coastal areas, which is bad news for corals trying to avoid getting poisoned by noxious plants. The nutrients in sewage are like Muscle Milk for seaweeds, beefing them up and helping them spread around so they can release the toxic chemicals that kill our corals. Ironically, most of the regions with the richest and most beautiful coral reef ecosystems – Southeast Asia, the Caribbean, and the Pacific – also have the worst sewage treatment systems: these areas discharge 80-90% of their wastewater untreated, according to the World Resources Institute. The sediment contributed by soil erosion, which is caused by deforestation and overgrazing by livestock, doesn’t help matters either. Add the 130 million tons of fertilizer that go into crops each year and end up in waterways to feed the lethal seaweed, and we’ve got ourselves a problem.

Rasher’s results may help answer the question of why coral recovery is so difficult, especially on reefs with lots of macroalgae. Even marine protected areas, which promote herbivorous creatures that eat up the seaweed and alleviate some of the pressure on corals, don’t always help; some seaweed species have toxins that also drive away herbivores. Given this constraint, the team suggests building marine reserves and establishing fishing bans that include species capable of consuming the chemically rich plants that kill corals. Sounds great, but it would be even better if we improved some of our land management policies and practiced more efficient agricultural methods – biology working against us doesn’t absolve us of our responsibility to lessen, rather than exacerbate, the problem. We’d do well to let evolution get it right on its own.

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Tara Thean is a TIME contributor. Find her on Twitter at @TaraThean. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.