Coral reefs are under pressure from hurricanes, pollution, bleaching and global warming, and scientists have now confirmed the extent of the threat from an aggressive alga.
The algae, known as peyssonnelid algal crusts (PAC), are colonising reefs in the Caribbean at such an aggressive rate that they are interfering with the reef’s natural ecosystem, according to new research.
Marine biologists from the University of Oxford, California State University Northridge and the Carnegie Institution for Science who have been studying the issue for four years describe the extent of the threat in the Scientific Reports.
The golden-brown, crust-like alga is rapidly overgrowing shallow reefs, and has been described as an “ecological winner” by Dr Bryan Wilson of the University of Oxford’s department of zoology.
“It aggressively occupies any vacant space on the reefs, rapidly overgrows and kills live corals, prevents free-swimming coral larvae from settling on the benthos and becoming adult colonies, and is unaffected by the regular destructive hurricanes that sweep through the area,”he says.
‘It is also seemingly resistant to grazing by fish, and as far as we know, is only fed upon by a single creature, the black spiny urchin (Diadema antillarum),”he said.
The spiny urchin was once abundant in the Caribbean, but was effectively wiped out in the 1980s by a mysterious disease, he says.
“Our research has initially looked into the microbiology of PAC and compared it with that of close relatives (the crustose coralline algae, or CCA) which are known to stimulate the recruitment of coral larvae to reefs,”Dr Wilson says.
A key finding was that the PAC alga manages to inhibit the growth of beneficial marine bacteria which otherwise produce chemical compounds that attract coral larvae to the seafloor.
This means that reefs colonised by the alga are unlikely to host corals again.
The alga is described as having a “dark brown and dirty orange veneer” which stands out among the white sands and light greens, pinks, yellows and other colours that make up the reef.
The scientists say it is unclear if PAC is made up of one algal species or several, and they don’t know what is causing its rapid spread – but describe it as an “alarming trend”
They say that the next stage of their research will be to unravel the alga's “complex physiological mechanisms for this ecological success”, through studying its genome. Ultimately, they say they “hope to find ways to mitigate against this new threat”.
