Along a seashore in Hong Kong yesterday, a vibrant blue glow was seen emanating from the water. Beautiful photographs show the shore glimmering, with the lights of the city sparkling in the background.

But this idyllic setting is potentially toxic.

The luminescence is an algal bloom created by Noctiluca scintillans, nicknamed “sea sparkle.” When disturbed by currents or waves, the bloom glows. “It looks like algae and can act like algae. But it’s not quite,” wrote Seth Borenstein of the Associated Press. “Noctiluca is a type of single-cell life that eats plankton and is eaten by other species.”

So why is it toxic? Such blooms are caused by farm pollution. “The plankton and Noctiluca become more abundant when nitrogen and phosphorous from farm run-off increase,” Borenstein wrote, “Noctiluca’s role as both prey and predator can eventually magnify the accumulation of algae toxins in the food chain.”

The beauty of Noctiluca scintillans is lethal. Such is the case in the Arabian Sea, where a biological dead zone the size of Texas is filled with the species. The resulting “emerald swirls” are large enough to been seen from space:

Pretty, right?

But Noctiluca scintillans can be lethal to plants and animals. Scientists think the dead zone was created when the species killed off native algae and compromised the fish population. Gwynn Guilford of Quartz wrote: “Few animals can survive ‘dead zones’ of oxygen-poor water. As the scientists discovered, N. scintillans thrives in these conditions…. And once a dead zone sets in, it’s hard for the ocean to recover.”

Noctiluca scintillans also forms red tides, a colorful red stain seen in Sydney and Hong Kong.