'Blood Rain' Mystery Solved: Traveling Microalgae Turned Local Waters Red

Fountains and tanks throughout villages of Zamora, Spain, appear to be filled with "blood rain." While the village scenery resembles that of Biblical plagues, the water was simply turned red by a green microalgae introduced to the area during last Autumn's rainfall, say researchers from the University of Salamanca who took a closer look at the phenomenon and found that the microalgae, Haematococcus pluvialis, specifically turns a reddish color when stressed. However, they are still unsure where the algae originated from, according to their news release.

When the water originally turned red in 2014, a resident named Joaquín Pérez from the village Ayoó de Vidriales took samples and monitored the blood rain throughout last autumn and winter. After noticing small particles along the edges and bottoms of the sample containers, Pérez sent them to scientists at the University of Salamanca.

"The reddish staining is caused by Haematococcus pluvialis, a freshwater green microalgae that is capable of synthesizing a red carotene pigment called astaxanthin when in a state of stress," Javier Fernández-Lozano, geologist and co-author of the study, explained in the release.

Using light microscopy, researchers observed the algae's life cycle. They concluded that under unfavorable circumstances, the algae forms cysts, or sacs filled with a liquid secretion, that ultimately turn them red. While this microalgae is known to exist in bodies of water in the Northern Hemisphere, their presence in environments such as Zamora is rare, researchers noted.

"What is interesting is that we have confirmed that this species does not appear in Sanabria Lake or in the reservoirs surrounding the village of Ayoó de Vidriales," Fernández-Lozano added, "so they must be coming from some other place."

In order to solve this part of the mystery, researchers used meteorological data, including wind patterns, to track corresponding days of blood rain. While they could not pinpoint an exact source, researchers suggest prevailing westerly winds during the autumn of 2014 could have carried the microalgae to areas of the Iberian Peninsula. However, since the microorganisms are known in North America, they could have even come from there, too.

Haematococcus pluvialis is not toxic and commonly used in the pharmaceutical industry for its antioxidant properties, as well as in food production to give salmon and trout a deeper red color. The motorcycke manufacturer Yamaha also recently discovered it could reduce carbon dioxide emissions in factories by injecting the gas into pools filled with the microalgae.

The recent findings were published in the Spanish Royal Society of Natural History Journal.

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