The rain in Spain was blood-red last year, at least in certain regions. Residents were disturbed to find crimson water pouring from the sky in the province of Zamora and, as of today, science can only partially explain why. Geologists have identified what made the rain red, but are still uncertain why it hit this particular region.

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A study published in the Spanish Royal Society of Natural History Journal documented the presence of a certain type of algae in the rain, called Haematococcus pluvialis. This microalgae is found in freshwater and is usually a greenish hue, yet when under stress it can turn red by synthesizing a pigment called astaxanthin. What caused the algae to become stressed in this case? The researchers cite how the organisms are “especially sensitive to variations in the intensity, quantity and quality of light, which can affect the distribution, size and morphology of its cells.”

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H. pluvialis is typically found in North America and some spots in Europe, yet has never made its way to Spain before. Scientists are still interested in tracing the algae’s original source – likely either wind or a body of water. Luckily for the residents of Ayoó de Vidriales and other villages where the rain hit, the algae is not harmful when ingested. In fact, there is some evidence the red microorganisms may improve cognitive functioning. The arrival of “blood rain,” in this case, is not as ominous as it seems, but mysterious all the same.

Via IFLScience

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