Several villagers in the Zamora province of Spain were shocked to see rain water in tanks and fountains turn red, like blood. Experts from the University of Salamanca found the red hue was caused by freshwater green microalgae called Haematococcus pluvialis which created red pigment when stressed.

The red pigment created by the microalgae is called astaxanthin, formed during a state of stress. A change in the water's salt level can cause a microalgae present to become stressed. Using light microscopy, a team of researchers led by Javier Fernández-Lozano from the University of Salamanca, followed a unicellular algae's complete life cycle.

During several life stages, the algae either move using their whip-like structure called a flagella or form lumps or cysts, which usually happens in harsh conditions. This stressful state leads to the production of a red pigment called astaxanthin, causing them to turn red. In sunlight, the red pigment becomes even more vibrant.

The organism that caused the stressful event had been discovered but scientists still cannot determine where it came from. Haematococcus pluvialis had only been present in water bodies along the European Atlantic coast and North America. Its appearance in Spain is its first documented appearance in a peri-Mediterranean climate such as the one in Zamora.

"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 so they must be coming from some other place," said study lead author Fernández-Lozano.

Analyzing meteorological data revealed a link to the prevalence of westerly winds which could have affected Iberian Peninsula's north-western area in the autumn of 2014 when another blood-rain incident was recorded. The researchers were not able to pinpoint the exact origin site but suggested that they could have originated from North America.

Haematococcus pluvialis is used in the pharmaceutical industry as an antioxidant. In the fish farming industry, the microalgae are used as food for trout and salmon which gives the fish a more vibrant red color. Flamingos' pinkish hue can also be traced to food sources laced with the microalgae.

The study was published in the Spanish Royal Society of Natural History Journal and can be accessed via Researchgate.

Photo: Holley And Chris Melton | Flickr