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HERMITAGE Castle near Hawick in the Scottish Borders is under siege from algae that turn the stonework orange. Historic Scotland, which looks after the castle, had always attributed the orange hue to the remains of coloured rendering. Now Melanie Jones and Rachel Wakefield of the Masonry Conservation Group at the Robert Gordon University in Aberdeen have shown that strains of the alga Trentepohlia are to blame. They say that while lichens, bacteria and fungi have long been known to degrade stonework, there is growing evidence that algae do not simply discolour stone but damage it as well. “It grows between the grains and seems to lever them apart,” says Wakefield. “We think the algae are quite butch.” The filaments are unusually thick and “armoured” with very strong cellulose walls. The researchers say that novel algicides are urgently needed to halt the damage.