A rusticle is a formation of rust similar to an icicle or stalactite in appearance that occurs underwater when wrought iron oxidizes. They may be familiar from underwater photographs of shipwrecks, such as the RMS Titanic and the German battleship Bismarck. Rusticles are created by microbes that consume iron.
The word rusticle is a portmanteau of the words rust and icicle and was coined by Robert Ballard, who first observed them on the wreck of the Titanic in 1986. Rusticles on the Titanic were the first investigated in 1996 by Roy Cullimore, based at the University of Regina in Canada. A new species of bacteria living inside the Titanic's rusticles called Halomonas titanicae was discovered in 2010. Henrietta Mann, the scientist who discovered the bacteria, has estimated that the Titanic may collapse circa 2030.
Rusticles can form on any submerged steel object and have been seen on other subsea structures such as mooring chains and subsea equipment. They form more rapidly in warmer climates and can form in water with little to no dissolved oxygen.
The rusticle consists of up to 35% iron compounds including iron oxides, iron carbonates and iron hydroxides. The remainder of the structure is actually a complex community of symbiotic or mutualistic microbes including bacteria Halomonas titanicae and fungi that use the rusting metal as a source of food, causing microbial corrosion and collectively producing the mineral compounds that form the rusticle as waste products.
Since rusticles are found on wrought iron rather than other ferrous metals, it is supposed that the microbes also use the sulfur and phosphorus impurities found in the metal.
Structurally, rusticles contain channels to allow water to flow through, and they seem to be built up in a ring structure similar to the growth rings of a tree. They are very delicate and can easily disintegrate into fine powder on even the slightest touch.
The outer surface of a rusticle is smooth red in appearance from the iron (III) oxide, while the core is bright orange due to the presence of crystals of goethite (a hydrated iron oxide).