Stone Conservation: An Overview of Current Research (2010)

Stone Decay 23have appeared determined to see bacteria and nothing else. A troublingnumber of authors have noted high numbers of bacteria in decayingstone, in comparison to low numbers in sound stone, and have concludedthat the bacteria cause the decay. However, an alternative explanationcould be that decayed stone presents a preferred habitat for the bacteria.Bacteria that attack stone chemically fall into two groups: autotrophicbacteria derive their carbon from carbon dioxide (CO 2 ), and mayderive their energy from light (photolithotrophs) or from chemical redoxreactions (chemolithotrophs). Heterotrophic bacteria, by contrast, utilizeorganic compounds on the stone to derive their carbon. Autotrophic bacteriainclude those that are capable of oxidizing sulfur and nitrogen compoundsto produce sulfuric acid and nitric acid, respectively. They are onemore means, therefore, by which air pollutants such as sulfur dioxideand nitrogen oxide are turned into sulfates and nitrates. This underlinesthe difficulty of separating out the individual causes of stone decay; severaldifferent factors may play integral roles in the overall decay process.The question remains of whether bacteria or catalyzing metal compounds,for example, are the main routes of sulfate production. However,if oxidation by both bacteria and metal compounds is rapid bycomparison with the rate at which sulfur dioxide arrives at the stonesurface, then the arrival rate will be the rate-determining step, not theroute taken. The synergistic effects of air pollution and biofilm formationhave been researched, with the finding that there is strong evidencethat biofilms enhance the absorption of air pollutants (Young 1996;Mansch and Bock 1998).Heterotrophic bacteria produce chelating agents and organicacids that are weaker than the inorganic acids produced by the sulfuroxidizingand nitrifying bacteria. They have received comparatively littleattention, but their role in deterioration is well established nonetheless(Lewis, May, and Bravery 1988; Saarela et al. 2004; Gorbushina 2007;Maruthamuthu et al. 2008).Recent work on bacteria has helped to quantify the effect theyhave on the dissolution of limestone, with one example showing a twofoldincrease in the laboratory dissolution rate compared to control limestonesamples (McNamara et al. 2005). Bacteria typically produce slimeor extracellular polymeric substances (EPS) as part of a complex biofilmmade up of polysaccharides, water, and proteins that has been shown tochange the dissolution rate and dissolution pit morphology on samples oflimestone (Perry et al. 2004). Damage from bacteria in the field has beendescribed by McNamara and others (2006) and Mansch and Bock (1998).Biodeterioration studies of important cave painting sites such asAltamira have resulted in recent advances in understanding. Researchershave found that cyanobacteria and algae (phototrophs) and networks ofheterotrophic bacteria increase stone deterioration through their metabolicproducts, biomediated dissolution, and mechanical alteration, suchas scaling (Cañaveras et al. 2001). As expected, the control of moisture,food, and light levels appears to be the most effective prevention method(Dornieden, Gorbushina, and Krumbein 2000; Zammit et al. 2008).Environmental control of cave environments has proven to becomplex and controversial, as in the cave at Lascaux, where efforts to