Increasing environmental pollution in urban areas has been endangering the survival of carbonate stones in monuments and statuary for many decades. Numerous conservation treatments have been applied for the protection and consolidation of these works of art. Most of them, however, either release dangerous gases during curing or show very little efficacy. Bacterially induced carbonate mineralization has been proposed as a novel and environmentally friendly strategy for the conservation of deteriorated ornamental stone. However, the method appeared to display insufficient consolidation and plugging of pores. Here we report that Myxococcus xanthus-induced calcium carbonate precipitation efficiently protects and consolidates porous ornamental limestone. The newly formed carbonate cements calcite grains by depositing on the walls of the pores without plugging them. Sonication tests demonstrate that these new carbonate crystals are strongly attached to the substratum, mostly due to epitaxial growth on preexisting calcite grains. The new crystals are more stress resistant than the calcite grains of the original stone because they are organic-inorganic composites. Variations in the phosphate concentrations of the culture medium lead to changes in local pH and bacterial productivity. These affect the structure of the new cement and the type of precipitated CaCO3 polymorph (vaterite or calcite). The manipulation of culture medium composition creates new ways of controlling bacterial biomineralization that in the future could be applied to the conservation of ornamental stone.

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Conservation of ornamental stone by Myxococcus xanthus-induced carbonate biomineralization.

Microbially induced calcium carbonate precipitation (MICCP) is a naturally occurring biological process in which microbes produce inorganic materials as part of their basic metabolic activities. This technology has been widely explored and promising with potential in various technical applications. In the present review, the detailed mechanism of production of calcium carbonate biominerals via ureolytic bacteria has been discussed along with role of bacteria and the sectors where these biominerals are being used. The review discusses the applications of bacterially produced carbonate biominerals for improving the durability of buildings, remediation of environment (water and soil), sequestration of atmospheric CO2, filler material in rubbers and plastics etc. The study also sheds light on benefits of bacterial biominerals over traditional agents and also the issues that lie in the path of successful commercialization of the technology of Microbially induced calcium carbonate precipitation from lab to field scale.

/pdf/biomineralization-of-calcium-carbonates-and-their-engineered-elsofi30be.pdf

Biomineralization of calcium carbonates and their engineered applications: a review

Exopolysaccharides (EPSs) are an important class of biopolymers with great ecological importance. In natural environments, they are a common feature of microbial biofilms, where they play key protective and structural roles. As the primary colonizers of constrained environments, such as desert soils and lithic and exposed substrates, cyanobacteria are the first contributors to the synthesis of the EPSs constituting the extracellular polymeric matrix that favors the formation of microbial associations with varying levels of complexity called biofilms. Cyanobacterial colonization represents the first step for the formation of biofilms with different levels of complexity. In all of the possible systems in which cyanobacteria are involved, the synthesis of EPSs contributes a structurally-stable and hydrated microenvironment, as well as chemical/physical protection against biotic and abiotic stress factors. Notwithstanding the important roles of cyanobacterial EPSs, many aspects related to their roles and the relative elicited biotic and abiotic factors have still to be clarified. The aim of this survey is to outline the state-of-the-art of the importance of the cyanobacterial EPS excretion, both for the producing cells and for the microbial associations in which cyanobacteria are a key component.

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Role of cyanobacterial exopolysaccharides in phototrophic biofilms and in complex microbial mats

The presence and deteriorating action of micro-organisms on monuments and stone works of art have received considerable attention in the last few years. Knowledge of the microbial populations living on stone materials is the starting point for successful conservation treatment and control. This paper reviews the literature on cyanobacteria and chlorophyta that cause deterioration of stone cultural heritage (outdoor monuments and stone works of art) in European countries of the Mediterranean Basin. Some 45 case studies from 32 scientific papers published between 1976 and 2009 were analysed. Six lithotypes were considered: marble, limestone, travertine, dolomite, sandstone and granite. A wide range of stone monuments in the Mediterranean Basin support considerable colonization of cyanobacteria and chlorophyta, showing notable biodiversity. About 172 taxa have been described by different authors, including 37 genera of cyanobacteria and 48 genera of chlorophyta. The most widespread and commonly reported taxa on the stone cultural heritage in the Mediterranean Basin are, among cyanobacteria, Gloeocapsa, Phormidium and Chroococcus and, among chlorophyta, Chlorella, Stichococcus and Chlorococcum. The results suggest that cyanobacteria and chlorophyta colonize a wide variety of substrata and that this is related primarily to the physical characteristics of the stone surface, microclimate and environmental conditions and secondarily to the lithotype.

/pdf/biodiversity-of-cyanobacteria-and-green-algae-on-monuments-5oyxjmlxyc.pdf

Biodiversity of cyanobacteria and green algae on monuments in the Mediterranean Basin: an overview

The role of microorganisms and biofilms in the breakdown and dissolution of quartz and glass

Over the last decade, the concept of stone decay being caused solely by one or two processes has been revised in favour of a more holistic notion that many operate together, are synergistic and possibly cyclical or seasonal. Certain of these processes may become predominant over others at particular times of the year or at specific periods in the history of the stone. Many researchers from different disciplines now agree that biological deterioration is of significance, and cross disciplinary work is becoming the rule rather than the exception. This short introduction gives an overall view of the principal mechanisms important in stone biodeterioration. The main groups of micro-organisms found on stone and their role in stone biodeterioration are reviewed. The final section discusses decay processes which can be brought about by many different groups of micro-organisms living together as a complex microbial community at the stone surface.

An introduction to stone colonizing micro-organisms and biodeterioration of building stone

The occurrence of a sub-aerial microbial community dominated by a species ofTrentepohlia, a red-orange pigmented filamentous green alga, growing on a 13th century sandstone building in the Scottish borders appears to be associated with unusual stone decay. Decay, in the form of extensive surface spalling and grain dissagregation over large areas of external stonework, are coincident with the distribution of the algal community on the north facade of the building. Comparison of S.E.M. images taken of fresh intact stone and spalled stone from decayed zones suggests that the mechanisms of decay may, in part, be a physical process which results in quartz crystals becoming separated. The investigations carried out revealed little evidence of chemical processes which may be involved in the decay. Large filaments ofTrentepohlia are clearly visible growing vertically between separate grains of stone and the amount of highly absorbent mucilage present makes the friable dry spall become soft and gelatinous on wetting. It is possible that the observed decay may have been created through the mechanical action of wet-dry cycling of mucilage and expansive stress from the growth of algal filaments.

Investigations of decayed sandstone colonised by a species ofTrentepohlia

A Scanning Fluorescence Microscope system for quantifying and mapping biological growth on stone is described. The system uses 488 nm light from an Argon-ion laser to excite the 685 nm fluorescence from the Chlorophyll-a in the algae and uses this as the means of mapping the biological material. The measured 685 nm fluorescence is shown to increase linearly with the quantity of algae on the stone surface. Application of the system to study the effectiveness of biocide treatment for the control of algae on stone is reported.

Melanie S. Jones

Papers

An introduction to stone colonizing micro-organisms and biodeterioration of building stone

Investigations of decayed sandstone colonised by a species ofTrentepohlia

In-situ monitoring of biological growth on stone