Organic Green Corrosion Inhibitors Derived from Natural and/or Biological Sources for Conservation of Metals Cultural Heritage

In the last decade, there has been an increase in research related to green corrosion inhibitors for conservation of metals cultural heritage to help promote sustainable practices in the field that are safe, environmentally friendly, and ecologically acceptable. The most common are organic substances derived either from natural and/or biological sources: plant extracts and oils, amino acids, microorganisms, and biopolymers. The chapter will provide a review of these substances as corrosion inhibitors for metals conservation, by discussing the state-of-the-art research to date, with a special focus on cysteine. Most of the research has focused on the examination of such inhibitors on metal coupons with or without corrosion products using electrochemical techniques or weight-loss measurements to determine their effectiveness. Some of these studies have also considered the conservation principles for practice, i.e., reversibility of the treatment and the visual aspect of the modification of the treated metal surface. However, before such green inhibitors can be routinely applied by conservators, more research is required on their application to real artefacts/monuments using in situ corrosion measurements. Furthermore, given that the composition of a green inhibitor is highly dependent on its extraction process, research must also involve identifying the specific adsorption models and involved mechanisms to ensure reproducibility of results.

Microbiota and Biochemical Processes Involved in Biodeterioration of Cultural Heritage and Protection

The world cultural heritage sites face new challenges for an effective protection and management because of destruction and damage initiated by both natural and anthropogenic causes. Fresh rock and sandstone surfaces of buildings are quickly colonized and covered by a layer of microorganisms, including phototrophs, lithotrophs, and heterotrophs to form a biofilm that alters the local conditions of the stone surfaces, especially under the favorable tropical climate conditions for autotrophic microorganisms and plants. Biofilms had been studied with indigenous or pure cultures of isolated microorganisms, but the selective ones that contribute to deterioration of the cultural heritage cannot be confirmed easily. Currently, high-throughput sequencing and metegenomics analyses are capable of obtaining microbial community and composition in great depth, but they also suffer from similar weakness unable to identify the culprits in the community. With these as background, this article presents a different approach by focusing on the biochemical processes and the responsible microorganisms involved to reveal the destruction processes for management and protection. Among these different functional groups of microorganisms, lichens are known as pioneering rock-decomposing microorganisms, and both sulfur-oxidizing bacteria and fungi participate in the decomposition of sandstone via sulfur cycling and initiation of salt attack of the stone afterward, resulting in defoliation and cracking of stone. Other microorganisms including ammonia-oxidizing bacteria and archaea, especially the latter, have been recently detected on sandstone monuments providing evidence on the new organisms involved in the deterioration of cultural heritage and buildings. In addition, fungi can colonize the surfaces of the matured biofilms and play a new role in the removal of them, which has a potential biotechnological application in conservation of cultural heritage. The new proposed approach by focusing the microorganisms with identified biochemical function is more productive than a description of the community composition and assembly when assessing cultural heritage biodeterioration, and this provides basic and useful information for effective protection strategies and management.

Molecular-Based Techniques for the Study of Microbial Communities in Artworks

Thanks to the revolutionary invention of the polymerase chain reaction and the sequencing of DNA and RNA by means of “Sanger sequencing” in the 1970th and 1980th, it became possible to detect microorganisms in art and cultural assets that do not grow on culture media or that are non-viable. The following generation of sequencing systems (next generation sequencing, NGS) already allowed the detection of microbial communities on objects without the intermediate step of cloning, but still most of the NGS technologies used for the study of microbial communities in objects of art rely on “target sequencing” linked to the selectivity of the primers used for amplification. Today, with the third generation of sequencing technology, whole genome and metagenome sequencing is possible, allowing the detection of taxonomic units of all domains and kingdoms as well as functional genes in the produced metagenome. Currently, Nanopore sequencing technology is a good, affordable, and simple way to characterize microbial communities, especially in the field of Heritage Science. It also has the advantage that a bioinformatic analysis can be performed automatically. In addition to genomics and metagenomics, other “-omics” techniques such as transcriptomics, proteomics, and metabolomics have a great potential for the study of processes in art and cultural heritage, but are still in their infancy as far as their application in this field is concerned.

Novel Antibiofilm Non-Biocidal Strategies

Subaerial biofilm (SAB) formation on cultural heritage objects is often considered an undesirable process in which microorganisms and their by-products, e.g., enzymes and pigments, cause damage or alteration to a surface. Since biofilms are widespread phenomena, there has been a high demand for preventive and control strategies that resist their formation or reduce their negative effects once formed. Up to date, the main strategy to control biofilms has been the use of biocides. Because of their intrinsic properties, biocidal products can pose risks to humans, animals, and the environment. In this chapter, the authors call “green” only those alternative strategies to biocides able to prevent/control biofilms but that do not kill microorganisms, i.e., irrespective of the use of natural compounds. Here, we describe some of the methods that are most commonly used to test the effectiveness of antibiofilm compounds with multiple-species biofilm model systems. A unified terminology and well described protocols and guidelines are still required to compare and test the effectiveness of traditional or novel compounds against biofilms retrieved on heritage surfaces.

Extreme Colonizers and Rapid Profiteers: The Challenging World of Microorganisms That Attack Paper and Parchment

Microorganisms form the backbone of life on Earth. Over billions of years, they have colonized and shaped every possible niche on the planet. Microbes have modelled both the land and the sea, and have created favourable conditions for multicellular organisms to thrive in. Our understanding of how microbial diversity is distributed across natural environments and how microbes affect ecosystems is constantly evolving as public databases are set up and new techniques based on massive sequencing are developed. The microbiome found in a particular anthropogenic environment is generally much less complex than those found in natural ones: there is less competition and the main actors are often linked to survival mechanisms regulated by a few limiting factors. Despite this simplicity, it is very difficult to link cause and effect when seeking to identify the role of individual organisms. In the case of biodeterioration of paper and parchment, even when analysing the individual components of a simple phenomenon, it is not always easy to understand the mechanisms at play. Works of art are unique objects and the elements that determine the arrival and establishment of one or more microorganisms and the direction that the biodeterioration process takes are always different. In some cases, however, there are common denominators and predictable mechanisms. The variables that come into play are examined below.

Protection and Consolidation of Stone Heritage by Bacterial Carbonatogenesis

For millennia, artists and architects around the world used natural stone for the carving of sculptures and the construction of monuments, such as Roman, Greek, and Maya temples, the European cathedrals, and the Taj Mahal, just to name a few. Currently, the survival of these irreplaceable cultural and historical assets is under threat due to their continued degradation caused by various biotic and abiotic weathering processes that affect not only the aesthetic appearance of these structures, but also their durability and survival. The natural precipitation of calcium carbonate minerals by bacteria has been proposed for conservative interventions in monument restoration. This chapter reviews the application of biomineralization by (indigenous) bacterial carbonatogenesis as a novel technology for the protection and consolidation of altered ornamental materials. Carbonatogenesis is based on the ability of some bacteria to induce calcium carbonate precipitation. Laboratory and in situ results support the efficacy of bacterial carbonatogenesis, since remarkable protection and consolidation are achieved on the surface and in depth, without alterations in color or porosity, and without fostering the development of microbiota that could be harmful to the stone material. A discussion on the advantages of this novel biotechnology is provided. Challenges and future work on bioconsolidation of stone artifacts are also outlined.

Bioremoval of Graffiti in the Context of Current Biocleaning Research

Some microorganisms can be used as bioremediation agents, in biocleaning treatments, to remove undesired sulphates, nitrates and organic matter from cultural heritage surfaces. Graffiti materials (mainly spray paints) are now included in the list of materials that can be biocleaned, with studies on this topic being initiated just over 5 years ago. Research on the bioremoval of graffiti is continuing and on a promising track. This chapter reports a critical analysis of studies of the bioremoval of graffiti carried out in recent years, which are compared with similar studies of the removal of salts (mainly nitrates and sulphates) and organic matter conducted in the last thirty years. Likewise, the present challenges and ways of overcoming them are addressed towards developing a complete protocol for the use of bioremediation to remove graffiti, with particular emphasis on the use of the method for cleaning facades and buildings.

Microbial Growth and its Effects on Inorganic Heritage Materials

Cultural heritage objects composed of inorganic materials, such as metals and stones, support microbial life. Many factors affect the growth of microorganisms: moisture, pH, light, temperature, nutrients. Their colonization relates closely to the nature of the substrata as well as to the characteristic of the surrounding environment. This chapter contains an overview of the complex relationships among microbial growth, materials, and the environment. It emphasizes issues on bioreceptivity of stones and the factors influencing biological colonization, focusing on the biological alteration of inorganic heritage objects and on the agents of biodeterioration. It outlines the effect of biofilms and lichens in terms of degradation of substrata and includes a discussion on an important topic, the bioprotection of stones by biofilms and lichens. In summary, this chapter aims to discuss these issues and review the recent literature on (i) biofilms and lichens colonizing inorganic materials, (ii) the limiting factors of this colonization, (iii) the deteriorative aspects, and (iv) the protective effects of the colonization.

Green Mitigation Strategy for Cultural Heritage Using Bacterial Biocides

The microbiota present in cultural heritage objects, made by diverse inorganic and organic materials and inserted into particular environment, represents a complex and dynamic ecosystem composed by bacteria, cyanobacteria, fungi, algae and lichens, which can induce decay by biological mechanisms. To control the microbial growth several methods are being applied such as mechanical and physical processes and chemical biocides. However, these methods have several weaknesses like be dangerous to handle, material incompatibility or produce environmental and health hazards. Therefore, the identification of effectively biodeteriogenic agents and the design of mitigation strategies directed to these agents without prejudice to historical materials, to the environment and to operators, taking into account the microbial community’s dynamics, is an important challenge to control biodeterioration of cultural heritage. Bacteria, in particular Bacillus spp. are worth for the creation of new green biocides solutions because they produce a great variety of secondary metabolites including ribosomally and non-ribosomally synthesized antimicrobial peptides, known to possess antagonistic activities against many biodeteriogenic fungi and bacteria. The discovery of new safe active compounds and green nanotechnology for direct application in cultural heritage safeguard can in a close future contribute to potentiate a new generation of biocides and safe sustainable methods for cultural heritage.

The Role of Microorganisms in the Removal of Nitrates and Sulfates on Artistic Stoneworks

This chapter will focus on the role of microorganisms in the removal of nitrates and sulfates on artistic stoneworks. The main groups of microbes and their metabolisms involved in bioremoval methods for the preservation and protection of cultural artifacts are reported. The aim is to offer a comprehensive view on the role and potentiality of virtuous microorganisms in the biocleaning and bioremoval of black crusts and salts altering CH stoneworks. We highlight the importance of the use of the selected microorganisms and the adoption of adequate carriers for the anaerobic metabolism of nitrate and sulfate reducers to be applied on the altered stone surfaces. The following characteristics of the delivery system are of great importance: the ability to guarantee water content for microbes, the absence of toxicity for the environment, no negative effects to the stone surfaces, easy to prepare, to apply, and to remove from different stone surfaces at the end of the treatment. We report an overview of the last 30 years on the biocleaning processes including diagnostic studies of the alterations, the assessment of associated risks, the effectiveness and efficacy of the proposed method, and the evaluation in terms of economic and environmental sustainability.