RESEARCH ARTICLE Weathering of a Roman Mosaic—A Biological and Quantitative Study on In Vitro Colonization of Calcareous Tesserae by Phototrophic Microorganisms Addolorata Marasco 1 *, Simona Nocerino 2 , Gabriele Pinto 3 , Antonino Pollio 3 , Giorgio Trojsi 4 , Antonino De Natale 3 1 Department of Mathematics and Applications, University of Naples Federico II, Complesso Universitario di Monte Sant’Angelo, Via Cintia, 80126 Napoli, Italy, 2 Via C. Ravizza, 20149 Milan, Italy, 3 Department of Biology, University of Naples Federico II, Complesso Universitario di Monte Sant’Angelo, Via Cintia, 80126 Napoli, Italy, 4 Università degli Studi Suor Orsola Benincasa, Via Santa Caterina da Siena, 80135 Napoli, Italy * marasco@unina.it Abstract The potential impact of cyanobacteria and microalgae on the weathering of calcareous tes- serae from a Roman mosaic of the II Century CE has been followed through in vitro experi- ments. Laboratory tests were carried out by inoculating mosaic tiles with single strains of Cyanobacteria or Chlorophyta to evaluate the roles of pioneer phototrophic microrganism on the resulting architecture of biofilms. The interaction between tesserae and strains was assessed at the whole substratum and micrometer scales, by image analysis and Confocal Laser Scanning (CLS) microscopy, respectively. The biofilm surface coverage on each tes- sera varied from 19% (Fischerella ambigua) to 97% (Microcoleus autumnalis). Cyanobacte- ria showed a better growth on calcareous tesserae, whereas the only green alga attaining a superficial coverage higher than 50% was Coelastrella rubescens. CLS microscopy evi- denced two different types of spatial arrangement of the phototrophic organisms on the tes- serae, that were defined as compact or porous, respectively. In the first one was measured a reduced number of empty spaces between cells or filaments, whereas in the second type, a reticulate texture allowed the presence of numerous empty volumes. The colonization pro- cesses observed are an intrinsic characteristic of each strain. We have proposed a coloniza- tion index I C as a sensible tool to describe, in a quantitative way, the pioneering attitude of each photosynthetic microorganism to colonize lithic substrates under laboratory conditions. Introduction Mosaics are one of the most typical works of art in Roman architecture. Built with small squares called tesserae and made with various materials (marble, glass paste and stones), they PLOS ONE | DOI:10.1371/journal.pone.0164487 October 26, 2016 1 / 19 a11111 OPEN ACCESS Citation: Marasco A, Nocerino S, Pinto G, Pollio A, Trojsi G, Natale AD (2016) Weathering of a Roman Mosaic—A Biological and Quantitative Study on In Vitro Colonization of Calcareous Tesserae by Phototrophic Microorganisms. PLoS ONE 11(10): e0164487. doi:10.1371/journal.pone.0164487 Editor: Brett Neilan, University of New South Wales, AUSTRALIA Received: April 15, 2016 Accepted: September 26, 2016 Published: October 26, 2016 Copyright: © 2016 Marasco et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Funding: A.M. has been partially supported by local Grant 2015-2016 “Analysis of Complex Biological Systems”. Competing Interests: The authors have declared that no competing interests exist.