Microbial communities on painted wet and dry external surfaces of a historic fortress in Niterói, Brazil

- Biofilms on painted dry (F1) and wet (F2) walls differed; potential deteriogens in both.
- Wet green biofilm predominantly Chloroflexi with few algae; diatoms in F1 and F2.
- Main colonisers of dry wall Alphaproteobacteria; major fungus Khuskia.
- Dry grey biofilm had greater genus richness and contained halophilic archaea.
- First time next generation sequencing used in paint biofilm wet vs dry analysis.

Bacteria, algae and fungi colonising dry (F1) and wet (F2) white painted walls in the fortress of Santa Cruz da Barra, Niterói, Brazil, were detected using field emission-scanning electron microscopy (FE-SEM) and next generation DNA sequencing (NGS) techniques. Major bacterial phyla Operational Taxonomic Units (OTUs) detected were Chloroflexi in the wet green biofilm F2 (38.85% compared with 7.56% in F1) and Proteobacteria in the dry grey biofilm F1 (57.17% compared with 28.69% in F2). Diatoms were detected at both sites by FE-SEM, but only at F1 by NGS. More algae and cyanobacteria were identified at F1, and this was the only biofilm containing Archaea, possibly related to the high level of salt efflorescences at this site. Although thinner, F1 biofilm showed considerably higher genus richness than the wet biofilm, F2. The thickness and appearance of the biofilms did not correlate with their genomic complexity. Ascomycetes of the Sordariomycetes were major fungi identified at both sites, Khuskia (3.33% OTUs) at F1 and Emericellopsis (7.99% OTUs) at F2, and few filamentous forms were seen by microscopy. However, many fungal OTUs could not be identified to phylum level. Potential bacterial and fungal paint deteriogens were detected at both sites. The results confirmed the importance of using a variety of techniques in the study of microbial communities.