Uncovering transcriptional processes in microbial communities adapted to differing saline conditions in salt-weathered historic buildings

BACKGROUND: Microbial colonization of architectural surfaces in historic buildings can cause not only aesthetic damage but also biodeterioration. One example is the colonizing microbiome on salt-weathered architectural surfaces. Halotolerant and halophilic communities on such surfaces produce colored pigments that visually alter cultural heritage sites and could potentially degrade organic binders used for mural paintings. Although the microorganisms involved in these deterioration processes have already been described, detailed information about the molecular processes that allow these communities to succeed, survive, and thrive under such extreme conditions is still lacking. RESULTS: A combined metagenome and metatranscriptome approach were employed to investigate three sampling sites located in two Austrian historic buildings displaying different environmental and saline compositions. The chapel of St. Virgil (Vienna) is a subsurface, climate-controlled environment. In contrast, the Charterhouse Mauerbach (Lower Austria) is exposed to natural fluctuations in temperature and humidity. DNA and total RNA were extracted from each sampling site simultaneously and sequenced. Two methods for gene assembly were compared and functionally evaluated. Results showed a minor bias in both methods, with improved results when they were combined. Comparison between DNA and RNA showed interesting variations in the taxonomic composition between the DNA- and RNA-based dataset, distinguishing the dormant from the active microbiome. The annotated halotolerance mechanisms in the metatranscriptomes indicated genome and proteome adaptations, showing high GC content, proteome acidification, with elevated aspartate and glutamate levels, and low isoelectric point profiles. Furthermore, the communities used both "salt-in" and "salt-out" osmoregulatory mechanisms. Pigment production was confirmed in all sampling points, revealing diverse pathways for carotenoid biosynthesis. Various protective mechanisms against oxidative stress were detected, such as those against reactive oxygen species (ROS), but also detoxification, protein folding, protein and DNA repair, and RNA chaperones. Key metabolic pathways revealed diverse pathways related to carbon, nitrogen, and sulfur cycling, linked to varying oxygen concentrations within biofilms. The results also highlighted the need for an in-depth analysis of the capabilities of the involved microorganisms. CONCLUSIONS: The study shows highly specialized and cooperative adaptations, using both "salt-in" and "salt-out" strategies, diverse phototrophic and redox metabolisms that tightly couple C-N-S cycling.