Hydrophobic surface development on heritage stones through Paracoccus denitrificans biofilms

Water is a major factor in the deterioration of stone-built heritage. Although significant advances have been made in stone-protective treatments, persistent trade-offs remain between performance, compatibility, durability, and environmental impact. This study advances bio-based conservation by investigating whether Paracoccus denitrificans biofilms can be deliberately steered to modify stone surface properties in a controlled and sustainable manner. Two lithotypes: Bentheimer sandstone and Euville limestone, were treated under polysaccharide-rich and lipid-rich nutrient regimes promoting different extracellular polymeric substances (EPS). Surface chemistry, morphology and mechanics were evaluated through contact-angle measurements, FTIR spectroscopy, roughness (Sa, Sz, Sdr) and surface hardness testing. As a result, lipid-rich conditions enhanced hydrophobicity (contact angle ≥ 90°) and decreased water absorption rate between 50% and 85% for both stone types due to change in surface composition. On the other hand, certain polysaccharide-rich conditions shifted surfaces to a more hydrophobic character linked to reduced macro-roughness (Sa, Sz) and increased micro-complexity (Sdr increased moderately to 2–5%.), evidencing morphological control of wetting. Surface hardness changes were minor and substrate-dependent, with slight consolidation effect in Euville limestone (max 15%). Together, these results highlight P. denitrificans as a biological tool capable of tuning stone-water interactions via dual chemical and physical pathways. The work positions microbial surface steering as a promising addition to the field of biological conservation for cultural heritage materials.