## 4 Discussion
### Effect of Microbial Colonization on Structures  
Deterioration of concrete is a complex process involving biophysical and biochemical interactions. The building surfaces in this study typically showed extensive discoloration in dark green and black, occasional rust-red tones, and green encrustations (Figure S2). The discoloration of building surfaces can be induced by the deposition of atmospheric pollutants and the excretion of biogenic pigments, initially leading to the aesthetic appearance issue. The common dark green color on the surface of the early architecture of Wuhan University is likely caused by photoautotrophic microorganisms. Photoautotrophic microorganisms, such as algae and cyanobacteria, can produce pigments. As morphologically diverse prokaryotic photoautotrophic microorganisms, cyanobacteria can synthesize pigments, including phycobilin and chlorophyll \(\alpha\)[43]. Photoautotrophic microorganisms, especially cyanobacteria, have the highest species richness in the green and blue biofilms of Angkor Wat [44]. The black and rust-red coloration of the surface from the early architecture of Wuhan University can be attributed to the actions of heterotrophic microorganisms and cyanobacteria. Fungi and actinobacteria are considered the most important biodetoriative organisms. Fungi can produce various pigments, such as melanin, \(\alpha\)-carotene, and \(\beta\)-carotene [45; 46]. Melanin produced by actinomycetes is responsible for the formation of black spots in structures [47]. The brown discoloration of paint surfaces has been reported to originate from minerals, fungi, and specific cyanobacteria [48]. In addition to microorganisms, the EPS in biofilm not only adheres to cells but also traps atmospheric dust and carbon residues from automobile traffic and pigments and minerals produced by microorganisms and contributes to the discoloration of buildings [49; 50]. In addition to pigments, microorganisms can produce organic acids and enzymes that trigger the biochemical corrosion of substratum material [51]. Microbial communities in the study area may produce oxal acid and sulfuric acid, leading to the occurrence of secondary minerals, including gypsum, thaumasite, and weddellite, on the surfaces of early architecture at Wuhan University. These secondary minerals crystallize under the effects of cyclic wetting and drying, creating internal stresses in the material and resulting in cracking, flaking, scaling, or detachment of construction material. The occurrence of gypsum and thaumasite was a result of the deposition by acidic air pollutants or biogenic sulfuric acid on structure facades, and deposition is commonly accompanied by the dissolution of calcareous construction materials [52]. As described in 3.2, relatively large amounts of gypsum, a typical deterioration product of calcareous construction materials, were found at four sampling sites [53; 54]. The corroding layer produced by the gypsum on the concrete surface can penetrate concrete, occupying a large space and generating stress because of the large density difference between gypsum and the concrete, and eventually leading to cracking, detachment, and destruction of external walls [55]. Additionally, some microorganisms require a certain amount of sulfur as a macronutrient [56]. [PERSON] et al. proved that gypsum can be a source of sulfur for microbial growth [57]. The production of gypsum not only affected the structural integrity of the concrete but also provided additional nutrients to microorganisms. Moreover, the occurrence of thaumasite in S-5 represented the thaumasite form of sulfate attack (TSA), which has attracted considerable interest in the past few decades as a newly discovered pattern of sulfate erosion of concrete. Unlike conventional sulfate attack, which leads to the expansion and destruction of concrete, TSA leads to a more insidious and destructive transformation of a concrete matrix inward to an incohesive mass from the surface inward [58].