Lichen colonization and associated biodeterioration processes on ancient bricks of the Gonbad-e Qābus tower, UNESCO World Heritage Site, Iran

Abstract

The impact of lichens on ceramic building materials has received little attention, particularly in the case of bricks and with reference to (semi-)arid bioclimatic areas (i.e. with steppe and desert climates), where they are a remarkable component of built cultural heritage. In this study, lichens were examined on the outer brick walls of the ten-pointed star-shaped tower of Gonbad-e Qābus (N-Iran; UNESCO World Heritage), investigating their diversity and distribution with respect to microclimatic features related to wall orientations, as well as physical and chemical interactions of dominant species with the substrate. Although lichens occurred widely across all the tower walls, higher specific diversity and percentages of colonized bricks were found on sides with higher surface moisture. This was related not only to prevailing wind directions, and consequent wind-driven rain, but also to the lower average values of solar radiation. Light and electron microscopy highlighted the penetration of mycobiont hyphae down to millimetric depths, associated with substrate disaggregation. Energy-Dispersive Spectroscopy, X-Ray Diffraction and Raman analyses documented elemental absorption by hyphae in contact with the substrate minerals, and the formation of calcium oxalate deposits at the interface between some species and the bricks. Such findings provide initial evidence at the microscopic scale of the strong biodeteriogenic impact of lichens on bricks in (semi-)arid climate areas. Further investigations are needed to clarify the balance between these biodeterioration phenomena and the potential counteracting role of thalli in protecting bricks from atmospheric agents, in order to advise on the need for lichen removal.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-025-20461-4.

Introduction

Saxicolous lichens grow directly on all natural and artificial stone substrates, including siliceous, calcareous and other less common lithologies, as well as bricks, concrete, mortars, tiles, and ceramic^1,2. Their morphological and physiological adaptations allow their survival and spread on and within mineral matrices, where the lack of nutrients and the stressful microclimatic conditions strongly limit the presence of higher plants, and they only have to compete with mosses and microbial biofilms^3,4. Accordingly, lichens are often the dominant component of lithobiontic communities in terms of diversity and biomass, on rock outcrops at all latitudes and altitudes, but also on the stone cultural heritage^5–8.

Since mid 18th century, lichen colonization of rocks has been mentioned as a remarkable factor driving the formation of soil and initiating vegetation succession³. Early qualitative observations were supported by experimental evidence of the physical action of epilithic lichens, with thalli developing on rock surfaces, but with mycobiont hyphae also penetrating deeply within the mineral substrate and being responsible for its disaggregation and the detachment of fragments⁹. The growth patterns of endolithic lichens (with thalli embedded in the rocks) were also characterized, showing their exploitation of internal cracks (chasmo-endolithic) or intrinsic porosities (crypto-endolithic), or their active dissolution of carbonate minerals (eu-endolithic)^2,9,10. Moreover, the role of primary (e.g. oxalic acid) and secondary (e.g. several depsides and depsidones) lichen metabolites bearing acidic and chelating functions in the chemical destabilization of original mineral constituents of rocks and the formation of secondary (bio-)minerals was repeatedly demonstrated^10,11.

Such biogeophysical and biogeochemical processes make lichens primary agents of biodeterioration when they occur on heritage stone surfaces^12,13. However, at least for certain combinations of species, lithologies and climate conditions, the role of lichens in bioprotecting rock surfaces from other decaying factors has been documented, mostly due to a physical-barrier (“umbrella”) effect of thalli and/or of (bio-)precipitated neoformed minerals^9,14. Accordingly, the impact of lichens on the stability of natural outcrops, and the conservation of heritage surfaces, likely differs from case to case depending on the equilibria between the activated biodeterioration processes and potential bioprotection from other decaying factors^9,15–17.

(Micro-)climate factors, including relative humidity, rain, wind, temperature, and sunlight, are crucial factors determining lichen colonization and distribution on heritage stone surfaces^18,19, and they also appear to regulate the biodeterioration-bioprotection balance¹⁰. Some experiments and models indicated a prevalence of bioprotection in temperate, wet environmental contexts, with infrequent temperature extremes, while biodeterioration should prevail in hot and dry environments¹⁵. Nevertheless, at the current state of knowledge, with a still limited number of deeply investigated combinations of species, lithologies, and climate conditions, decisions on lichen removal in the framework of conservation and restoration plans should not be taken without acquiring fulfill data about the extent, the (micro-)climate regulating factors, and the actual effect(s) of colonization on the monuments under investigation.

In comparison to natural lithologies, few studies have considered lithobiontic colonization and related biodeteriorative and/or bioprotective effects on artificial stone surfaces, including bricks^20,21. Burnt or fired bricks (thus excluding sun-dried bricks or mud-brick), tiles and ceramics could be defined as metamorphic rocks that are stable only after temperature and final conditions of pressure of the artificial firing²². Fired bricks usually display a porous texture; their moisture absorption is mostly governed by capillary uptake and evaporation processes. The high capillarity of bricks usually determines a rapid absorption of large amounts of water and a low-rate release. In the case of composite materials, such as masonries of mortar and brick, moisture transport from the former to the latter (and vice versa) is dependent on differences in their pore dimensions and distributions²³. Lichen colonization of brick monuments, including lichen diversity and interactions with the substrate, has been particularly neglected^20,21. Lichen growth on (historical) bricks is a widespread phenomenon in areas of W-Europe (e.g. the Netherlands, Belgium), where they are a common substrate due to the scarceness in natural stone materials¹. In this context, some older studies correlated lichen penetration with an enhanced brick porosity and surface discoloration²⁴. With respect to ancient bricks, lichen diversity was characterized on vertical walls of the archaeological Roman site of Ostia (Italy), in the Mediterranean climatic area²⁵, but biodeteriorative patterns were not investigated. The presence of calcium oxalate deposits at the interface between lichens (Dirina massiliensis f. sorediata) and Roman bricks was documented in a desert site near Alexandria (Egypt)²⁶. However, to the best of our knowledge, no other investigation is available for (semi-)arid bioclimatic areas (i.e. with steppe and desert climates, sensu²⁷), where bricks represent a remarkable component of the built cultural heritage, as in the case of Iran²⁸. In this latter country, in particular, lichen diversity and deterioration patterns have been the subject of deep investigations focused on natural stone materials^8,29–31, while biodeterioration of bricks was only recently considered and with a focus on microbial biofilms rather than on lichens^32,33.In the rainy season, the bricks of Gonbad-e Qābus tower (UNESCO World Heritage list n. 1398), in the cold desert zone of N-Iran, become wet, resulting in conditions highly favourable to microbial biofilms and showing a reduced mechanical resistance, revealed by blackening and brick breakages, respectively. Such phenomena mostly affect the most shaded, horizontal surfaces (terraces, tops of walls), although vertical ones are also subjected to water run-off. The presence of lichens on the ancient bricks also impacts the aesthetics of the heritage wall surfaces and may contribute to their biodeterioration, but no investigations were carried out until now. This work aimed to characterize the diversity of lichens on the brick walls of the ten-pointed star-shaped Gonbad-e Qābus tower (Fig. 1), their distribution with respect to microclimatic features related to wall orientations, and their specific patterns of interaction with the substrate. In particular, we aimed to test the hypothesis that lichen colonization (Supplementary Fig. 1) is responsible for the biodeterioration of brick masonries, in order to address conservation strategies extendable to historical towers, bridges, and castles in northern Iran.

Fig. 1.

Location and geometric features of the Gonbad-e Qābus tower UNESCO World Heritage Site, Iran. (A-B) Geographical location of Gonbad-e Qābus city (red mark), and (C-D) Gonbad-e Qābus tower (red mark and square) [A-D, map data ©2024 Google]. (E-F) General view and map of the Gonbad-e Qābus brick tower, displaying its ten-pointed star-shaped plan and the entrance at the SE side.

Results

(Micro-)climatic conditions on differently oriented walls

Prevailing wind-driven rain and high solar elevation conditions registered around the tower are visualized in Fig. 2. In Gonbad-e Qābus, rainfall is concentrated in the wet period between November and April (approx. 90 mm, av. 18.3 total rainy days; Supplementary Fig. 2A), when southwestern wind prevails (av. 2.43 m s^− 1; Supplementary Fig. 2B). In the dry period, from May to October (approx. 30 mm, av. 7.3 rainy days), the northwestern wind direction is prominent (av. 2.57 m s^− 1). The sun mostly shines on the southern and eastern building sides (no. 8, 9, 10, 1, 2). The other sides are generally either in the shade or exposed to less sunlight. In particular, from November to March, solar elevation higher than 20° is mostly combined with a southern azimuth, and a (north-)western azimuth is only associated to the last hours of daylight (with sun elevation < 20°; Supplementary Fig. 2C). Accordingly, average values of highest intensities of solar radiation quantified on SE-facing sides (no. 9, 10, 1, 2) was higher than 1100 W m^− 2, while on the NW-facing sides (no. 4, 5, 6, 7) it was lower than 1000 W m^− 2 (Supplementary Table 1). These radiation conditions showed a significant inverse correlation with brick surface moisture (%) monitored on each side (Pearson test: ρ = -0.953, p < 0.0001; Supplementary Tables 2 and Supplementary Fig. 3), with av. annual values in the range 41–46% for SE-facing sides and 33–36% for NW-facing ones, and sides 3 and 8 showing intermediate conditions (Supplementary Table 1).

Fig. 2.

Distribution of lichens on the building surfaces from a vertical view and its relationship with environmental factors. The green colors indicate the distribution of lichens on each side and on the roof. The prevailing directions of wind-driven rain (wind and rain symbols) and high solar elevation (sun symbols) are specified, together with the percentage abundance of colonized bricks on each side (black numbers).

Lichen colonization

More than 80% of bricks surveyed on the different tower sides, from the ground up to a height of three meters, were colonized by lichens. Thirteen lichen taxa were listed (Table 1; Supplementary Fig. 4). An unidentified cyanolichen (likely a new taxon of genus Peltula on the basis of the identification work still in progress) was the most common species, colonizing 60% of surveyed bricks, followed by Calogaya decipiens and Kuettlingeria teicholyta (approx. 40%).

Table 1.

Lichen flora on the Gonbad-e Qābus tower.

Lichen taxa	Colonized bricks (n per tower sides 1–10, n total, % of surveyed bricks)
	1	2	3	4	5	6	7	8	9	10	Total	%
Calogaya decipiens (Arnold) Arup, Frödén & Søchting	5	14	100	80	208	172	262	155	45	45	1086	41.7
Candelariella medians (Nyl.) A.L. Sm.	-	-	1	15	15	1	10	1	5	-	48	1.8
Candelariella vitellina (Hoffm.) Müll. Arg.	-	-	1	30	-	-	-	-	-	-	31	1.1
Kuettlingeria teicholyta (Ach.) Trevis.	92	100	300	160	174	-	-	162	58	65	1131	42.4
Flavoplaca citrina (Hoffm.) Arup, Frödén & Søchting	-	-	1	10	-	1	-	-	-	-	12	0.4
Peltula obscurans (Nyl.) Gyeln.	-	2	-	3	30	15	-	-	-	-	50	1.9
Polyozosia albescens (Hoffm.) S.Y. Kondr., Lőkös & Farkas	-	-	2	20	-	159	1	1	-	-	183	6.9
Protoparmeliopsis muralis (Schreb.) M. Choisy.	-	1	3	-	-	5	-	60	-	-	69	2.6
Staurothele monicae (Zahlbr.) Wetmore	-	-	-	3	10	1	-	5	-	-	19	0.7
unidentified cyanolichen (cfr. Peltula sp.)	70	150	100	150	240	238	285	223	165	-	1621	61.8
Verrucaria macrostoma DC.	-	2	2	-	-	2	1	1	-	1	8	0.3
Verrucaria nigrescens Pers.	20	70	18	5	10	1	8	10	8	15	165	6.3
Total lichen counts	187	339	528	476	687	594	567	618	281	126	4403
Total bricks colonized by lichens	179	208	233	250	257	245	263	236	192	125	2188	83.5
Total number of surveyed bricks	250	278	272	277	276	254	276	273	276	187	2619
Lichen colonized bricks (%)	71.6	74.8	85.6	90.2	93.1	96.4	95.2	86.4	69.5	66.8

Abundance of each taxon is expressed in terms of colonized bricks (n) per each side (1–10) of the building. Total specific counts, total and percentage of colonized bricks, and total surveyed bricks per tower side are reported.

Lichen distribution was not homogeneous on the different sides of the tower, with the percentage of colonized bricks ranging from a minimum of 66.8% to a maximum 96.4% (Fig. 2). The lowest percentages were associated to SE-facing sides no. 9, 10, 2 and 1, which received the maximum sun radiation and were opposite to the main directions of wind. Oppositely, the highest presence of lichens characterized sides no. 4. 5. 6, and 7, in the NW side of the building. With respect to the monitored environmental parameters, lichen abundance (colonized bricks %) showed linear correlation with both brick surface moisture and sun radiation (Supplementary Fig. 3). In particular, Pearson’s correlation coefficients showed that lichen abundance is positively correlated with brick surface moisture (ρ = 0.950, p < 0.0001) and inversely with highest intensity of solar radiation (ρ=-0.993, p < 0.0001) (Supplementary Table 2). In this respect, Fig. 3 visualizes the distribution of lichens on the examined surfaces of the different tower sides and its relationships with the brick surface moisture monitored (in the spring) during field surveys and prominent at the base of the walls, and with architectural features, such as the inscriptions.

Fig. 3.

Distribution of lichens on the building surface from a horizontal view, and its relationship with brick surface moisture monitored for sides No. 1–10. For each side, the blue colors indicate the brick surface moisture (A; intense continuous blue: >60%; light discontinuous blue: <40%) and the green colors indicate the presence of lichens (B; intense continuous green: continuous lichen cover, light discontinuous green: discontinuous lichen cover).

Lichens also colonized the roof of the tower (Fig. 2), but the characterization of these communities was beyond the aims of the work due to the limited roof accessibility.

Biogeophysical and biogeochemical lichen-brick interactions

Microscopic observations of brick samples showed biodeterioration phenomena, including penetration of mycobiont structures within the substrate, cracks and defoliations likely due to the growth and hydration-dehydration cycles of thalli, and deposits of biogenic minerals related to the reactivity of lichen-secreted metabolites (Fig. 4). In particular, polarizing microscope observations were performed on cross sectioned fragments obtained from bricks colonized by Verrucaria nigrescens (Fig. 4A) and Calogaya decipiens (Fig. 4B). Both the species showed mycobiont penetration within the stone substrate down to millimetric depths. In particular, the hyphal penetration component of V. nigrescens was observed down to depths of 1.0–2.5 mm within the brick fragments from both the dry and wet sides. Differently, C. decipiens showed a greater penetration within the bricks of the wet sides (4.0–5.0 mm), with respect to those of the dry sides (2.0–3.0 mm; Fig. 4C). In the case of C. decipiens, fine-grained mineral deposits were observed from the thallus-brick interface down to depths of 50–100 μm (Fig. 4D). Scanning Electron Microscopy (SEM) observations displayed a firm adhesion of lichen thalli to the brick surface and the underneath penetration of a hyphal network, together with evidence of consequent substrate fragmentation and loss of the brick material (Fig. 4E, F). Penetrating hyphal bundles were also observed beneath the thalli of the unidentified cyanolichen (Supplementary Fig. 5A). Energy Dispersive Spectroscopy (EDS) analyses showed some accumulation of calcium, aluminum, silicon, iron and magnesium in the hyphae in contact with the mineral component with respect to the central part of the hyphal bundles (Supplementary Figs. 5B-F and 6; Supplementary Table 3).

Fig. 4.

Lichens as agents of biodeterioration on the Gonbad-e Qābus tower bricks. (A) Verrucaria nigrescens and (B) Calogaya decipiens on the bricks. (C-D) C. decipiens-brick interface (thallus and rock-surface on the right side), observed by transmitted light microscopy (C) and under double polarized light (D), displaying the penetration of mycobiont hyphae within the substrate ($; C) and fine-grained mineral deposits (*; D). (E-F) V. nigrescens-brick interface, observed by scanning electron microscopy, showing the hyphal network within the brick substrate, determining its disaggregation (E, F). Scale bars: 200 μm (C, D), 10 μm (E, F).

At the interface between Kuettlingeria teicholyta and the brick substrate, deposits of fine-grained minerals (SEM images in Supplementary Fig. 7A, B) were mainly composed of calcium (EDS analyses in Supplementary Fig. 7C). X-Ray Diffraction (XRD) analyses carried out on the same materials indicated the presence of whewellite (calcium oxalate monohydrate; CaC₂O₄.H₂O) (Supplementary Fig. 7D and Supplementary Table 4). The occurrence of whewellite at the thallus-brick interface of K. teicholyta was also confirmed by Raman portable spectroscopy, together with carotenoids (Supplementary Fig. 8).

Discussion

Knowledge of the biodeterioration potency of lithobiontic communities is crucial to plan conservation strategies, but it is still limited for certain groups of (micro-)organisms, particularly with respect to certain lithologies and/or (micro-)climatic conditions^10,34. This is the case of lichens in (semi-)arid environments, where the threats they pose to stone conservation are still less known in comparison with the huge literature available for other bioclimatic areas, particularly temperate ones^8,35,36, and still need to be unveiled in the case of brick heritage surfaces.

In the examined case of the Gonbad-e Qābus brick tower in Iran, the more abundant lichen colonization on the NW- facing sides was related to the prevalent western winds, and, consequently, wind-driven rain, similar to the situation on stone surfaces in Pompeii, in a temperate Mediterranean climate¹⁹. Nevertheless, northwestern winds only prevail in the dry season, with those from South-West dominant in the wet season. Accordingly, it seems that the combination of NW wind-driven rain with the lowest solar radiation accounts for the higher brick surface moisture seasonally recorded on the NW-facing sides, and its significant correlation with the percentage of colonized bricks.

Surface moisture is a crucial factor in determining the bioreceptivity of stone materials and the distribution of lithobiontic communities³⁷. The more humid condition of the NW-sides accounts for the higher number of lichen species³⁸. Even on the S- and SE-facing sides, however, lichen colonization was widespread, at least up to three meters above the ground, in agreement with the observation in other dry sites of a remarkable lichen presence also on more xeric and sun-exposed surfaces^8,36. In the case of the Gonbad-e Qābus tower, the very high percentage of colonized surfaces, whatever the orientation of the walls, likely relates to the high porosity and surface roughness of the unglazed ceramic materials, as bricks, which favor water absorption and, consequently, bioreceptivity^20,21, particularly at the base of the walls.

This study also verified the hypothesis that the growth of some widespread lichens (Verrucaria nigrescens, Calogaya decipiens, Kuettlingeria teicholyta, and an unidentified cyanolichen, possibly a new species of the genus Peltula) may affect the stability of the brick substrate, causing erosion processes and the precipitation of neoformed minerals (patterns observed for each species are summarized in Supplementary Table 5). Results obtained on heritage buildings in Europe similarly showed species of genera Verrucaria, Calogaya and Kuettlingeria as abundant colonizers of ceramic materials, including tiles and bricks^20,25,39. Their interactions with ceramic substrates were recognized as a cause of biodeterioration^21,40, and the same behavior is confirmed here for dry climates. A remarkable expansion and contraction of thalli and penetrating hyphae following changes in available humidity (wetting and drying cycles) mechanically disturb the brick substrate²⁰, likely yielding the observed detachment from the substrate and inclusion of mineral fragments within the thallus. The mycobiont penetration through the small pores of the bricks observed for V. nigrescens, determines the occurrence of a hyphal network down to depths (1.0–2.5 mm) analogous to those observed for V. nigrescens within the ceramic material of Roman dolia in N-Italy²¹. The hyphal penetration depth of Calogaya decipiens (2.0–5.0 mm) was instead remarkably higher than that reported for the same species within unglazed tiles in Spain, where it was negligible, possibly due to different microstructural and mineralogical features³⁹. However, the very different conditions of the temperate and the cold, arid climates may also account for the detected incongruency¹⁵. On the other hand, in the case of C. decipiens, higher penetration observed on the wet sides appears in contrast with the expectation of higher deterioration effects in drier conditions¹⁵. Similar penetration depths observed for V. nigrescens on the NW- and SE-facing sides are instead in agreement with observations of the hyphal penetration component of four lichen species colonizing schists with opposite orientations and not showing significant differences in depth⁴¹.

As in the case of natural stone materials³, lichen colonization and penetration within the brick substrate were associated with chemical changes in the original minerals and the neoformation of biogenic minerals. The biochemical weathering, highlighted by the elemental absorption (e.g. Ca, Fe) by mycobiont structures in contact with the minerals, may lead to limited compositional shifts, to etching, or to complete dissolution^42,43. The brown and black color of the brick surface was putatively attributed to the past biogeochemical activity of lichens and, particularly, to the absorption of iron from the brick bulk to the surface, driven by metal complexing metabolites, as described for other substrates⁴⁴. On the other hand, with respect to Verrucaria, hyphae penetrating within the micro-holes and matrix of ceramic substrates, in addition to physical changes, can modify the color from red to black by shifting the percentage of clay elements and, thus, worsening the heritage surface appearance^39,43. Moreover, oxalates in the form of whewellite (CaC₂O₄. H₂O) detected by XRD at the interface between K. teicholyta and the substrate -likely constituting also the fine-grained mineral deposits beneath C. decipiens- account for oxalic acid-driven complexolysis, which further yields biodeterioration of the brick substrate, and consequent erosion processes^45–48. Oxalates are formed due to the chemical reaction between oxalic acid secreted by lichen and calcium present in the original mineral constituents of the brick substrate, primarily calcite (CaCO₃). Higher occurrence of K. teicholyta on drier sides of the tower agrees with reports indicating a prominent accumulation of oxalates on rock outcrops with southeastern orientation⁴¹, but C. decipiens prevails on the opposite side.

In comparison to other stone surfaces colonized by lichens, bricks appear even more porous and disintegrated, suggesting the necessity of their removal, but also of extreme care in planning conservation strategies. Removing lichens after they have already affected their substrate may indeed cause more exposure to damage and the conservation worsen if adequate protective treatments are not planned^3,7,38. Although the physical and chemical interactions of lichens and stone materials imply biodeterioration phenomena, lichen colonization does not always threaten conservation. Studies by several researchers in Europe and the United States, and also in the continental semi-arid steppe of Central Anatolia, showed (or argued) that lichen crusts protect stone surfaces from atmospheric agents^17,49–51. Some species of lichens, at least in temperate climate conditions, were shown to improve the physical properties of ceramic materials by reducing water absorption²¹. In (semi-)arid regions, whewellite (and calcite) were shown to promote secondary cementation of limestone surfaces⁴³. Decisions on the management of lithobionts on brick monuments in the arid climate regions have thus to correctly balance findings on microscopic biodeterioration, general macroscopic surface stability and aesthetic evaluations, to choose between preserving or removing lichens. With this regard, the removal of lichens from selected parcels, distributed on the different tower sides, and a subsequent monitoring of the stability of their brick surface with respect to areas maintaining lichens may offer an experimental support to management decisions, and suggest the opportunity of additional treatments to protect the brick surfaces if/where lichens are removed⁷.

Materials and methods

Study site

One of the most unique and prominent relics of Iranian architecture in the early Islamic period is the Gonbad-e Qābus eben-e Voshmgir brick tower. It is located three kilometres northeastern from the ancient city of Jorjan, the capital of the Ziyarids dynasty, where it is now surrounded by the current large city of Gonbad-e Qābus in Golestan Province (N37°15′28.9″E 55°10′8.4″; Fig. 1A-D). The site lies in the cold desert climate zone (BWk, B = dry, W = arid desert, k = cold, according to the Köppen Geiger climate classification²⁷, with average temperatures of approx. 10 °C in winter and 30 °C in summer, and with 120 mm rainfall yr^{− 1} (https://weatherspark.com/y/105489/Average-Weather-in-Gonbad-e-K%C4%81v%C5%ABs-Iran-Year-Round).

The brick tower of Gonbad-e Qābus⁵² has been registered as Iranian National Monument on 6 Jan 1932. It was officially registered as a UNESCO World Heritage site on 31 July 2012. The monument is 53 m high from the foundation to the top roof (Fig. 1E). The body of the building is 37 m high and has a ten-pointed star-shaped plan, that is ten sides, numbered from 1 to 10. The internal circumference is 30 m and the external is 60 m; the conical dome is 16 m high and its eastern part has an aperture 1.9 m high. The entrance of the building is on its southeast side (side 10) and its crescent-shaped vault is adorned with two handsome Muqarnas (Fig. 1F). This is one the prototype of Muqarnas, later used as a decorative device in traditional Islamic architecture. The bricks -primarily composed of quartz, feldspar, calcite and hematite- are square-shaped, mostly display dimensions of approx. 20 × 20 × 7 cm, and are bound with lime mortar.

Monitoring of environmental parameters

Environmental parameters possibly related to the lichen distribution and biodeterioration activity, including wind driven rain, solar elevation and azimuth, solar intensity, and brick surface moisture, were evaluated for the different sides of the tower.

Wind-driven rain on the different tower sides was estimated on the basis of rainfall and wind direction data available in the Gonbad-e Qābus meteorological station database (available online at: weatherspark.com/y/105489/Average-Weather-in-Gonbad-e-K%C4%81v%C5%ABs-Iran-Year-Round; Supplementary Fig. 2A-B). Wind-driven rain was further evaluated with field observations of wet surfaces, weekly collected for the different tower sides). Solar elevation and azimuth through the year were also obtained from the online database (Supplementary Fig. 2C).

Brick surface moisture was quantified every two weeks from Oct 2018 to Oct 2019 using a handheld Protimeter BLD9800-C MMS3 (Protimeter, USA) in the pin moisture meter mode, according to the instruction manual. For each tower side, measures were performed on 25 bricks (three measures per brick) randomly selected in its central part from the ground up to a height of 3 m. Such evaluation was combined with the measure of the highest intensity of solar radiation, obtained using ten TES-1333R pyranometer data loggers (TES, Taiwan), positioned on each side of the tower in their central part, at approx. 1.5 m from the ground.

Lichen survey

Lichen diversity and distribution were surveyed on the different sides of the tower, from the ground up to a height of 3 m -using a scaffolding-, in the period Jan 2018 - Jan 2020. Different taxa were visually distinguished in the field and their identity was then checked in the laboratory of the Iranian Research Organization for Science and Technology (IROST). Morpho-anatomical features were microscopically examined, and the secondary metabolite contents were characterized by thin layer chromatography⁵³. Lichen identification was based on monographic keys and descriptions. Nomenclature follows Kirk and colleagues⁵⁴ principles of the Index Fungorum database (www.indexfungorum.org)⁴⁸.

For each tower side, the number of bricks colonized by the different species up to the height of 3 m was evaluated, and calculated as percentage with respect to the total number of surveyed bricks (from 187 to 278, depending on the side).

The Pearson correlation coefficients between lichen percentage colonization and average values of brick surface moisture and highest intensity of solar radiation measured for each side were also calculated and their significance tested with Bonferroni’s correction. The correlation of the distribution of lichens with surface moisture related to architectural features of the different tower sides was also visualized using AutoCAD software (version 1.4).

Biodeterioration assessment

Physical and chemical interactions between lichens and bricks were investigated by light and scanning electron microscopy observations and by spectroscopic (EDS, Raman) and diffractometric (XRD) analyses. Such investigations were distributed on brick fragments colonized by the dominant species on the tower, collected from wet and/or dry sides (Verrucaria nigrescens and Calogaya decipiens, n = 6 samples each, from wet sides 5, 6 and 7, and dry sides 9, 10, and 1; unidentified cyanolichen, and Kuettlingeria teicholyta, n = 3 samples each from wet side 6).

Mycobiont penetration within the brick substrate was characterized by transmitted light polarizing microscope and SEM observations. The former were run on thin cross sections, obtained after the dehumidification in an oven (12 h; 70 °C) of brick samples colonized by lichens, and their covering with epoxy resin in the vacuum. The thin sections were cut in different directions using a microtome machine, then placed on a microscope slide, and observed using a Nikon Eclipse 50i. For electron microscope observations, brick samples were cut transversely, fixed in glutaraldehyde and then in osmium tetroxide, dehydrated in a series of ethanol solutions, and embedded in resin. Blocks of resin-embedded brick samples were finely polished and carbon coated. The sections were observed by scanning electron microscopy with secondary electron (SE) and backscattered electron (BSE) imaging, using a FEI INSPECT SEM microscope. Some sectioned materials were also analyzed using an energy dispersive X-ray spectrometer equipped with a Link ISIS micro-analytical system. In particular, the amount of elements inside the lichen thallus, at the thallus-brick interface and inside brick was investigated, and the results were reported as elemental weight%. The microscope operating conditions were 15 kV of acceleration potential, 1–5 nA of probe, and 6–25 mm of working distance.

The mineralogical composition of the bricks -in the bulk and at the lichen interface- and the mineral content of lichen thalli were characterized by X-ray diffraction analyses and Raman spectroscopy. Samples (approx. 2 × 3 × 2 cm, and 5 g each) were collected with the aid of a scalpel and crushed to a size of 75 μm for XRD analysis, and to a size of 2–3 mm for the Raman-based specific identification of some minerals. Diffractometric analyses were performed by means of a Bruker AXS Advance D8 diffractometer (Bruker company; Karlsruhe, Germany), using copper anode radiation with a wavelength of Kα = 1.54 Å. Raman analyses were performed using an ultra-mobile (handheld) spectrometer (InnoRam BWTEKINC) with a 785 nm excitation laser and a 20× lens. First, the probe was installed in a manual mobile platform and placed directly on the surface of the desired area, which made it possible to perform microanalyses. The spectra were recorded between 100 and 1800 cm^-1.

Conclusion

This research presents biodeterioration processes on the bricks of the Gonbad-e Qābus with respect to environmental conditions. These latter control the distribution of lichen species and influence biodeterioration on different building sides. Solar elevation and azimuth, wind-driven rain, and consequent solar radiation and moisture level at the brick surface are factors conditioning the abundance of lichens and related biogeophysical and biogeochemical processes, as the penetration of mycobiont hyphae promoting substrate disaggregation, and the neoformation of biogenic minerals (oxalates). Microscopic and spectroscopic investigations demonstrate the high bioreceptivity of historical brick buildings in (semi-)arid areas to lichen colonization, and their consequent susceptibility to biodeterioration, likely favored by the high porosity and mineral composition. Nevertheless, such patterns observed at the microscopic scale do not exclude a possible bioprotective role of lichens from atmospheric agents. Accordingly, continuous monitoring of environmental factors and their impact on surface erosion and/or lichen spread is needed to address future conservation strategies for the brick tower.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

M.Z. : Conception and experimental design, collection and assembly of data, data analysis, interpretation, and manuscript writing; M.S. :Supervision, Conception and experimental design, interpretation, financial support; S.F.L.: Supervision, conception, experimental design, and interpretation; A. N. : Supervision, conception, experimental design, and interpretation.

Funding

This study was partially supported by the Iranian Research Organization for Science and Technology (IROST).

Data availability

The datasets used and analyzed during the current study available from the corresponding author on reasonable request.

Competing interests

The authors declare no competing interests.