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extract was sent out to LGC genomics GmbH (Berlin, Germany) for 16S rRNA gene sequencing on an Illumina MiSeq platform and library preparation. For the bacteria it followed the same procedure as De Paepe et al. (2017) with 35 PCR cycles. Additionally, the archaea were determined, on three samples of each location, using a nested approach (De Vrieze et al., 2018).

      Furthermore, one powdered sample of each location was used as inoculum for isolations: R2A agar for heterotrophic bacteria and thiosulphate plates for sulphur oxidizers containing (per Litre) 980 mL fresh water basal mineral medium, 9.7 g Na₂SO₄, 6 g Na₂S₂O_3, 10 g agar, 0.02 g bromothymol blue, 10 mL 971 M MOPS buffer, 10 mL 1 M NaHCO₃, 1 mL SL-10 trace element solution and 1 mL 7-vitamin solution The plates were incubated between two and six weeks at room temperature. For anaerobic isolations 2 g/L NaNO₃ was added. The isolates have been characterized by Sanger sequencing (LGC Genomics GMbH, Berlin, Germany) using 27 F and 1492R LGC primers. The resulting sequences were blasted with NCBI’s BLAST and with RDP Seqmatch. The gypsum crusts were characterized by their soluble ions. These have been extracted from powdered crust and underlying rock with Milli-Q water with a 1 : 5 ratio. The cations Na⁺, K⁺, Ca²⁺, Mg²⁺ and the anions Cl^–, NO₃^–, NO₂^–, SO₄^2–, PO₄^3– have been quantified on a 930 Compact Ion Chromatograph Flex (Methrohm, Switzerland) with a conductivity detector. From the measured concentrations, the amount of soluble ions was calculated. RUNSALT was used to model the phases of the salt mixture in function of relative humidity (Price, 2000; Bionda, 2005).

      The potential discolouration of Arthrobacter agilis, one of the isolates was tested on the French oolitic Savonnières limestone. This stone is extensively used as a replacement for Lede stone (Dewanckele et al., 2014) This was tested by a water run-off test with a similar setup as De Muynck et al. (2009). Arthrobacter agilis was grown in R2A broth at 20 °C, after which it was sprinkled during two hours over six slaps of Savonnières limestone and dried during 24 hours. This was repeated two times more and in between its colour at two spots on the rock was examined with a point measurement using the CM-2600d spectrophotometer.

       Results and Discussion

      Gypsum crusts were present both in the urban and rural environment. These were thick botryoidal and black in Ghent, while in Berlare those were thinner and laminar with a rusty colour. The occurrence of the gypsum crusts has also been confirmed by the soluble salt analysis (see Table 1 for the anions). In every sample, there was a high concentration

      of soluble SO₄^2– ranging between 1 mg/g_rock and 10 mg/g_rock. Although highly variable between the samples, overall, the soluble salt content was higher in the City Hall of Ghent compared to the samples of Berlare. This was also the case for the Cl^– and NO₃^–. Sample B6 was the only sample of Berlare with more Cl^– and NO₃^–, because of its sheltered position underneath a windowsill. Nitrite and phosphate were not found or only available in a very low amount. The compatible cations were primarily calcium (0.6−5 mg/g_rock) and in a lesser extend also potassium and magnesium. Sodium was most of the times slightly present, but could reach a high concentration (about 5 mg/g_rock for G4) combined with a high amount of soluble chloride. Based on the modelling performed using RUNSALT the accompanying salts (except gypsum), for the samples with high soluble salts content, at 20 °C and low RH were among others carnallite (KCl·MgCl₂.6H₂O), niter (KNO₃), nitromagnesite, (Mg(NO₃)₂.6H₂O), halite (NaCl), nitratine (NaNO₃) and Ca(NO₃)₂. These salts would be in solution at a relative humidity of about 60 % and more. For the other samples, the result of the model was more complex.

      Within and underneath the gypsum crusts, 16S rRNA gene sequencing successfully identified the prokaryotic community on the City hall of Ghent and the Castle of Berlare, except for one sample (G1). Sulphur oxidizing, sulphur reducing and nitrogen oxidizers have been identified in some of the samples. Genera of nitrifying bacteria from different taxa were present: Nitrolancea (0.03 % 98in G2), Nitrospira (0.06 % in G2), Nitrosomonadaceae with Nitrosospira (0.002 % in G4, 0.7 % G6 and 0.0003 % in B1) and Nitrosomonas (0.002 % in G6). Archaea have been found in only one sample (G6) belonging to the nitrifying Nitrososphaeraceae.

Table 1: Soluble anions of the different samples in the urban and rural environment (mg/grock).

Sample	Cl–	NO3–	SO42–
G1	0.351	1.626	6.795
G2	1.014	5.404	6.858
G3	0.103	0.261	7.337
G4	5.348	12.537	9.651
G5	0.112	0.238	6.948
G6	0.087	0.306	6.884
B1	0.004	0.006	4.000
B2	0.207	0.129	7.156
B3	0.005	0.004	4.952
B4	0.004	0.003	1.261
B5	0.005	0.005	4.632
B6	0.801	1.150	7.221
B7	0.008	0.005	2.851

      Some sulphur oxidizing bacteria were found as well, belonging to the “purple non-sulphur” bacteria (PNS). Despite their name, members of this group can oxidize low concentrations of sulphur (Hunter et al., 2009). PNS were abundant in G5 with 16 % of Rhodoplanes. Furthermore, there might be more PNS present in the samples as the order Rhodospirillales has been detected in G4 and G6 with an unclassified chemolithoautotrophic Magnetospira (0.2 % in G4) and with an unclassified genus of the family Rhodospirillaceae (0.3 % in G4). Furthermore, potential PNS in Rhodobacteraceae were represented as well, however again unclassified (0.5 % in G2, 0.002 % in G4, 0.4 % in G6 and 0.0003 % in B1) (Hunter et al., 2009; Williams et al., 2012). Beside sulphur oxidation, some identified bacteria belonged to the sulphur/sulphate reducing bacteria such as Desulfuromonadales (0.005

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