Читать книгу Monument Future - Siegfried Siegesmund - Страница 126

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 % in G3) and 4 % Geoalkalibacter in G5, but also Desulfobacteraceae, with Desulfofrigus (0.008 % in G5 and 0.0006 % in B4), Desulforhopalus (0.0006 % in B4) and unclassified genera (0.0009 % in G6, 0.003 % in B4 and 0.003 % in B6) (Greene, Patel and Yacob, 2009; Rosenberg et al., 2013).

There is a strong link between the location and the occurrence of the nitrogen oxidizing and sulphur bacteria. Both are present in selected samples across both locations, but mainly in the City Hall of Ghent, where in one sample they even reach high abundance. There is also a link between the occurrence of the nitrifying and sulphur prokaryotes and the higher soluble salt content of respectively nitrate or sulphate. However, also here there are some exceptions. The occurrence of sulphate reducing bacteria followed the same trend. These bacteria can cause further deterioration (Krumbein and Gorbushina, 2009) but besides inducing gypsum crust formation, they can reduce the sulphate and precipitate calcium carbonate. This process also produces sulphur and H₂S (Castanier, Le Métayer-Levrel and Perthuisot, 1999). As one sample in the city (G5) contains both a high concentration of PNS and sulphate reducing bacteria, it is not impossible that both groups are strongly linked together.

These results combine some of the findings of previous research by Mansch and Bock (1998) & Villa et al. (2015). The former found more nitrifying bacteria in the urban environment and the latter more sulphur oxidizing bacteria. It also confirms the findings of Li et al. (2016). However, members of sulphur compound oxidizing genera of which it is known that they deteriorate building stones, such as Thiobacillus (and related genera) have not been found (Krumbein and Gorbushina, 2009). This contrasts with the earlier findings of Villa et al. (2015). Furthermore, it is unclear if the PNS bacteria would affect natural building stones the same way as Thiobacillus. The presence of sulphur bacteria that only tolerate lower amounts of sulphur can be related to the decreasing SO₂ concentrations in the atmosphere. The low abundance of sulphur and nitrogen oxidizers, combined with the absence of those groups in several samples, indicates that other chemical factors are most likely still dominating gypsum crust formation. Although those gypsum crust have been formed since decades, when the SO₂ and NO_x concentrations were higher, it cannot be excluded that back then more sulphur and nitrogen oxidizers were present affecting crust formation.

Besides the metagenomic approach, also an isolation campaign has been performed. This did not succeed to retrieve chemolithoautotrophic prokaryotes. However, 20 genera belonging to 31 species of heterotrophic bacteria were successfully isolated and identified. Growth occurred mainly aerobic and denitrification occurred barely. Many isolates contained a red, orange, pink or yellow pigment and one of them, Arthrobacter agilis, has been successfully applied on the water run-off test (Figure 2). After one cycle of two hours, the rocks became significantly red and this has been confirmed by the spectrophotometric data. There was a dip between 450 and 560 nm in the reflectance, leading to an increase visibility of the complementary 99red/orange colours. During this test, we applied bacterial concentrations that do not occur in nature. However, it shows how easy it can discolour a natural building stone and the potential of some of the bacteria to cause aesthetic changes. Besides natural oxidation of iron-bearing minerals, these bacteria could also contribute to the rusty/red colour on the Castle of Berlare.

Figure 2: A) Spectral data showing the initial state of the Savonnières limestone (blank) and the progress of the colour change after adding Arthrobacter agilis during the different cycles (Cycle 1–3). B) Savonnières stone after three cycles showing the discolouration compared to the initial state at the top section.