Bacterial bioconsolidation of deteriorated limestone exposed to tropical conditions

Abstract

Biologically-based treatments are increasingly attracting the interest of the restoration community in Mexico. A microbially induced precipitation (MICP) strategy was implemented as a proof-of-concept based on locally isolated epilithic bacteria that were screened for biocarbonatogenic activity in vitro. Six percent (12) of the 203 tested isolates displayed urease activity and produced crystals with two different media. Isolates TM1B-407, TM1B-475, TM1B-494 and TM1B-508 were the most active isolates on both tests. The most efficient isolate was identified by 16S rRNA gene sequencing as a close relative of Rothia halotolerans strain YIM 90716. Scanning Electron Microscopy (SEM) showed that Rothia halotolerans TM1B-475 produced crystals, which by μ-X-ray fluorescence (XRF) analysis indicated a calcium carbonate composition; X-ray Photoelectron Spectroscopy (XPS) analysis confirmed that aragonite was the dominant polymorph. Efficiency of the consolidation treatments by Rothia halotolerans TM1B-475, M-3P medium (designed to stimulate the endogenous carbonatogenic community), and Ca(OH)₂ was measured by cohesion (Peeling test) and surface hardness (Equotip) on damaged limestone exposed to tropical conditions and statistically analyzed after 15, 30, 45, 60, and 120 days. There was a clear trend of estimated mean detachment (95 % confidence intervals) for all treatments. On the other hand, there was not a clear trend regarding surface hardness among treatments, but a better performance of the bacterial treatment at the end of the experiment, with an increased hardness of the limestone surface was observed. This study provides the first evidence that the MICP can be used for consolidation purposes in the Yucatan Peninsula on degraded limestone under tropical conditions.