Secondary metabolites and natural rock substrate alteration as biosignatures in microbe-mineral alterationA Case Study of Lichens-Rock Surfaces interaction in Hasan Beg and Grdmandil mountains, Kurdistan, Iraq

Abstract:

Lichens can produce significant textural and mineralogical changes in the natural rock substrates they colonize. These alterations are investigated here in view of mineral substrate variation, diagenetic character, and any biosignatures of microbe-mineral interaction. Three species of lichens were collected and identified as Lecanora Muralis, Rhizocarpon Macrosporum, Aspicilia Leprosescens. The rock substrates were sampled and prepared for scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). The SEM imaging of micro-cross sections of the lichen-rock interface revealed microbially induced diagenetic alterations such as cementation, original matrix and mineral dissolution, micro-porosity formation, grain-grain bridging, open space filling, and neo-mineral formation. EDX mapping showed new element distribution in the substrate where the fungal hyphae, as the lichen mycobiont had penetrated indicating element mobilization and neo-mineral formation. XRD showed a bulk difference in mineral phases between non-colonized and lichen colonized substrates where neo-minerals formed comprised mainly of Ca/Mg oxalates and calcite. Although, lichen-mineral substrate interactions have previously been studied as a bioweathering process, our detailed study suggests important micro-textural and mineralogical diagenetic features that are similar to characteristic early microbial diagenesis in sediments. Due to their distinctive characteristics, we propose that they can be considered as biosignatures of microbe-rock interactions. In addition, secondary metabolites (SM) were methanol-extracted from all studied lichen species Lecanora Muralis, Rhizocarpon Macrosporum and Aspicilia Leprosescens colonizing different rock substrates. A GCMS used for screening of lichen SM and the concentrations of the lichen metabolites were assessed using UV-Vis Spectrophotometer. The result from all examined lichen species showed that the composition of lichen SM varied with mineral composition of rock substrate for the same species of lichen, which further suggests substrate-lichen interaction specificity.

Keywords: Lichens, Fungi, Metabolites, Diagenesis, Biosignatures