Colonization and weathering of engineering materials by marine microorganisms: an SEM study

Abstract

Microorganisms are a ubiquitous feature of most hard substrata on Earth and their role in the geomorphological alteration of rock and stone is widely recognized. The role of microorganisms in the modification of engineering materials introduced into the intertidal zone through the construction of hard coastal defences is less well understood. Here we use scanning electron microscopy (SEM) to examine microbial colonization and micro-scale geomorphological features on experimental blocks of limestone, granite and marine concrete after eight months' exposure in the intertidal zone in Cornwall, UK.Significant differences in the occurrence of microbial growth features, and micro-scale weathering and erosion features were observed between material types (ANOVA p < 0·000). Exposed limestone blocks were characterized by euendolithic borehole erosion (99% occurrence) within the upper 34·0 Ϯ 12·3 mm of the surface. Beneath the zone of boring, inorganic weathering (chemical dissolution and salt action) had occurred to a depth of 125·0 Ϯ 39·0 mm. Boring at the surface of concrete was less common (27% occurrence), while bio-chemical crusting was abundant (94% occurrence, mean thickness 45·1 Ϯ 27·7 mm). Crusts consisted of biological cells, salts and other chemical precipitates. Evidence of cryptoendolithic growth was also observed in limestone and concrete, beneath the upper zone of weathering. On granite, biological activity was restricted to thin epilithic films (<10 mm thickness) with some limited evidence of mechanical breakdown.Results presented here demonstrate the influence of substratum lithology, hardness and texture on the nature of early micro-scale colonization, and the susceptibility of different engineering materials to organic weathering and erosion processes in the intertidal zone. The implications of differences in initial biogeomorphic responses of materials for long-term rock weathering, ecology and engineering durability are discussed.

Cool barnacles: Do common biogenic structures enhance or retard rates of deterioration of intertidal rocks and concrete?

“…These interactions are further moderated by bioerosive microorganisms, which act to increase near-surface pore space (particularly in calcareous rocks) once exposed in the intertidal zone (Coombes et al, 2011). With respect to moisture retention, the influence of surface orientation warrants further investigation given that retention (both on the surface and within biological structures) may vary considerably between horizontal (as simulated here) and vertical or sloping surfaces.…”

“…Sample blocks (50 x 50 x 30 mm) of Portland limestone, Cornish granite and marine grade concrete were attached to two rocky shore platforms in Cornwall, UK, at mean tide level (see Coombes et al, 2011). After 32 months of exposure, five blocks of each material type that had been colonised by barnacles (Chthamalus spp.)…”

“…This can be partly explained by the development of bio-chemical crusts after relatively short periods of intertidal exposure (Coombes et al, 2013b;Coombes et al, 2011). These crusts (which were absent from limestone and granite) limit water uptake and release relative to unexposed (i.e., fresh) concrete, and thereby moderate thermal gain and loss .…”

“…Richard Thompson and Dr Stephen Roast are thanked for their support in setting up the field experiments (Coombes et al 2011) from which the materials used here originally derive. DeLank Quarry, Stone Firms Ltd., Leslie Randle and Steve…”

Rock armour for birds and their prey: ecological enhancement of coastal engineering

“…Carboniferous limestone rock armour was added into the upper areas of the revetment to create the necessary engineering void ratio and a more ecologically favourable substrate than granite over the short term (e.g. Coombes et al, 2011) and long term (e.g. Plymouth breakwater).…”