The significance of calcite (travertine) formation by algae in a moss-dominated travertine from Matlock Bath, England

Abstract

An investigation of epiphytic algae on the leaves of the bryophyte Cratoneuron commutatum var. commutatum from a thermal travertine-depositing spring revealed 18 species of diatoms and two cyanobacteria. Algae and calcite were detected on leaves immediately after their expansion, with diatoms Achnanthes minutissima aggr., Denticula tenuis and Navicula cari var. cincta the most numerous. In terms of biomass (as biovolume) and surface area, Rhopalodia gibba was the most important epiphyte. The algal epiphytes attained maximum numbers 20-30 mm below the moss cushion surface and thereafter declined due to lack of light. Travertine (calcite) deposition increased in a linear fashion with cushion depth over the first 80 mm but with a lag in deposition at the stem apex. About 1 μg calcite was deposited per mm 2 leaf/day. A significant association was found between the presence of the large diatom Rhopalodia gibba and calcite deposition but no associations were found with the remaining algae. The results suggest that Rhopalodia exerts a positive effect on calcite deposition by presenting a favourable physico-chemical microenvironment through biochemical (including photosynthetically-driven) processes. Travertine deposition continued well below the level where the moss leaves were photosynthetically active and below the level where the algae attained maximum abundance. Although diatoms have been shown to influence calcite precipitation, the deposition of travertine was considered to be controlled mainly by inorganic processes.