Ultrastructure and geochemistry of endolithic microorganisms in limestone of the Niagara Escarpment

Abstract

Endolithic microbial communities dominated by photosynthetic cyanobacteria occur at a depth of 0.5 – 3.0 mm inside dolomitic limestone that forms the vertical cliffs of the Niagara Escarpment in Ontario, Canada. Coccoid Gloeocapsa was the most prominent cyanobacterial species in colonized rock samples examined by differential interference contrast and episcopic fluorescent microscopy. Filamentous varieties of cyanobacteria were less common than coccoid forms but could be easily distinguished in all slide mount preparations. Scanning electron microscopy showed that the endoliths occurred in communal populations situated within the open pore spaces of the limestone. Heterotrophic bacteria were also observed growing as epiphytes on dead and living cyanobacteria, and in epilithic biofilms on pore space walls. In thin-sectioned specimens examined by transmission electron microscopy, these adherent bacteria commonly exhibited extensive accumulations of fibrous extracellular polymeric materials external to their cell walls. Biomass concentrations, estimated from the organic phosphorus content of the samples, generally paralleled rock porosity and ranged from 3.3 to 17.1% dry weight. Carbon isotope (14C) measurements indicated that atmospheric carbon dioxide is used by the endolithic cyanobacteria, rather than dissolved inorganic carbon from the weathering of carbonate minerals in the limestone host rock. In addition, whole rock multielement analyses revealed an enrichment of some elements (e.g., phosphorus, barium, lead, and zinc) in the endolith zone over the host rock, while other elements (e.g., magnesium, calcium, iron, and copper) were depleted. The implication is that the endolithic microorganisms play an active role in the biogeochemical cycling of nutrient and trace elements in cliff face ecosystems of the Niagara Escarpment.Key words: limestone, endolith, biofilm, cyanobacteria, geochemistry.