Microbial excavation of solid carbonates powered by P-type ATPase-mediated transcellular Ca ²⁺ transport

Abstract

Some microbes, among them a few species of cyanobacteria, are able to excavate carbonate minerals, from limestone to biogenic carbonates, including coral reefs, in a bioerosive activity that directly links biological and geological parts of the global carbon cycle. The physiological mechanisms that enable such endolithic cyanobacteria to bore, however, remain unknown. In fact, their boring constitutes a geochemical paradox, in that photoautotrophic metabolism will tend to precipitate carbonates, not dissolve them. We developed a stable microbe/mineral boring system based on a cyanobacterial isolate, strain BC008, with which to study the process of microbial excavation directly in the laboratory. Measurements of boring into calcite under different light regimes, and an analysis of photopigment content and photosynthetic rates along boring filaments, helped us reject mechanisms based on the spatial or temporal separation of alkali versus Acid-generating metabolism (i.e., photosynthesis and respiration). Instead, extracellular Ca 2+ imaging of boring cultures in vivo showed that BC008 was able to take up Ca 2+ at the excavation front, decreasing the local extracellular ion activity product of calcium carbonate enough to promote spontaneous dissolution there. Intracellular Ca 2+ was then transported away along the multicellular cyanobacterial trichomes and excreted at the distal borehole opening into the external medium. Inhibition assays and gene expression analyses indicate that the uptake and transport was driven by P-type Ca 2+ -ATPases. We believe such a chemically simple and biologically sophisticated mechanism for boring to be unparalleled among bacteria.bioerosion | calcium metabolism | endoliths | microbialites

Extreme cellular adaptations and cell differentiation required by a cyanobacterium for carbonate excavation

“…The model also assumed that protons constitute the counter ions exchanged for calcium to maintain charge. The concentrations of Ca 2+ i attained during transport, which are of paramount importance for cell physiology, given the ion's toxicity in most cellular systems, including cyanobacteria (14,15), remain unknown, but there is a potential that a borer's cells might experience prolonged, very high Ca 2+ i concentrations (10). The experimental evidence in support of the current model was obtained using selective inhibitors and an extracellular calcium fluorophore to study Ca 2+ dynamics during boring.…”

“…If transport were driven exclusively by distal apical pumps, then severing a filament midway should not result in leakage of calcium at the breakpoint, given that the extracellular concentration in seawater medium (10 mM) is much higher than physiologically plausible (15). However, if pumps existed along the entire filament, they would remain active, and localized regions of calcium supersaturation in the medium would be observed adjacent to filament breakpoints, just as it is seen close to the apical cells that extrude Ca 2+ to the outside medium (10). Mechanical severing of boring filaments at various filament locations after making them accessible by cleaving the mineral substrate along the direction of excavation did indeed result in substantial and localized increases in Ca 2+ in regions directly adjacent to damaged filaments (Fig.…”

“…The inhibition of calcium extrusion in BC008 by P-type calcium ATPase-binding antagonists, such as thapsigargin (10,16), had revealed their agency in the process of calcium extrusion during boring. In an attempt to localize the exact sites of activity of these enzymes, we used fluorescently labeled thapsigargin (BDTH), which has been shown to bind specifically to eukaryotic P-type calcium ATPases (17,18).…”

“…In our laboratory, we have recently developed a conceptual model of the physiology of cyanobacterial carbonate excavation based on experiments carried out with the model filamentous euendolith, Mastigocoleus testarum strain BC008 (10). Similar to many filamentous cyanobacteria, this strain is a true multicellular microbe: it displays complex morphological plasticity, which includes cell differentiation of lateral heterocysts dedicated to nitrogen fixation, motile multicelled hormogonia for dispersal, true branching of the main thallus, and the ability to alter filament morphology during boring into solid substrates (11).…”

Investigating the role of microbes in mineral weathering: Nanometre-scale characterisation of the cell–mineral interface using FIB and TEM

“…Cyanobacteria are photosynthetic oxygen-evolving bacteria distributed in many environments worldwide (Madigan et al, 2003), including surfaces and internal spaces of rocks (Gorbushina, 2007). It is known that cyanobacteria are capable of the alteration of some calcium containing minerals (Garcia-Pichel, 2006;Garcia-Pichel et al, 2010), however despite their common presence on rock surfaces in natural environments their potential role in silicate mineral weathering and nutrient release is at present poorly understood. Only a small number of laboratory-based studies on cyanobacterial bioweathering of silicate minerals exist and none of the studies specifically looked at the cell-mineral interface and the potential chemical changes occurring there (Brehm et al, 2005;Büdel et al, 2004;Chizhikova et al, 2009;Gorbushina and Palinska, 1999;Olsson-Francis and Cockell, 2010).…”