Abstract
Some cyanobacteria, including Candidatus Gloeomargarita lithophora, which was isolated from Lake Alchichica (Mexico), can form intracellular carbonates. This contradicts the common paradigm that cyanobacterial calcification is always extracellular and suggests that calcification might be controlled by these cyanobacterial species. Intracellular carbonates have several peculiar characteristics: they are relatively small (between 60 and 500nm), they are poorly crystalline, and they have Sr/Ca and Ba/Ca ratios much higher than the solution in which the cells grow. It is therefore crucial to understand whether these unique features may indicate the involvement of specific biological processes. Here, in vitro abiotic syntheses were performed to synthesize Mg-, Ca-, Sr- and Ba-containing carbonates with compositions, crystallinities and sizes close to those observed in intracellularly calcifying cyanobacteria. Precipitates were characterized by scanning and transmission electron microscopies coupled with energy dispersive X-ray spectroscopy, thermogravimetric analysis and X-ray diffraction. The size and the poor crystallinity of cyanobacterial intracellular carbonates could be mimicked under these abiotic conditions. It was shown that similarly to Mg, elements such as Sr and Ba can favor stabilization of poorly crystalline carbonates. In contrast, the differential partitioning of Sr, Ba and Ca between the solution and the solids as observed in cyanobacteria could not be mimicked in vitro. This provides keys to a better understanding of biological processes involved in the formation of intracellular carbonates by some cyanobacteria, including the involvement of membrane transporters.
Highlights
Cyanobacteria are photosynthetic prokaryotes that occupy a wide array of terrestrial, marine, and freshwater habitats (Badger et al, 2006)
In a first experiment, called “stoichiometric conditions”, MgCl2, CaCl2, SrCl2, BaCl2 and Na2CO3 were mixed in a solution with relative proportions similar to those measured by Scanning electron microscopy (SEM)-Energy dispersive x-ray spectrometry (EDXS) by Couradeau et al (2012) in G. lithophora intracellular inclusions (i.e., Mg = 18%, Ca = 53%, Sr = 8%, Ba = 21%, in atomic ratios relative to the total cation content)
The diffractogram revealed the presence of two phases: (1) a poorly crystalline phase with broad peaks centered at 30°, 45° and 65° (2θ) attributed to a (Ca, Mg, Sr, Ba)-carbonate phase consistently with Transmission electron microscopy (TEM) observations and former x-ray diffraction (XRD) analyses of amorphous calcium carbonates (ACC), and (2) a crystalline phase with narrow peaks centered at 27.4°, 31.7°, 45.5°, 56.5°, 66.3° and 75.3° (2θ), assigned to NaCl
Summary
Cyanobacteria are photosynthetic prokaryotes that occupy a wide array of terrestrial, marine, and freshwater habitats (Badger et al, 2006) They contribute significantly to the global carbon cycle by assimilating CO2 to organic carbon (Fuhrman, 2003) and by triggering CaCO3 precipitation (e.g., Verrecchia et al, 1995; Kamennaya et al, 2012). The general reaction of CaCO3 precipitation can be written as: Ca2+ aq + CO32- aq = CaCO3, solid (1) This is coupled with the acid-base equilibrium of carbonates, e.g. CO2 is fixed by the RuBisCO enzyme and transformed into organic carbon while CO32- and/or OHproduced by the carbonic anhydrase reaction are exported to the extracellular medium (e.g., (Kamennaya et al, 2012)). An additional source of oversaturation is provided by an active export of Ca2+ from the cells coupled with an import of H+ (Thompson et al, 1997)
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