Characterization of calcium oxalates generated as biominerals in cacti

Abstract

The chemical composition and morphology of solid material isolated from various Cactaceae species have been analyzed. All of the tested specimens deposited high-purity calcium oxalate crystals in their succulent modified stems. These deposits occurred most frequently as round-shaped druses that sometimes coexist with abundant crystal sand in the tissue. The biominerals were identified either as CaC(2)O(4).2H(2)O (weddellite) or as CaC(2)O(4).H(2)O (whewellite). Seven different species from the Opuntioideae subfamily showed the presence of whewellite, and an equal number of species from the Cereoideae subfamily showed the deposition of weddellite. The chemical nature of these deposits was assessed by infrared spectroscopy. The crystal morphology of the crystals was visualized by both conventional light and scanning electron microscopy. Weddellite druses were made up of tetragonal crystallites, whereas those from whewellite were most often recognized by their acute points and general star-like shape. These studies clearly demonstrated that members from the main traditional subfamilies of the Cactaceae family could synthesize different chemical forms of calcium oxalate, suggesting a definite but different genetic control. The direct relationship established between a given Cactaceae species and a definite calcium oxalate biomineral seems to be a useful tool for plant identification and chemotaxonomy.

Figure 1

Portion of calcium oxalate druse biominerals from two representative Cactaceae spp. that emphasize microcrystal appearance and arrangement at the surface of single druses. A, Weddellite from G. platense (Cereoideae). B, Whewellite from O. penicilligera (Opuntioideae).

Figure 2

Infrared spectra, in the range 4,000 to 400 cm⁻¹, of whewellite from Puna clavaroides (A) and weddellite from Pyrrhocactus strausianus (B).

Figure 3

Weddellite druses from Cereoideae: light microscopy. Crystalline aggregates from Chamaecereus silvestrii (A), Cleistocactus baumanii (B), Wigginsia tephracantha (C), and P. strausianus (D) (scale bars = 200 μm). C, Inset, a purified sample of crystal sand isolated from W. tephracantha. The inset in D tries to lay stress on developmental stages toward the formation of a spherical crystalline cluster in P. strausianus. Small mineral fragments seem to be deposited from a starting point at the center of prismatic solid skeletons. Scale bars = 200 μm.

Figure 4

Weddellite druses from Cereoideae: scanning electron microscopy. Fine structure of typical freshly isolated deposits from W. tephracantha (A), G. platense (B), C. silvestrii (C), and P. strausianus (D). Individual tetragonal crystallites are usually tightly clustered to form a more or less spherical dense mineral body. A, Inset, area-restricted EDAX spectrum from the above druse. The expected peaks for calcium and oxygen in the calcium oxalate are denoted. Scale bars = 60 μm.

Figure 5

Gill-like weddellite druses from the Cereoideae R. margarethae. A and B, Light microscopy (scale bars = 80 μm). C and D, Scanning electron microscopy. Most druses were made up of very thin tightly packaged flat crystalline lamella. The general appearance of these agglomerates is noteworthy.

Figure 6

Whewellite from the Opuntioideae subfamily: stellate druses from the genera Opuntia. Crystals from these genera were found visually indistinguishable among the examined species. A, SEM micrograph of a representative whewellite druse from Opuntia auranthiaca (above) and area-restricted EDAX-spectrum from the same crystal (below) confirming the sole presence of calcium and oxygen in the aggregate. B, Detail of a typical stellate druse from Opuntia longispina (light microscopy, 500× magnification). C, Population of purified stellate druses from O. penicilligera (light microscopy, 100× magnification). Scale bars from light microscopy = 80 μm.

Figure 7

Whewellite druses from other Opuntioideae members: conventional light photographs of purified crystal preparations of druses from Tephrocactus articulatus (A), Maihueniopsis glomerata (B), and Puna clavarioides (C). These whewellite deposits were much smaller in diameter than that of the Opuntia spp. Morphological differences were also evident. Scale bars = 100 μm.

Figure 8

SEM photographs of whewellite druses from other Opuntioideae members: M. glomerata (A) and P. clavarioides (B).