Biominerals in the Cacteae tribe: Chemical-morphological characterization, distribution and systematic perspectives

Abstract

• The occurrence of silica bodies in the hypodermis of cacti are diagnostic for all the species of Echinocactus , and the exclusion of E. grusonii of this genus is supported by the lack of silica bodies as well as the occurrence of calcium oxalate druses. • Six biomineral habits were found in the Cacteae species studies, being spherulites and prisms the most common. • Extracellular sandstones occur in six species of Mammillaria , commonly in the outer periclinal wall of the epidermis. • For the first time the structure of the biominerals of seven genera ( Cumarinia, Echinomastus, Escobaria, Obregonia, Sclerocactus , Stenocactus) of Cacteae are characterized. Biominerals precipitate by biological activity and are found in various organisms. In the Cactaceae family, biominerals have been described for numerous genera and species and their contribution to systematics has recently been demonstrated. The aims of this study are to characterize the biominerals in the stem of members of Cacteae by describing their shape, position and mineral composition. Twenty-eight genera and 149 species of Cacteae were studied by bright-field and polarized microscopy. The composition of biominerals was analyzed by scanning electron microscopy and energy-dispersive X-ray spectroscopy in 34 species representing the Cacteae clades. The biominerals of seven genera out of 28 are described for the first time. Only six species of Mammillaria have extracellular biominerals. Sixteen genera and 91 species possess intracellular biominerals in hypodermis, where species of Echinocactus and Ferocactus showed silica bodies. Spherulites, prisms and druses are present in 28 genera, and are distributed in the cortex of 141 species and in the pith of 94 species. Statistical analysis revealed differences between genera for the maximum length of biominerals in the hypodermis and cortex. Notably, Kadenicarpus possesses the smallest biominerals in the cortex and pith (< 35 µm in mean) and the largest mean in the pith for Astrophytum (212 µm). In the Cacteae, calcium oxalates were the most predominant biominerals. Other elements such as magnesium, manganese, and aluminum were also identified. Lophophora is characterized by crystalline aggregates in the cortex of both species with calcium oxalate and traces of silicon and iron, which are not found in other crystalline aggregates of other taxa. We detected a unique combination of biominerals that allowed for us to distinguish the species of some genera, such as Astrophytum and Echinocactus . In conclusion, the occurrence, structure and size of biominerals provide valuable information for systematics at the generic level in Cacteae.