Non-Skeletal Biomineralization by Eukaryotes: Matters of Moment and Gravity

Abstract

Skeletal biomineralisation by microbial eukaryotes significantly affects the global biogeochemical cycles of carbon, silicon and calcium. Non-skeletal biomineralisation by eukaryotic cells, with precipitates retained within the cell interior, can duplicate some of the functions of skeletal minerals, e.g. increased cell density, but not the mechanical and antibiophage functions of extracellular biominerals. However, skeletal biomineralisation does not duplicate many of the functions of non-skeletal biominerals. These functions include magnetotaxis (magnetite), gravity sensing (intracellular barite, bassanite, celestite and gypsum), buffering and storage of elements in an osmotically inactive form (calcium as carbonate, oxalate 2 polyphosphate and sulfate; phosphate as polyphosphate) and acid-base regulation, disposing of excess hydroxyl ions via an osmotically inactive product (calcium carbonate, calcium oxalate).While polyphosphate has a wide phylogenetic distribution among microbial eukaryotes, other non-skeletal minerals have more restricted distributions, and as yet there seems to be no definitive evidence that the alkaline earth components (Ba and Sr) of barite and celestite are essential for completion of the life cycle in organisms that produce these minerals.

Environmental controls on coccolithophore calcification

“…Buitenhuis et al (1999) (Paasche 1964, Herfort et al 2004, Trimborn et al 2007, Leonardos et al 2009, Xu et al 2011. The intracellular precipitation of calcite by coccolithophores when the bulk medium is undersaturated has parallels in the intracellular deposition of celestite in acantharians (Raven & Knoll 2010) and of silica (opal) by diatoms (Raven & Waite 2004). However, in the case of celestite and silica, the present surface ocean is well below the saturation value for these 2 minerals, and for silica this has been the case since (at least) soon after the appearance of diatoms in the fossil record (Raven & Waite 2004, Raven & Knoll 2010.…”

Competition between Silicifiers and Non-silicifiers in the Past and Present Ocean and Its Evolutionary Impacts

“…Based on Ensikat et al (2016), Gal et al (2012), Knoll (2003), Knoll and Kotrc (2015), Marron et al (2016b), Raven and Knoll (2010), Weich et al (1989), and references therein. present in an undissociated form (Si(OH) 4 ) under seawater pH conditions (Del Amo and Brzezinski, 1999), it is possible for silicic acid to diffuse across the cell membrane (Raven, 1983;Thamatrakoln and Hildebrand, 2008). In the high silicic acid Precambrian ocean, diffusion from the surrounding Si-saturated seawater could result in silica precipitating freely within the cytoplasm, interfering with cellular processes and disabling the functioning of the cell.…”

“…Raven (1983) calculated that the energetic costs of silicic acid uptake and silica structure formation is substantially more efficient than forming the same volume of an organic structure (∼20x more for lignin and 10x for polysaccharides like cellulose). Based on the structural model of biogenic silica of Hecky et al (1973), Lobel et al (1996) identified by biochemical modeling a low-energy reaction pathway for nucleation and growth of silica.…”

Protistan Skeletons: A Geologic History of Evolution and Constraint

“…Calcite mineralizes the tests of some foraminiferans (Lipps 1973) and the scales of coccolithophorid algae (Young and Henriksen 2003), and rare occurrences of carbonate biomineralization have been reported among dinoflagellates and in at least one amoebozoan. Phosphatic biomineralization is exceedingly rare in protists, being reported only in the scales of a single freshwater green phytoflagellate (Domozych et al 1991, but see Raven and Knoll 2010) and in the test lining of a freshwater amoebozoan (Hedley et al 1977). In contrast, amorphous silica occurs in diverse protistan clades (Preisig 1994, Fig.…”

“…Eukaryotic organisms precipitate a variety of minerals intracellularly, serving a range of functions that includes the regulation of buoyancy, gravity sensing, magnetotaxis, and storage (Raven and Knoll 2010). Of particular interest are those eukaryotes, large and small, that precipitate mineralized skeletons: tests, scales, spicules, sclerites, shells and bones.…”