Biomineralization in perforate foraminifera

Abstract

a b s t r a c t a r t i c l e i n f oIn this paper, we review the current understanding of biomineralization in perforate foraminifera. Ideas on the mechanisms responsible for the flux of Ca 2+ and inorganic carbon from seawater into the test were originally based on light and electron microscopic observations of calcifying foraminifera. From the 1980s onward, tracer experiments, fluorescent microscopy and high-resolution test geochemical analysis have added to existing calcification models. Despite recent insights, no general consensus on the physiological basis of foraminiferal biomineralization exists. Current models include seawater vacuolization, transmembrane ion transport, involvement of organic matrices and/or pH regulation, although the magnitude of these controls remain to be quantified. Disagreement between currently available models may be caused by the use of different foraminiferal species as subject for biomineralization experiments and/or lack of a more systematic approach to study (dis)similarities between taxa. In order to understand foraminiferal controls on element incorporation and isotope fractionation, and thereby improve the value of foraminifera as paleoceanographic proxies, it is necessary to identify key processes in foraminiferal biomineralization and formulate hypotheses regarding the involved physiological pathways to provide directions for future research.

Chamber formation leads to Mg/Ca banding in the planktonic foraminifer Neogloboquadrina pachyderma

“…We can therefore reasonably conclude that the Mg/Ca banding in N. pachyderma is also unlikely to be result of organic layering in the test wall. As such, consistent with most recent hypotheses on Mg/Ca banding (de Nooijer et al, 2014;Spero et al, 2015), we propose that such banding is intrinsic to the calcification mechanism of planktonic foraminifera. Below we discuss Mg/Ca heterogeneity in the lamellar and crust calcite, as well as the paleoceanographic implications of this intratest compositional variability.…”

“…Bentov and Erez, 2006;de Nooijer et al, 2014). Planktonic foraminifera build their skeleton by progressively adding chambers.…”

“…However, Mg/Catemperature relationships are species specific and remain (largely) empirical and many studies have shown a tight biological control on Mg incorporation in foraminiferal calcite (Anand et al, 2003;Bentov and Erez, 2006;de Nooijer et al, 2014;Jonkers et al, 2012;Jonkers et al, 2013a;Lea et al, 1999;Spero et al, 2015). In particular, the Mg distribution within single foraminifera tests shows large, non-temperature related variability (Eggins et al, 2003;Eggins et al, 2004;Kunioka et al, 2006;Sadekov et al, 2005).…”

“…Alternative hypotheses involve the participation of a tiny amount of seawater (which has high Mg concentrations) at the initiation of chamber formation, which would result in a high Mg band at the beginning (inner edge) of each lamina (de Nooijer et al, 2014), or an active role for mitochondria in Mg removal from the site of calcification, such that their reduced activity or density, possibly diurnally modulated, may cause high Mg bands (Spero et al, 2015). Nevertheless, the origin of Mg-banding is not yet fully understood and the suggested mechanisms are to a large extent based on observations from a single tropical and symbiont-bearing species O. universa, which forms a unique spherical outer chamber and hence differs from other species of planktonic foraminifera (Spero, 1988).…”

Reconsidering the role of carbonate ion concentration in calcification by marine organisms

“…Here, organisms do not exclusively rely on direct inorganic carbon utilization from seawater but supplement calcification to a variable degree with CO 2 gained intracellularly from respired biomass. This CO 2 utilization may be further strengthened (1) when metabolic CO 2 is "trapped" inside the organisms through the establishment of pH gradients, which limit the diffusive loss of CO 2 (Bentov et al, 2009, Glas et al, 2012b, or (2) when CO 2 is transported actively towards the site of calcification (de Nooijer et al, 2014). Thus, CO 2 reacting with H 2 O to form HCO − 3 and H + (catalyzed by the ubiquitous enzyme carbonic anhydrase) could be an alternative inorganic carbon source for calcification in particular taxa.…”