GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 16, NO. 1, 10.1029/2001GB001408, 2002

5. Implications for Biomineralization

[20]   Previous studies suggest that calcification in E. huxleyi is related to photosynthetic carbon fixation [Simkiss and Wilbur, 1989; Sikes and Fabry, 1994; Brownlee et al., 1995], although identifying the exact interactions by which the two processes are linked is still contentious [e.g., McConnaughey, 1993; Brownlee et al., 1995]. How photosynthetic carbon fixation and the incorporation of Sr into the calcite coccolith are connected is an even more challenging question. By exploring the mechanism of biomineralization of coccolithophorids, it may be possible to answer this question and gain insight into why calcifying algae developed in the first place.

[21]   A physical mechanism for the assembly and extrusion of a coccolith has been proposed by Westbroek et al. [1984] and detailed by Simkiss and Wilbur [1989] and Lowenstam and Weiner [1989]. The coccolith forms in a vesicle closely associated with the nucleus in E. huxleyi. Attached to this coccolith vesicle is a reticular body, which is thought to pass matrix material and calcium, supplied by Golgi vesicles, to the forming coccolith. Assembly of the E. huxleyi coccolith, which consists of a cycle of radially and vertically oriented calcite crystals, involves a folded acidic polysaccharide matrix [Young et al., 1992]. This polysaccharide matrix is thought to promote and mould calcification by providing uranic acid groups as nucleation sites for Ca²⁺ but also inhibits crystal growth by adhering to the surface of the coccolith when construction is complete [Westbroek et al., 1984]. Once the coccolith is formed, the reticular body detaches and degenerates while the coccolith vesicle moves to the surface of the cell and exocytoses the mineralized body onto the surface to form an interlocking sphere of coccoliths.

[22]   As calcification occurs in an intracellular vesicle, it is likely that the biological control on the relative rates of supply of Ca²⁺ and Sr²⁺ ions to the vesicle outweigh any inorganic kinetic controls [Lorens, 1981] on the composition of the precipitating calcite. This is exemplified by the greater sensitivity of the cultured coccolith Sr/Ca to changes in growth rate compared to inorganic experiments, although absolute comparison of values is difficult because of the inorganic data depending on the mass of seed added. In inorganic systems a 100-fold increase in precipitation rate is associated with a 3.5-fold increase in D_Sr, whereas in culture an increase in precipitation rate by only 1 order of magnitude results in a sixfold increase in D_Sr. All of the values of D_Sr published for coccolithophorids are higher than those quoted for inorganic systems, which implies that biology must be controlling the chemistry of the coccolith.

[23]   In order to understand how Sr may be affected by the biomineralization process, it is necessary to review intracellular Ca and Sr transport. Calcium plays a dual role in E. huxleyi as both a substrate for calcification and an intracellular regulator, and the cytosol concentration of free calcium is rigorously controlled and maintained at a very low level. By contrast, Sr has no specific biological role in a coccolithophore. Sr is more chemically similar to Ca than any other trace element, and so it is possible that it substitutes for Ca and is transported via the same mechanism but at a different rate as Ca across each membrane. The Ca²⁺ ions necessary for the formation of coccoliths diffuse from seawater through Ca²⁺-selective channels into the cytosol of the coccolithophore driven by a potential difference and by a very low calcium ion activity in the cytosol (0.1 µM). This low cytosolic concentration of Ca means that Ca must be pumped against a concentration gradient at some stage during its transport to the coccolith vesicle in order to attain saturation within the coccolith vesicle sufficient for the precipitation of calcite. The rate-dependent discrimination between the biological transport of Sr²⁺ and Ca²⁺ suggested by our data could be associated with passive transport through ion channels or active pumping via carrier proteins. It should be noted that in a biologically more complex hermatypic coral, the precipitation of Sr and Ca may involve different biochemical mechanisms [Ip and Krishnaveni, 1991].

[24]   Carrier proteins, which transport Ca against a concentration gradient, act in a similar way to an enzyme-substrate reaction and have a binding site specific to the transported ion. One possibility is that the rate-dependent discrimination between Sr and Ca is controlled by the pumping of these specific carrier proteins. The carrier proteins could bind Ca²⁺ ions noncovalently more strongly than the Sr²⁺ ions because of the marginally higher charge density, as has been suggested by Stephan and Hasselbach [1991]. This means that the maximal rate of transport by the carrier protein V_max would be attained at a lower concentration of Ca than Sr (Figure 4). As the rate of transport increases, the concentration of transported Sr increases proportionally more than the Ca. Therefore the Sr/Ca transported to the vesicle and available for precipitation will increase with increased rates of pumping. This pumping of Ca against a concentration gradient requires an input of energy, which is most probably derived from photosynthetic products. In addition, a supplementary source of carbon dioxide for photosynthesis may be derived from the calcification reactions occurring within the coccolith vesicle such that higher ion pumping rates and calcification lead to higher rates of photosynthetic carbon fixation [Sikes et al., 1980; Sikes and Wheeler, 1982; Brownlee et al., 1995; McConnaughey, 1993]. These scenarios provide a plausible explanation for the relationship between µ_PIC and D_Sr.

[25]   Alternatively, the rate-dependent discrimination between the transport of Sr and Ca may occur during diffusion through the Ca²⁺ channels. There are indications that cytosolic Ca²⁺ controls this diffusion and also regulates directly the uptake of the bicarbonate ion (HCO₃^-) [Brownlee et al., 1995]. HCO₃^- can be considered to move passively into the cell, and at a pH of 7.0 a proportion will be protonated to form CO₂ for photosynthesis, and an equivalent amount is transported into the coccolith vesicle for calcification reactions [Anning et al., 1996]. Therefore, at high rates of photosynthetic carbon fixation and calcification, more HCO₃^- is required by the coccolithophore. Assuming that cytosolic Ca²⁺ is regulating HCO₃^- transport, then increased HCO₃^- could be achieved by an increased rate of supply of Ca through the ion pumps. If the Ca²⁺-channels transport ions more quickly, the selectivity against Sr may become less efficient such that the Sr/Ca of the cytosol and, ultimately, the coccolith could increase accordingly.

[26]   Each of the above mechanisms can be considered as a biological analogue of the inorganic model proposed by Lorens [1981]. In his model he proposes a rate-dependent discrimination by the crystal against the larger ions. We propose that strontium incorporation into coccoliths is controlled by a rate-dependent discrimination by the cell against the biological pumping of the larger ions.

Citation: Rickaby, R. E. M., D. P. Schrag, I. Zondervan, and U. Riebesell, Growth rate dependence of Sr incorporation during calcification of Emiliania huxleyi, Global Biogeochem. Cycles, 16(1), 10.1029/2001GB001408, 2002.

Copyright 2002 by the American Geophysical Union.