Sr in coccoliths of Scyphosphaera apsteinii: Partitioning behavior and role in coccolith morphogenesis

Abstract

Coccolithophores are important contributors to global calcium carbonate through their species-specific production of calcite coccoliths. Nannofossil coccolith calcite remains an important tool for paleoreconstructions through geochemical analysis of isotopic and trace element incorporation including Sr, which is a potential indicator of past surface ocean temperature and productivity. Scyphosphaera apsteinii (Zygodiscales) exhibits an unusually high Sr/Ca ratio and correspondingly high partitioning coefficient (DSr = 2.5) in their two morphologically distinct types of coccoliths. Whether or not this reflects mechanistic differences in calcification compared to other coccolithophores is unknown. We therefore examined the possible role of Sr in S. apsteinii calcification by growing cells in deplete, ambient, and higher than ambient Sr conditions (between 0.33-140 mmol/mol Sr/Ca). The effects on growth, quantum efficiency of photosystem II (Fv/Fm), coccolith morphology, and calcite DSr were evaluated. No effect on S. apsteinii growth rate or Fv/Fm was observed when cells were grown in Sr/Ca between 0.33-36 mmol/mol. However, at 72 mmol/mol Sr/Ca growth rate was significantly reduced, although Fv/Fm was unaffected. At 140 mmol/mol cultures failed to grow. Reducing the Sr/Ca from ambient (9 mmol/mol) did not significantly alter the frequency of malformed and aberrant muroliths and lopadoliths, but at higher than ambient Sr/Ca conditions coccolith morphology was significantly disrupted. This implies that Sr is not a critical determining factor in normal coccolith calcite morphology in this dimorphic species. Using electron dispersive spectroscopy (EDS) we observed an increase in [Sr] and decrease in DSr of coccoliths as the Sr/Ca of the growth medium increased. Interestingly, muroliths had significantly lower Sr/Ca than lopadoliths at ambient and elevated [Sr], and lopadolith tips had lower Sr than bases in ambient conditions. In summary, the Sr fractionation behavior of S. apsteinii is unusual because of an overall high DSr, and an inter-and intra-coccolith variability in

Evolutionary Rates in the Haptophyta: Exploring Molecular and Phenotypic Diversity

“…The latter, together with the Braarudosphaeraceae, do not calcify in the haploid phase but produce coccoliths with high-Mg calcite [54] during the diploid phase, suggesting different cellular ion transport pathways or rates of ion uptake compared to other coccolithophores. In a similar vein, strontium is an interesting element that has been demonstrated to have relatively low concentrations in the coccoliths of most taxa, except for one striking example, the Pontosphaeraceae [130,131] and an extinct Jurassic morphospecies [132]. A silicon dependency in the heterococcolith formation appears to be correlated with the status of "obligate calcifier" [75], whereas those that lack the same can survive without calcifying (Figure 8).…”

Evidence of high Sr/Ca in a Middle Jurassic murolith coccolith species

“…The current knowledge on coccolithophore phylogeny and Sr/Ca cannot exclude one or the other solutions, hence a possible ion pump or polysaccharide could have favored the high Sr/Ca in C. crassus. Recently, Meyer et al (2020) observed that Sr 2+ and Ca 2+ fluxes in the coccolith vesicle for coccolithogenesis does not solely govern Sr incorporation in the coccolith speculating than that possible species-specific polysaccharides dedicated to a peculiar coccolith morphology may act in the high Sr/Ca observed in Scyphosphaera.…”

Unexpected silicon localization in calcium carbonate exoskeleton of cultured and fossil coccolithophores

“…In order to better understand the mechanism of coccolithogenesis, it is important to obtain information about the presence and distribution of minor elements as well as organic compounds in coccoliths 14 . While the distribution of minor elements in general is helpful, the one of Si is particularly important because Si plays a role in morphogenesis on Si-requiring species, contrary to other minor elements such as Sr 7 . Unfortunately, very little is known about the presence of Si in coccoliths, and even less about its distribution.…”

“…Until recently, the study of coccolithophore calcification physiology has mainly focused on the structural components of coccolith calcite, i.e. calcium ion (Ca 2+ ) and dissolved inorganic carbon 1 , 5 , 6 , and on the fractionation of certain minor elements such as strontium (Sr) 7 – 9 . Since silicon is not a component of calcite, the Si requirement of some coccolithophore species had not been expected and was not realized until Durak et al 10 showed that some species need Si to calcify.…”