The requirement for calcification differs between ecologically important coccolithophore species

Abstract

Summary Coccolithophores are globally distributed unicellular marine algae that are characterized by their covering of calcite coccoliths. Calcification by coccolithophores contributes significantly to global biogeochemical cycles. However, the physiological requirement for calcification remains poorly understood as non‐calcifying strains of some commonly used model species, such as Emiliania huxleyi, grow normally in laboratory culture.To determine whether the requirement for calcification differs between coccolithophore species, we utilized multiple independent methodologies to disrupt calcification in two important species of coccolithophore: E. huxleyi and Coccolithus braarudii. We investigated their physiological response and used time‐lapse imaging to visualize the processes of calcification and cell division in individual cells.Disruption of calcification resulted in major growth defects in C. braarudii, but not in E. huxleyi. We found no evidence that calcification supports photosynthesis in C. braarudii, but showed that an inability to maintain an intact coccosphere results in cell cycle arrest.We found that C. braarudii is very different from E. huxleyi as it exhibits an obligate requirement for calcification. The identification of a growth defect in C. braarudii resulting from disruption of the coccosphere may be important in considering their response to future changes in ocean carbonate chemistry.

An Extracellular Polysaccharide-Rich Organic Layer Contributes to Organization of the Coccosphere in Coccolithophores

“…Some structural diversity was observed in the polysaccharide layers between species. C. braarudii exhibited a granular polysaccharide layer with distinct regularly spaced areas in which staining was absent (Figure 2A), as observed previously (Walker et al, 2018). These regular ellipsoidal regions likely correspond to the position of the coccoliths.…”

“…We have recently demonstrated that polysaccharides can be visualized around the coccolithophore C. braarudii using fluorescent dyes conjugated to lectins (Walker et al, 2018). In the present study, we use fluorescence microscopy to examine the extracellular polysaccharides associated with coccoliths in five coccolithophore species.…”

“…As the oval-shaped regions are preserved even after decalcification, it suggests that the polysaccharide-rich organic layer in C. braarudii possesses structural properties that may contribute to the organization of the coccosphere. We have previously shown that the nonstained regions in the organic layer are reduced in number or absent when coccolith production is disrupted by treatment with germanium (Ge) (Walker et al, 2018), suggesting that they represent apertures in the polysaccharide layer corresponding to the position of the coccoliths. To test this hypothesis we partially decalcified C. braarudii cells so that we could image the position of the coccoliths in the polysaccharide layer.…”

A 15 million-year long record of phenotypic evolution in the heavily calcified coccolithophore <i>Helicosphaera</i> and its biogeochemical implications

“…Coccolithus and Helicosphaera, and non-obligate calcifiers such as Emiliania huxleyi (Durak et al, 2016;Walker et al, 2018).…”

Looking away from the streetlight – new insights into marine calcification

“…In this issue of New Phytologist , Walker et al . (pp. 147–162) present a study that addresses the link between coccolith formation and cell biology.…”