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

Abstract

Coccolithophores are globally abundant marine microalgae characterized by their ability to form calcite platelets (coccoliths). The coccoliths are produced internally in a Golgiderived vesicle. Mature coccoliths are extruded from the cell to form a protective covering on the cell surface, known as the coccosphere. Current evidence indicates that calcite precipitation in the coccolith vesicle (CV) is modulated by coccolith-associated polysaccharides (CAPs). Whilst previous research into CAPs has focussed on their roles in calcite precipitation within the CV, little is known of their extracellular roles. Using fluorescent lectins, we visualize the extracellular polysaccharide-rich organic layer associated with external coccoliths and demonstrate that it differs between species in structure and composition. Biochemical analysis of polysaccharide extracted from coccoliths indicated substantial differences between species in monosaccharide composition and uronic acid content. In Coccolithus braarudii our studies indicate that polysaccharide-rich material is extruded with the coccoliths, where it plays a role in the adhesion of the coccoliths to the cell surface and contributes to the overall organization of the coccosphere. Together, these results highlight the important extracellular roles of CAPs and their contribution to the dynamic nature of the coccosphere.

Acidic Monosaccharides become Incorporated into Calcite Single Crystals**

“…Later,t he presence of uronic acids in the PSs of coccoliths was detected in several strains and species. [79][80][81] Interestingly,t wo recent papers attribute lattice distortions in calcite to the incorporation of coccolith macromolecules. [82,83] However,n oneo ft he reports firmly establishes the types of molecules, purified or not, responsible for this effect.…”

“…Further biochemical characterization showed that the galacturonic acid residues, through their carboxy groups and not their ester sulfate groups, are solely responsible for the inhibition of CaCO 3 precipitation, [78] a property that establishes uronic acids as key players in the interaction with calcium carbonate. Later, the presence of uronic acids in the PSs of coccoliths was detected in several strains and species [79–81] . Interestingly, two recent papers attribute lattice distortions in calcite to the incorporation of coccolith macromolecules [82, 83] .…”

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

“…For example, some CAP influence crystal morphology by site-specific attachment to crystallographic steps (Henriksen et al, 2004), but others might not. S. apsteinii have CAP that are known to differ from those of some placolith bearing species, such as C. braarudii (Walker et al, 2018a). We propose that the difference in CAP between S. apsteinii and other species could contribute to different Sr fractionation behavior in at least three ways: 1) intra coccolith vesicle CAP could alter the vesicle Ca 2+ concentration needed to achieve super-saturation, which would in turn affect Sr fractionation (Langer et al, 2006).…”

The Polar Lipidome of Cultured Emiliania huxleyi: A Source of Bioactive Lipids with Relevance for Biotechnological Applications

“…Emiliania huxleyi biomass (2 mg × 3 replicates) was subjected to the same prehydrolysis and hydrolysis conditions described for phenol-sulfuric acid method. For determination of uronic acids (UA) content by m-phenylphenol colorimetric method [ 36 ], a volume of 500 µL of biomass hydrolysate was recovered after 1 h of hydrolysis at 100 °C with 1 M H 2 SO 4 and diluted in 1 mL of distilled water. d -Galacturonic acid solutions (0–80 µg mL −1 ) were used to construct a calibration curve.…”