Effects of nitrogen and phosphorus availability on the expression of the coccolith-vesicle V-ATPase (subunit c) of Pleurochrysis (Haptophyta)
The coccolithophores Emiliania and Pleurochrysis demonstrate increased coccolith production when growth is reduced by nitrate or phosphate limitation. The function of enhanced coccolith production under these conditions and its regulation have not been resolved. Studies at the molecular level are ideally suited to determine the exact relationship between calcification and other cellular functions. In a previous study we provided evidence for the presence of a vacuolar H+-ATPase on coccolith vesicle membranes of P. carterae. These trans-Golgi-derived vesicles are the sites of coccolith production, with each vesicle containing one coccolith. In this study, expression of the vap gene, encoding subunit c of the vacuolar H+-ATPase, was investigated. Our objective was to explore potential relationships between vap expression, nutrient-dependent growth, and calcification. Specifically, we monitored vap expression relative to two genes, fcp and pcna, whose expression was previously shown to vary with growth conditions; fcp encodes a fucoxanthin chl a/c-binding protein, and pcna encodes the proliferating cell nuclear antigen. Relative to the expression of pcna and fcp, vap expression was highest at nutrient concentrations where growth curves and chl a patterns indicated arrest of cell division. Our results indicate that the level of vap expression does not decrease when cell growth diminishes.