English: A role for silicon in coccolith formation.
(a) Representative SEM micrographs demonstrating the effects of Si limitation and Ge addition on coccolith production. C. braarudii, S. apsteinii and C. leptoporus were incubated for 72 h in very low Si seawater (<0.1 μM), which was amended with Ge (1 μM for S. apsteinii or 5 μM for the other two species). C. braarudii and C. leptoporus cells grown in very low Si appeared superficially similar to cells grown in Si-replete seawater (100 μM Si), but closer inspection revealed that many ‘blocky’ coccoliths are apparent (arrowed), indicating a calcification defect related to the lack of Si. The addition of Ge resulted in the production of highly aberrant coccoliths in all three species. In C. braarudii these aberrant liths fail to integrate fully into the coccosphere and were often shed into the media. Both types of heterococcolith in S. apsteinii (the large cup-shaped lopadoliths and the small plate-like muroliths) exhibit extensive malformations. In C. leptoporus the aberrant coccoliths are all co-localized, suggesting that the newly formed liths in this species are secreted in a similar position in the coccosphere. The addition of 100 μM Si to Ge-treated cells markedly reduced the inhibitory effects on calcification. Scale bar, 5 μm. (b) Quantification of the production of aberrant coccoliths in C. braarudii grown in very low Si media for 24 and 72 h, amended with 5 μM Ge or 5 μM Ge+100 μM Si. For this experiment, only highly aberrant coccoliths were scored and more subtle coccolith malformations such as the ‘blocky’ coccoliths observed under low Si were not scored. n=40 cells. For discarded liths 4–7 fields of view were scored containing at least 40 cells. The experiment was repeated four times and representative results are shown. (c) Quantification of the production of aberrant lopadoliths in S. apsteinii grown in low Si media for 72 h, amended with 1 μM Ge, or 1 μM Ge+100 μM Si. n=40 cells. The experiment was repeated four times and representative results are shown. Error bars denote s.e.