UNCOUPLING OF SILICON COMPARED WITH CARBON AND NITROGEN METABOLISMS AND THE ROLE OF THE CELL CYCLE IN CONTINUOUS CULTURES OFTHALASSIOSIRA PSEUDONANA(BACILLARIOPHYCEAE) UNDER LIGHT, NITROGEN, AND PHOSPHORUS CONTROL¹

Abstract

The elemental composition and the cell cycle stages of the marine diatom Thalassiosira pseudonana Hasle and Heimdal were studied in continuous cultures over a range of different light‐ (E), nitrogen‐ (N), and phosphorus‐ (P) limited growth rates. In all growth conditions investigated, the decrease in the growth rate was linked with a higher relative contribution of the G2+M phase. The other phases of the cell cycle, G1 and S, showed different patterns, depending on the type of limitation. All experiments showed a highly significant increase in the amount of biogenic silica per cell and per cell surface with decreasing growth rates. At low growth rates, the G2+M elongation allowed an increase of the silicification of the cells. This pattern could be explained by the major uptake of silicon during the G2+M phase and by the independence of this process on the requirements of the other elements. This was illustrated by the elemental ratios Si/C and Si/N that increased from 2‐ to 6‐fold, depending of the type of limitation, whereas the C/N ratio decreased by 10% (E limitation) or increased by 50% (P limitation). The variations of the ratios clearly demonstrate the uncoupling of the Si metabolism compared with the C and N metabolisms. This uncoupling enabled us to explain that in any of the growth condition investigated, the silicification of the cells increased at low growth rates, whereas carbon and nitrogen cellular content are differently regulated, depending of the growth conditions.

Effects of changing pCO2 and phosphate availability on domoic acid production and physiology of the marine harmful bloom diatom Pseudo‐nitzschia multiseries

“…Claquin et al (2002) showed that diatom cellular Si quotas increase with decreasing growth rates, along with an increase in cell size and elongation of the gap phase G and mitosis. In our study, an increase in Si quota also coincides with a reduced growth rate in low pCO 2 and P-limited cultures.…”

Salinity-dependent diatom biosilicification implies an important role of external ionic strength

“…Nevertheless, the observed nanostructural changes indicate that external ionic strength governs diatom biosilica formation in the way described in classical silica chemistry (19,20); it is apparently independent of physiological processes (18,25,30). At the salinity levels usually found offshore (psu Ͼ30), diatoms may thus be expected to produce more hydrated, less condensed biosilica (Fig.…”

“…Indeed, we have shown that growth was not impaired at the two applied culture conditions (Table 1), but the difference of the biosilica nanostructure of cells cultured at higher and lower salinity is evident. Because silicon metabolism in diatoms is independent of the major cellular metabolic pathways for growth [N and P metabolism and photosynthesis (18,25)], the salinitydependent nanostructural changes in diatom biosilica could also have occurred without the interference of cellular processes to compensate for differences in osmotic pressure at the applied salinities. In other words, cellular osmoregulatory responses may have had no effect on the silica polymerization reaction inside the SDV.…”

“…Indeed, reduction in growth rate increases the cellular biogenic silica content (18). ANOVA analysis of growth for both species indicated that there was no significant difference in growth between the two applied salinities; apparently, growth was not hampered, so changes in biogenic silica content as a consequence of a reduced growth rate could be excluded.…”

The relationship between light intensity and nutrient uptake kinetics in six freshwater diatoms

“…Krause et al (2012) pointed that the kinetics parameters of Si uptake (values of V m(Si) and V m(Si) :K m(Si) ) in phytoplankton were coupled with ambient light availability and they increased with the increasing of irradiance at two of the three field stations. Some studies also found that the light stress led to high Si uptake rates in diatoms (Claquin et al, 2002;Davis, 1976;Pondaven et al, 2000). However, other studies showed that Si uptake (Mongin et al, 2003) and Si metabolisms (Brzezinski, 1992) were independent of the ambient light intensity and the rate of Si transport (Blank and Sullivan, 1979) during the day and night was similar in field studies.…”