Growth rates, half‐saturation constants, and silicate, nitrate, and phosphate depletion in relation to iron availability of four large, open‐ocean diatoms from the Southern Ocean

Abstract

Four large, open-ocean diatoms from the Southern Ocean (Actinocyclus sp., Thalassiosira sp., Fragilariopsis kerguelensis, and Corethron pennatum) were grown in natural (low iron) Southern ocean seawater with increasing Fe concentrations. With increasing dissolved iron (Fe diss ) concentrations, the growth rates increased three-to sixfold. The species with the smallest cells had the highest growth rates. The half-saturation constants (K m ) for growth were low (0.19-1.14 nmol L Ϫ1 Fe diss ), and close to the ambient Fe diss concentrations of 0.2 nmol L Ϫ1 . The range in K m with respect to Fe diss also varied with the size of the diatoms: the smallest species had the lowest K m and the largest species had the highest K m . As Fe diss concentrations decreased, silicate consumption per cell increased, but nitrate consumption per cell decreased. Phosphate consumption per cell varied without clear relation to the dissolved iron concentrations. The differences in nutrient consumption per cell resulted in marked differences in elemental depletion ratios in relation to Fe diss concentrations, with the depletion ratios being most affected by iron limitation in the largest cells. These experimental findings are in agreement with previous laboratory and field studies, showing the relatively high requirements of large diatoms for Fe. The size-dependent response of the diatoms with respect to nutrient depletion is a good illustration of the effects of Fe on silicate, nitrate, and phosphate metabolism.

Transformation of iodate to iodide in marine phytoplankton driven by cell senescence

“…The behaviour and growth of the phytoplankton was not affected by the added iodate. Measured chl a and cell densities appeared similar to untreated samples, and species showed normal growth rates of µ = 0.11 to 0.31 cell doublings d -1 (Timmermans et al 2004) through- out the experiments, suggesting that the added iodate neither enhanced nor inhibited cell growth. Bacterial densities are expressed in bacterial carbon (µg C l -1 ) and are considered in relation to the phytoplankton biomass also expressed in micrograms of carbon per litre.…”

Effects of iron on the elemental stoichiometry during EIFEX and in the diatoms <i>Fragilariopsis kerguelensis</i> and <i>Chaetoceros dichaeta</i>

“…Besides the impact on POC, PON, and POP composition, one important effect of iron limitation on the elemental stoichiometry is an increase in the BSi:POC, BSi:PON, and BSi:POP ratio of diatoms (Hutchins and Bruland, 1998;Price, 2005;Takeda, 1998;Timmermans et al, 2004;. While this can be caused by decreased cellular Si levels upon release from iron stress (Hutchins and Bruland, 1998;Takeda, 1998) other studies show the effect to be driven mainly by increase in cellular nitrogen and carbon with little or no change in cellular Si (Franck et al, 2003;Takeda, 1998).…”

Southern Ocean phytoplankton physiology in a changing climate

“…Diatoms exhibit the highest sensitivity to Fe limitation (Miller et al, 1991;Morel et al, 1991) with shifts to larger sizes (>10 m) in response to Fe fertilisation (De Baar et al, 2005). Experiments carried out with cultured diatoms in the laboratory showed a relationship between Fe, the surface/volume ratio (S/V) and the iron biological requirement for growth (De Baar et al, 2005;Timmermans et al, 2004;Sarthou et al, 2005), with larger diatoms being associated with greater iron requirement. Therefore, Fe bioavailability is not only influenced by its chemical forms, but also by the different uptake strategies, biological requirements and interactions of the phyto-and bacterio-plankton communities (e.g., Barbeau et al, 1996;Hutchins et al, 1999).…”