The Chemical Form of Dissolved Si Taken Up by Marine Diatoms

Abstract

Resultsof past studies of the pH-dependent Si uptake kinetics of Phaeodactylum tricornutum Bohlin suggested that the anion SiO(OH) is the chemical Ϫ 3 form of dissolved Si taken up by marine diatoms. We determined the chemical form of Si taken up by three other marine diatom species and P. tricornutum by examining the kinetics of Si use under two dramatically different SiO(OH) :Si(OH) 4 ratios in sea-Ϫ 3 water by varying pH from ഠ8 to ഠ9.6. Uptake rates were determined using a precise and sensitive 32 Si tracer methodology. The pH-dependent uptake kinetics obtained for all species except P. tricornutum suggest that marine diatoms transport Si(OH) 4 . The half-saturation constant (K m ) varies strongly as a function of pH for all species when the substrate of transport is assumed to be SiO(OH) . Kinetic Ϫ 3 curves for Thalassiosira pseudonana (Hustedt) Hasle et Heimdal, Thalassiosira weissflogii (Grunow) G. Fryxell et Hasle, and Cylindrotheca fusiformis Reimann et Lewin have statistically identical values of K m at each pH when the substrate for transport is assumed to be Si(OH) 4 (T. pseudonana and T. weissflogii) or total dissolved silicon (C. fusiformis). In contrast, P. tricornutum exhibits unusual biphasic uptake kinetics: uptake conforms to Michaelis-Menten kinetics up to 15 to 25 M, above which uptake increases linearly. This enigmatic response may have biased conclusions drawn from past experiments using this species. However, based on the consistency of the results for the three other species, a new model of Si transport in marine diatoms is proposed on the basis of the direct formation of a complex between the Si-transport protein and Si(OH) 4 .

Initiation of winter phytoplankton blooms within the Gironde plume waters in the Bay of Biscay

“…Because phosphate, nitrate and silicate concentrations in the intermediate and turbid plume waters were significantly higher than their respective classical half saturation constant K s for uptake (< 0.1, 0.5 and 2 µM; Thingstad et al 1993, Del Amo & Brzezinski 1999, Kudela & Cochlan 2000 at BIOMET 2 (Table 2, Fig. 5), nutrients were not limiting for phytoplankton growth in early winter in these waters.…”

The place of diatoms in the biofuels industry

“…After the two daughter cells separate, individual cells can sequentially synthesize new girdle bands, which enable elongation of the cell. The source of Si for diatoms is dissolved silicic acid, Si(OH) 4 [73], which is taken up at low concentrations by specific silicic acid transport proteins, and at high concentration by diffusion across the plasma membrane [74].…”

Calculation of 29Si NMR shifts of silicate complexes with carbohydrates, amino acids, and muhicarboxylic acids: potential role in biological silica utilization

“…The preferential uptake of H 4 SiO 4 , relative to H 3 SiO 4 Ϫ by many, albeit not all, diatom species has been confirmed experimentally (Amo and Brzezinski, 1999). Thus, neutral silicic acid is the most likely form of silicon transported from the ambient aqueous medium across the cell membrane.…”

“…This idea is supported by the recent discovery that the enzyme, silaffin, and polyamine chains, isolated from diatoms are capable of polymerizing silicic acid at neutral to acidic pHs (Mitzutani et al, 1998;Kroger et al, 1999Kroger et al, , 2000Pohnert, 2002) characteristic of freshwater (pH ϳ 7) and of the silica deposition vesicle of the diatom (pH ϳ 5-6) (Vrieling et al, 1999a). The uptake of silicic acid across the cell membrane from extracellular seawater with a mildly basic pH of ϳ8.5 has also been shown in several diatom species (Amo and Brzezinski, 1999). A different perspective is that the starting compound is a silicate-organic compound.…”