Del Amo, Y. University of California Santa Barbara/Marine Science Institute, email@example.com
Brzezinski, M. University of California Santa Barbara/Marine Science Institute, firstname.lastname@example.org
DETERMINATION OF THE CHEMICAL FORM OF DISSOLVED Si TAKEN UP BY DIATOMS
Results of past studies of the pH-dependent Si uptake kinetics of Phaeodactylum tricornutum suggest that the anion SiO(OH)3- is the chemical form of dissolved Si taken up by diatoms rather than undissociated silicic acid Si(OH)4. We examined the kinetics of Si use in 4 diatom species, including P. tricornutum, under the dramatically different SiO(OH)3-:Si(OH)4 ratios that exist in seawater depending upon pH. Uptake rates were determined using a precise and sensitive 32Si tracer methodology.
The pH-dependent uptake kinetics obtained for all species except P. tricornutum suggest that the form of Si transported into the cell is total dissolved silicon or Si(OH)4. The half saturation constant (Km) varies strongly as a function of pH for all species when the substrate for transport is assumed to be SiO(OH)3-. Kinetic curves for Thalassiosira pseudonana, Thalassiosira weissflogii and Cylindrotheca fusiformis have statistically identical values of Km independent of pH when the substrate for transport is assumed to be total dissolved silicon or Si(OH)4. In contrast, P. tricornutum exhibits unusual biphasic uptake kinetics: uptake conforms to Michaelis-Menten kinetics up to 30 micromolar, above which uptake increases linearly. This enigmatic response may have biased conclusions drawn from past experiments using this species.
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