The chemical form of dissolved SI taken up by marine diatoms

Results of past studies of the pH-dependent Si uptake kinetics of Phaeodact ylum tricornutum Bohlin suggested that the anion SiO(OH)(3)(-) is the chemi cal form of dissolved Si taken up by marine diatoms, We determined the chem ical form of Si taken up by three other marine diatom species and P. tricor nutum by examining the kinetics of Si use under two dramatically different SiO(OH)(3)(-):Si(OH)(4) ratios in seawater by varying pH from approximate t o 8 to approximate to 9.6. Uptake rates were determined using a precise and sensitive Si-32 tracer methodology. The pH-dependent uptake kinetics obtai ned for all species except P. tricornutum suggest that marine diatoms trans port Si(OH)(4). The half-saturation constant (K-m) varies strongly as a fun ction of pH for all species when the substrate of transport is assumed to b e SiO(OH)(4). Kinetic curves for Thalassiosira pseudonana (Hustedt) Hasle e t Heimdal, Thalassiosira weissflogii (Grunow) G. Fryxell et Hasle, and Cyli ndrotheca fusiformis Reimann et Lewin have statistically identical values o f 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.fusiform is). In contrast, P. tricornutum exhibits unusual biphasic uptake kinetics: uptake conforms to Michaelis-Menten kinetics up to 15 to 25 mu M, above wh ich uptake increases linearly. This enigmatic response may have biased conc lusions drawn from past experiments using this species. However, based on t he consistency of the results for the three other species, a new model of S i transport in marine diatoms is proposed on the basis of the direct format ion of a complex between the Si-transport protein and Si(OH)(4).