Premium
DIATOM MINERALIZATION OK SILICIC ACID. I Si(OH) 4 TRANSPORT CHARACTERISTICS IN NAVICULA PELLICULOSA
Author(s) -
Sullivan C. W.
Publication year - 1976
Publication title -
journal of phycology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.85
H-Index - 127
eISSN - 1529-8817
pISSN - 0022-3646
DOI - 10.1111/j.1529-8817.1976.tb02862.x
Subject(s) - silicic acid , silicic , nuclear chemistry , biology , valinomycin , kinetics , intracellular , biochemistry , biophysics , chemistry , membrane potential , organic chemistry , physics , paleontology , volcano , quantum mechanics
SUMMARY Silicic acid transport was studied in the photosynthetic diatom Navicula pelliculosa (Bréb.) Hilse using [ 68 Ge] germanic acid ( 68 Ge(OH) 4 ) as a tracer of silicic acid (Si(OH) 4 ). The initial uptake rate of Si(OH) 4 was dependent on cell number, pH, temperature, light and was promoted by certain monovalent cations in the medium. Na + was more effective than K + , whereas Li + and NH + 4 were ineffective at promoting uptake. Uncouplers and inhibitors of oxidative phosphorylation and of photophosphorylation reduced uptake by 40–99% of control values. Uptake was also especially sensitive to the sulfhydryl blocking agents at 10 −5 M and to the ionophorous compound valinomycin (10 −7 M) which inhibited uptake by 82%. The Si(OH) 4 transport system displayed Michaelis‐Menten‐type saturation kinetics with kinetic parameters of K S = 4.4 p. mol Si(OH) 4 · 1 −1 , V max = 334 pmol Si(OH) 4 · 10 6 cells −1 · min −1 . Calculations of the acid soluble silicic acid pool size based on 60 s uptake at 20 μM Si(OH) 4 suggested that intracellular levels of Si could reach 20 mM and as much as 5 mM could exist as free silicic acid, representing maintenance of a 250‐fold concentration gradient compared with the medium. Efflux from preloaded cells was dependent on temperature and the Si(OH) 4 concentration of the external medium. In the presence of 100 μMM “cold” Si(OH) 4 , approximately 30% of the Si(OH) 4 in preloaded cells was exchanged in 20 min. The initial uptake rate of Si(OH) 4 in logarithmic phase cells was constant, but the uptake rate increased in a linear fashion for 6 h in stationary phase cells. These results suggest that the first step in silica mineralization by diatoms is the active transmembrane transport of Si(OH) 4 by an energy dependent, saturable, membrane‐carrier mechanism which requires the monovalent cations Na + and K + and is sensitive to sulfhydryl blocking agents. Silicic acid transport activity also appears to be regulated during different growth stages of the diatom.