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[AOS22-02] Comparison of dissolution rate of silica frustule among coastal diatoms
Keywords:diatom frustule, biogenic silica, dissolution, planktonic diatoms, benthic diatoms
Diatoms are the major primary producer particularly in coastal environments. They have a cell wall (frustule) made by amorphous silica. Silica frustule (biogenic silica; BSi) dissolves after the death of diatom. Previous studies documented that dissolution rate of the frustule differs among species. However, only some species were tested and it is still unclear what factors mainly cause the difference is. In addition, only planktonic diatoms are focused as the target species despite the fact that benthic diatoms also significantly consume silicic acid in coastal areas. In this study, dissolution rate of silica frustules of 14 coastal diatoms including benthic species are tested. The aim is to reveal the differences among the species and the factors affecting for the dissolution.
Diatoms obtained by pure culture incubation were harvested, digested by acid to remove organic materials, centrifuged, and dried. The purified frustules of each species were suspended in filtrated seawater for approximately 30 days under continuously shaking condition in the dark at 20ºC. A small volume of the seawater was subsampled arbitrary to calculate residual percentage of silica frustule (as BSi) to the initially added amount. Then, microscopic observation of the frustule was occasionally conducted using a scanning electron microscope.
Dissolution of the silica frustule with time was roughly divided into two stages irrespective of species. As reported by Kamatani & Riley (1979), relationship between the residual BSi% and time in each stage was expressed approximately as exponential function: residual BSi% = aexp^(-Kx*t), where a is the intercept and K is dissolution rate coefficient at the first (K1) or second stage (K2). The value of K1 was larger than the K2 in all species, indicating silica frustule dissolved rapidly at the initial and thereafter moderately. The values of K1 varied largely among the species, while the differences in K2 were much smaller than the K1. Relatively labile fraction of BSi (i.e. BSi showing dissolution rate of K1) occupied a major portion of the total BSi at each species, but the percentage varied among the species. Some benthic diatoms tended to show lower K1 with lower percentages of the labile fraction as compared to those of the planktonic ones, though there were large variations even in the same group. Microscopic observation indicated that some specific part had high resistance to dissolve in some species.
Diatoms obtained by pure culture incubation were harvested, digested by acid to remove organic materials, centrifuged, and dried. The purified frustules of each species were suspended in filtrated seawater for approximately 30 days under continuously shaking condition in the dark at 20ºC. A small volume of the seawater was subsampled arbitrary to calculate residual percentage of silica frustule (as BSi) to the initially added amount. Then, microscopic observation of the frustule was occasionally conducted using a scanning electron microscope.
Dissolution of the silica frustule with time was roughly divided into two stages irrespective of species. As reported by Kamatani & Riley (1979), relationship between the residual BSi% and time in each stage was expressed approximately as exponential function: residual BSi% = aexp^(-Kx*t), where a is the intercept and K is dissolution rate coefficient at the first (K1) or second stage (K2). The value of K1 was larger than the K2 in all species, indicating silica frustule dissolved rapidly at the initial and thereafter moderately. The values of K1 varied largely among the species, while the differences in K2 were much smaller than the K1. Relatively labile fraction of BSi (i.e. BSi showing dissolution rate of K1) occupied a major portion of the total BSi at each species, but the percentage varied among the species. Some benthic diatoms tended to show lower K1 with lower percentages of the labile fraction as compared to those of the planktonic ones, though there were large variations even in the same group. Microscopic observation indicated that some specific part had high resistance to dissolve in some species.