Differences in Dissolution Characteristics of Biogenic Silica Among Diatom Species
Differences in Dissolution Characteristics of Biogenic Silica Among Diatom Species
Abstract:
Dissolution of biogenic silica (BSi) initiates immediately after the death of marine diatoms, and most of BSi are decomposed into dissolved silica (DSi) in the euphotic layer. However, a negligible portion of BSi produced in the ocean is buried in sediments by escaping the decomposition. It is believed that dissolution rates of BSi are different among diatom species, and the relatively limited species contribute largely to the removal of BSi from the ocean. In this study, we examined differences in dissolution of BSi among diatom species. We cultured five marine diatoms, including three planktonic, one tychopelagic, and one benthic species. Their silica frustules (i.e. BSi) were obtained by removing the attached organic matter through acid digestion. The silica frustules of each species were suspended in filtrated seawater, and incubated at 20ºC in the dark during a month. The seawater was subsampled periodically to monitor changes in the DSi concentration with the time. Dissolution of BSi during the incubation period was roughly divided into two stages irrespective of species. The estimated dissolution rate coefficient at the first stage (K1) was higher than that of the second one (K2) in all the five species. The K1 of the benthic diatom, Achnanthes kuwaitensis, was much lower than those of the other four species (5×10-3/h vs. 3-6×10-2/h), but the K2 showed similar levels in all the five diatoms (0.9-2×10-3/h). Relatively faster dissolution portion of BSi (i.e. BSi showing dissolution rate of K1) was estimated to be only about 50% in Achnanthes. By contrast, those of the other four diatoms were calculated to be around 90%. Therefore, the observed differences in BSi dissolution between the diatoms were explained largely by differences in K1 and the ratio between K1 and K2 portion of BSi in the silica frustule. We will also introduce the results of morphological changes in the silica frustules during the incubation period in the presentation.