Mineralogical and Biogeochemical Characteristics of Rhodolith from Wu Island, Jeju-do, Korea

This study focused on investigation of mineralogical and biogeochemical characteristics of the rhodoliths and examination of the rhodolith origin whether they were formed by chemical or biological processes. Rhodoliths are free-living forms of calcareous, coralline red algae formed under specific environmental conditions, but their formation processes are still controversial. The rhodoliths and sea water were sampled at Seogwang-ri coast in the western part of Wu Island, Jeju-do, Korea. The chemical compositions and pHs of sea water were measured by ICP-AES and pH meter. Microorganisms enriched from rhodoliths were aerobically cultured at room temperature in D-1 media containing various concentrations (0, 30, 100 mM) of Ca and Mg-acetate, and the microorganisms were analyzed by 16S rRNA gene DGGE analysis to confirm microbial diversity. Mineralogical characteristics of the rhodoliths and precipitates formed by the enriched microorganisms were determined by XRF, XRD, and SEM-EDS analyses. The sea water had a range of pH 6 to 7, and consisted of approximately 400 mg/L of Ca and 1200 mg/L of Mg which are low to form carbonate minerals by chemical process in natural environments. XRF and XRD analyses showed the rhodoliths mainly consisted of 46% CaO and 5 % MgO and mineralogy is Mg-rich calcite. A 16S rRNA sequence analysis showed the enriched microorganisms contained a carbonate forming microorganism, Proteus mirabilis. The enriched microorganisms precipitated carbonate minerals using D-1 media containing Ca- and Mg-acetate (30, 100 mM) and mineralogy of the precipitated carbonate mineral was Mg-rich calcite, whereas the microorganisms did not form carbonate minerals without Ca- and Mg-acetate in D-1 media. SEM-EDS analyses showed that the Mg-rich calcite formed by the microorganisms had a rhombohedron shape. And the Mg-rich calcite consisted of Ca, Si and Mg with extracellular polymeric substance (EPS). These results indicate that the rhodoliths at Seogwang-ri coast could be formed biologically and the microorganisms induce precipitation of Mg-rich calcite on the cell walls and EPS via the accumulation of Ca and/or Mg ions on the cells. The study of the natural analogues, rhodolith beach, complements laboratory experiments and possibly provides opportunity to constrain the mechanisms for CO2-bearing phases to form via biological processes.