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[BCG06-01] Origins of spherulites observed in reefal microbial crusts in Kumejima, Okinawa
Microbialites have recorded biological activities and the earth environment for about 3.5 billion years, and are recently drawing attention in terms of oil reservoirs. Microbialites often contain spherulites, which are sometimes interpreted to be microbial origin. In fact, many spherulites were found from microbialites composing huge oil reservoirs. However, the interpretation that the spherulites are microbial origin is mainly based on morphological observations by scanning electron microscope (SEM), and lacks more direct evidence. In addition, the interpretation of "spherulites" in Phanerozoic microbialites requires special caution, because ascidians form spicules resembling spherulites. Therefore, this study examined reefal microbial crusts (RMCs), which are marine microbialites, and ascidian spicules from the family Didemnidae, with the aim of distinguishing between spherulites of microbial origin and ascidian spicules.
The RMCs samples were collected from storm rocks washed up at Nishimezaki, Kumejima, Okinawa Prefecture. Ascidian spicules samples were also collected from the sea around Kumejima. These samples were subjected to morphological observation and elemental composition analysis, by using polarizing microscope and SEM. For spherulites which are suspected to be microbial origin, ultra thin section were prepared by focused ion beam (FIB) processing from conventional thin sections, and observed and analyzed using scanning transmission X-ray microscope (STXM) and transmission electron microscope (TEM).
By polarizing microscope observations, ascidian spicules observed have a globular–stellate shape, and extinction patterns showed that they were composed of bundles of parallel acicular crystals. Grains showing similar characteristics were also recognized in RMCs, indicating their ascidian origin. On the other hands, some spherulites adjacent to RMCs were composed of thin acicular and radial crystals, and they were tightly packed to exhibit anhedral appearance. In addition, they contain filamentous structures of about 1 mm in a diameter, and they sometimes radiated from the center of spherulites. These characteristics are significantly different from ascidian spicules, suggesting their microbial origin. In fact, STXM analysis detected spectra characterizing carboxy groups and amorphous calcium carbonate (ACC) at the filament rim, suggesting microbial extracellular polymeric substances-mediated carbonate precipitation. Elemental composition analysis by SEM showed that the S/Ca ratio of the spherulites of microbial origin was significantly lower than that of the ascidian spicules, which may indicate that the spherulite was formed in a reductive environment inhabited by sulfate-reducing bacteria.
The RMCs samples were collected from storm rocks washed up at Nishimezaki, Kumejima, Okinawa Prefecture. Ascidian spicules samples were also collected from the sea around Kumejima. These samples were subjected to morphological observation and elemental composition analysis, by using polarizing microscope and SEM. For spherulites which are suspected to be microbial origin, ultra thin section were prepared by focused ion beam (FIB) processing from conventional thin sections, and observed and analyzed using scanning transmission X-ray microscope (STXM) and transmission electron microscope (TEM).
By polarizing microscope observations, ascidian spicules observed have a globular–stellate shape, and extinction patterns showed that they were composed of bundles of parallel acicular crystals. Grains showing similar characteristics were also recognized in RMCs, indicating their ascidian origin. On the other hands, some spherulites adjacent to RMCs were composed of thin acicular and radial crystals, and they were tightly packed to exhibit anhedral appearance. In addition, they contain filamentous structures of about 1 mm in a diameter, and they sometimes radiated from the center of spherulites. These characteristics are significantly different from ascidian spicules, suggesting their microbial origin. In fact, STXM analysis detected spectra characterizing carboxy groups and amorphous calcium carbonate (ACC) at the filament rim, suggesting microbial extracellular polymeric substances-mediated carbonate precipitation. Elemental composition analysis by SEM showed that the S/Ca ratio of the spherulites of microbial origin was significantly lower than that of the ascidian spicules, which may indicate that the spherulite was formed in a reductive environment inhabited by sulfate-reducing bacteria.