In this study, fragments presumed to be marine microbial remnants other than clay minerals were found in the mud powder. These microflakes were observed in various sizes depending on the degree of breakage. The shape of the piece was a plate-like structure observed with a thickness of 2.5 μm and a length of up to 30 μm, and many circular pores were formed at regular intervals. Circular pores with a diameter of about 1 μm were regularly arranged on the surface.
Baschini et al. (2010) reported that amorphous siliceous microbial residues exist in the mud component, most of which are diatom frustules. The shell, the cell wall of diatoms, has a colorful shape and design, and it is amorphous hydrated silica (SiO2–(H2O)n) produced by biomineralization of the cell itself (Habchi et al. 2006). A diatom belonging to the genus Coscinodiscus is a single-celled algae that forms a silica shell or frustule in which pores with a size of 1 μm or less are arranged at regular intervals (Jeffryes et al. 2008; Zaman et al. 2020).
Diatoms are unicellular photosynthetic plants that inhabit freshwater and seawater and are amorphous bio-silica components in which pores are systematically and uniformly arranged in the shell (Xing et al. 2017). In this study, the remnants of marine microorganisms were identified as typical diatom shells, as reported by Baschini et al. (2010), and holes on the frustule surface were found to be uniformly and systematically arranged with pores.
In addition, in the scanning electron microscope at a high magnification, the surface of the shell was formed of amorphous biosilica with some irregularities. The appearance of irregularities on the surface is considered to be formed by accumulation of silica in the form of fine granules as the mineralization progresses.
The diatom frustule having such microstructural characteristics and chemical composition of silica has been applied in various industrial fields. In addition, excellent physical filtration and transport properties of the porous cytoskeleton have been applied to studies, such as light harvesting through needle-like pores, separation of photons and molecules, and drug delivery (Hale and Mitchell 2001; Jeffryes et al. 2008; Gnanamoorthy et al. 2014; Kieu et al. 2014; Dong et al. 2016).
The organic compounds, ions, and inorganic soluble complexes contained in the mud powder act as physicochemical factors that determine the efficacy of the mud (Fioravanti et al. 2011). Organic compounds are accumulated in the mud after the growth of cyanobacteria, a type of diatoms and blue-green algae. Glycolipids and sulfoglycolipids are the organic molecules secreted by these microorganisms. Organic molecules are a major factor in mud aggregation and have anti-inflammatory properties (Sukenik et al. 1999; Tolomio et al. 1999; Carretero et al. 2010; Trabelsi et al. 2016). In the scanning electron microscopic observation of the mud powder used in this study, cyanobacteria, photosynthetic prokaryotes, were not observed, and diatoms were observed in the form of fine fragments with pulverized shells.
The reason why the microstructure of cyanobacteria and bacteria is not observed in the mud mixture sample is that the purity of the mud collected from the shore is high, and it undergoes a heat sterilization process, milling process, and wet separation for ease of use (Viseras et al. 2007); it is considered that the morphological characteristics cannot be confirmed because the organic material is decomposed or separated.
As a result of the analysis of the constituent elements of the diatom shell in this study, Si, Al, Fe, K, Na, Mg, and Ti components were detected, but Ca and S element components found in clay minerals were not detected.
The Si element was found to be the main constituent element of the frustule with a content of 72.99%. Al element was detected with a content of 10.67%, and Fe, K, Na, Mg, and Ti elements were detected in small amounts. The content of Si element in the diatom shell was higher than the Si element content contained in the whole mud powder. This result indicates that silica is the main component of the diatom frustule. It is considered that the detection elements other than Si are caused by the influence of clay mineral particles present in trace amounts in the analysis sample.
The supply of silica to marine sediments falls into three categories. First, it is divided into land-influent silica, which is mainly produced by entering rivers from land; second, bio-derived silica, mainly obtained from biological structures or skeletal remnants of microorganisms and secretions of siliceous organisms; and third, hydrothermal silica, usually associated with hydrothermal fluids (Xu et al. 2021).