# Micromechanical Characterization of Diatom Frustules of Multiple Origin
## 2 State-of-the-Art
### Taplonomic Processes in Diatom Frustules  
Taphonomy studies the transition \"biosphere--lithosphere\" in organic remains. These analyses start with an organism's death or the detachment of some of its parts. Taphonomy studies facilitate the interpretation of fossil records. Unlike organic remains, the number of living organisms preserved as fossils is minimal [34]. Diagenetic variations are technically defined as taphonomic processes. Some Japanese marine diatomaceous soils (micro remnants) present particles (plant and animal) usually less than 1 mm in size [13]. The formation of frustules occurs by the polymerization of silicic acid within the deposition vesicles (low pH environment) [2]; this process involves cell components such as the cytoskeleton, the mitochondria, the endoplasmic reticulum, the Golgi apparatus and the plasmalemma membrane [35]. Silica spheres conforming to diatom frustules are considered impermeable and solid excrement. The frustule's evolutionary adaptations have made it dominant in cell physiology and morphogenesis [9]. The extent of silicification of frustules depends on the rate of Silica fixation, either by the availability in the environment or by the effectiveness and genomic characteristics of its transporters [36; 37], except for exceptional cases that do not require higher Silica content for cell development. Silica concentration defines frustules' growth; its presence generates ballast necessary in the sedimentation process. Silica concentrates mainly in the photic zone of aquatic environments [38]. Silica in aquatic environments is found mainly in the form of silicic acid Si(OH)\({}_{4}\). It is usually transformed into solid silicon oxide, SiO\({}_{2}\), or other siliceous compounds in the presence of organic substances by condensation processes [39] or by diffusion through the membranes of diatoms. When low silicic acid concentrations are in the aquatic environment, cells activate transporters that facilitate fixation [30; 40]. Unlike nitrogen and phosphorus metabolization, silica uptake in diatoms mainly relates to aerobic respiration processes and not so much to photosynthetic processes [30; 41]. The acid accumulates temporarily in the deposition vesicles. Then, by the interaction of specific proteins, polyamines and elements of the cytoskeleton, the amorphous Silica is deposited in the form of small spheres, together with some organic matter (organic components can lead to the formation of bacteria, which retard cell growth [30]), in the strict ordering patterns presented by the frustules. This process involves a relatively low energy expenditure (2% of the diatom's energy reserves due to a frustule with 25%silica). It is primarily concentrated in the stage of silicic acid transport across the plasma membrane [2]. Frustules are multilayered porous structures with remarkable mechanical properties (Elastic Modulus 22.4 GPa) and restorative properties when imposed loads and deformations are applied. They have allowed the conservation of diatoms over the last tens of millions of years [10; 11]. There are two diatom growth environments: deep and shallow. The first is described by high oxygen content, low silica concentration, and planktonic conditions. The second refers to high silica proportion and benthic characteristics. High concentrations of dissolved silicic acid supply nutrients (phosphates and nitrates), which promote diatom development [42]. Planktonic forms float freely, while benthic forms are attached to substrates, rock surfaces, or plants [23]. The amount of light absorbed influences the structure of the frustule. The size and shape of the phytoplankton influence the amount of light absorbed. Small cells are more efficient in absorption than large cells with the same concentration and type of pigment. There are records of species that can grow in the dark [43]. Some diatoms catalyze bicarbonate by protonation in the areas surrounding the cell with low CO\({}_{2}\) contents.