some of them are signalling substances producing acquired immunity. When deprived, some plants will substitute with increased production of other defensive substances. Life on Earth is largely composed of carbon, but astrobiology considers that extraterrestrial life may have other hypothetical types of biochemistry. Silicon is considered an alternative to carbon, as it can create complex and stable molecules with four covalent bonds, required for a DNA-analog, and it is available in large quantities. Marine microbial influences Diatoms uses silicon in the biogenic silica () form, which is taken up by the silicon transport protein (SIT) to be predominantly used in the cell wall structure as frustules. Silicon enters the ocean in a dissolved form such as silicic acid or silicate. Since diatoms are one of the main users of these forms of silicon, they contribute greatly to the concentration of silicon throughout the ocean. Silicon forms a nutrient-like profile in the ocean due to the diatom productivity in shallow depths. Therefore, less concentration of silicon in the upper ocean and more concentrations of silicon in the deep/lower ocean. Diatom productivity in the upper ocean contribute to the amount of silicon exported to the lower ocean. When diatom cells are lysed in the upper ocean, their nutrients such as iron, zinc, and silicon, are brought to the lower ocean through a process called marine snow. Marine snow involves the downward transfer of particulate organic matter by vertical mixing of dissolved organic matter. It has been suggested that silicon is considered crucial to diatom productivity and as long as there is silicic acid available for diatoms to use, the diatoms can contribute to other important nutrient concentrations in the deep ocean as well. In coastal zones, diatoms serve as the major phytoplanktonic organisms and greatly contribute to biogenic silica production. In the open ocean, however, diatoms have a reduced role in global annual silica production. Diatoms in North Atlantic and North Pacific subtropical gyres only contribute about 5-7% of global annual marine silica production. The Southern Ocean produces about one-third of global marine biogenic silica. The Southern Ocean is referred to as having a "biogeochemical divide" since only minuscule amounts of silicon are transported out of this region. Human nutrition There is some evidence that silicon is important to human health for their nail, hair, bone, and skin tissues, for example, in studies that demonstrate that premenopausal women with higher dietary silicon