Si is the second most abundant element on earth, yet the essentiality of Si in many organisms has been the subject of debate in the literature for decades. Si has a required structural or functional role in many different organisms, from diatoms to humans, yet its essentiality in other organisms, like plants, is unclear.

In diatoms, radiolarians, and some sponges, Si provides a structural role and is necessary for growth. Diatoms make silica-based cell walls, an intensive and fascinating process that entails building a mineral-based cell wall inside the cell and then exporting it out (31308524). Recent studies have determined that Si limitation facilitates viral infection and subsequent death in diatoms (31308524). 

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Si has also been found to have a functional role in plant stress responses, growth, and defense mechanisms, but Si deficiency has not been found to disrupt the life cycle of the plants (26617630). Thus, it is generally viewed as a non-essential element for plants that increases vigor and responses to stress when present. Recent studies examining the role of Si in the alleviation of plant drought stress identify Si as an enhancer of water uptake, a regulator of stoma and water loss, a mediator of the accumulation of solutes and osmoregulatory substances, and an inducer of plant defense that contributes to maintenance of the water balance of the plant (34848683). Certain plants also accumulate Si, which associates with plant tissues and the cell wall in a process called silicification (32344677). 

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In humans, Si has been found to have a role in strengthening structural elements and the immune system (26617630). The major dietary source of Si in humans is from plants. Si is involved in several biological functions, including bone and connective tissue metabolism as well as immune and inflammatory responses. Prolonged deficiency in Si results in bone disabilities and potential cognitive impairment due to increased availability of Al.