CS Knowledge Base

The silica cycle is the biogeochemical cycle in which biogenic silica is transported between the Earth's systems. Silicon is one of the most abundant elements on Earth, and is considered necessary for life.^[2][3] The silica cycle has significant overlap with the carbon cycle (see carbonate–silicate cycle) and plays an important role in the sequestration of carbon through continental weathering, biogenic export and burial as oozes on geologic timescales.^[4]

Silicon is the eighth most abundant element in the universe and the second most abundant element in the Earth's crust (the most abundant is oxygen). The weathering of the Earth's crust by rainwater rich in carbon dioxide is a key process in the control of atmospheric carbon dioxide.^[5][6] It results in the generation of silicic acid in aqueous environments. Silicic acid, Si(OH)₄, is a hydrated form of silica found only as an unstable solution in water, yet it plays a central role in the silica cycle.^[1]

Silicifiers are organisms that use silicic acid to precipitate biogenic silica, SiO₂. Biogenic silica, also referred to as opal, is precipitated by silicifiers as internal structures^[7] and/or external structures.^[8] Silicifiers are among the most important aquatic organisms. They include micro-organisms such as diatoms, rhizarians, silicoflagellates and several species of choanoflagellates, as well as macro-organisms such as siliceous sponges. Phototrophic silicifiers, such as diatoms, globally consume vast amounts of silicon along with nitrogen (N), phosphorus (P), and inorganic carbon (C), connecting the biogeochemistry of these elements and contributing to the sequestration of atmospheric carbon dioxide in the ocean.^[9] Heterotrophic organisms like rhizarians, choanoflagellates, and sponges produce biogenic silica independently of the photoautotrophic processing of C and N.^{[10][8][11][1]}

The diatoms dominate the fixation and export of particulate matter in the contemporary marine silica cycle. This includes the export of organic carbon from the euphotic zone to the deep ocean via the biological carbon pump. As a result, diatoms, and other silica-secreting organisms play crucial roles in the global carbon cycle by sequestering carbon in the ocean. The connection between biogenic silica and organic carbon, together with the significantly higher preservation potential of biogenic siliceous compounds compared to organic carbon makes opal accumulation records of interest in paleoceanography and paleoclimatology.

Understanding the silica cycle is important for understanding the functioning of marine food webs, biogeochemical cycles, and the biological pump. Silicic acid is delivered to the ocean through six pathways as illustrated in the diagram above, which all ultimately derive from the weathering of the Earth's crust.^[12][1]

Silica is an important nutrient utilized by plants, trees, and grasses in the terrestrial biosphere. Silicate is transported by rivers and can be deposited in soils in the form of various siliceous polymorphs. Plants can readily uptake silicate in the form of H₄SiO₄ for the formation of phytoliths. Phytoliths are tiny rigid structures found within plant cells that aid in the structural integrity of the plant.^[2] Phytoliths also serve to protect the plants from consumption by herbivores who are unable to consume and digest silica-rich plants efficiently.^[2] Silica release from phytolith degradation or dissolution is estimated to occur at a rate double that of global silicate mineral weathering.^[3] Considering biogeochemical cycling within ecosystems, the import and export of silica to and from terrestrial ecosystems is small.