displacement of siliceous sponges from the shelves, suggests that this takeover began in the Cretaceous (146 Ma to 66 Ma), while evidence from radiolarians suggests "take-over" did not begin until the Cenozoic (66 Ma to present). Carbon cycle The diagram depicts some mechanisms by which marine diatoms contribute to the biological carbon pump and influence the ocean carbon cycle. The anthropogenic CO2 emission to the atmosphere (mainly generated by fossil fuel burning and deforestation) is nearly 11 gigatonne carbon (GtC) per year, of which almost 2.5 GtC is taken up by the surface ocean. In surface seawater (pH 8.1–8.4), bicarbonate () and carbonate ions () constitute nearly 90 and <10% of dissolved inorganic carbon (DIC) respectively, while dissolved CO2 (CO2 aqueous) contributes <1%. Despite this low level of CO2 in the ocean and its slow diffusion rate in water, diatoms fix 10–20 GtC annually via photosynthesis thanks to their carbon dioxide concentrating mechanisms, allowing them to sustain marine food chains. In addition, 0.1–1% of this organic material produced in the euphotic layer sinks down as particles, thus transferring the surface carbon toward the deep ocean and sequestering atmospheric CO2 for thousands of years or longer. The remaining organic matter is remineralized through respiration. Thus, diatoms are one of the main players in this biological carbon pump, which is arguably the most important biological mechanism in the Earth System allowing CO2 to be removed from the carbon cycle for very long period. Urea cycle A feature of diatoms is the urea cycle, which links them evolutionarily to animals. In 2011, Allen et al. established that diatoms have a functioning urea cycle. This result was significant, since prior to this, the urea cycle was thought to have originated with the metazoans which appeared several hundreds of millions of years before the diatoms. Their study demonstrated that while diatoms and animals use the urea cycle for different ends, they are seen to be evolutionarily linked in such a way that animals and plants are not. While often overlooked in photosynthetic organisms, the mitochondria also play critical roles in energy balance. Two nitrogen-related pathways are relevant and they may also change under ammonium () nutrition compared with nitrate () nutrition. First, in diatoms, and likely some other algae, there is a urea cycle. The long-known function of the urea cycle in animals is to excrete excess nitrogen produced by amino acid catabolism; like photorespiration,