structural:scholar_edition
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 (NH) nutrition compared with nitrate (NO) 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, the urea cycle had long been considered a waste pathway. However, in diatoms the urea cycle appears to play a role in exchange of nutrients between the mitochondria and the cytoplasm, and potentially the plastid and may help to regulate ammonium metabolism. Because of this cycle, marine diatoms, in contrast to chlorophytes, also have acquired a mitochondrial urea transporter and, in fact, based on bioinformatics, a complete mitochondrial GS-GOGAT cycle has been hypothesised.