Richelia are made up of filaments called trichomes, which are fine hair-like structures that grow out of a myriad of plant species, though their presence as free-living organisms in the marine environment is rare.
[3] The heterocyst is characterized by a thick glycolipid layer which minimizes oxygen's ability to interfere with nitrogen fixation.
[2] This is important to Richelia’s function as oxygen can bind to nitrogenase and inhibit the cyanobacteria's nitrogen fixing abilities.
In many regions of the world's oceans the availability of inorganic nitrogen such as nitrate and ammonium limits the rate of photosynthesis (primary productivity).
[7] Day-night cycles potentially play a role in coordination of resource exchange and cell division between a diazotroph and its diatom host.
Photosynthesis, nitrogen fixation, and resource acquisition related genes show day-night fluctuations in their expression pattern in Richelia.
[8] The genus name of Richelia is in honour of Andreas du Plessis de Richelieu (1852–1932), who was a Danish naval officer and businessman who became a Siamese admiral and minister of the Royal Thai Navy.
[10] The diatom-Richelia symbiotic relationships that are confirmed and most well-known are as follows: Richelia are most commonly found and best understood within host diatoms.
While transfer of the Richelia trichome to daughter cells can occur before division, this method will eventually end as it limits vegetative growth due to the progressive reduction in the size of the host diatom.
[16] Thus far, many observations have been subject to criticism due to issues of misidentifying hosts and the associated diazotrophs, and demonstrating symbiotic relationships overall.
[16] Warm, silicate-rich conditions, such as those found in the Amazon River plume, allow for high Richelia growth rates.
[16] Richelia cells also decrease in abundance as inorganic nitrogen increases because they are at a competitive disadvantage when nitrate concentrations are high.
[3][18] Because the Eastern Mediterranean Sea has oligotrophic conditions due to a large transport of nutrients out to the North Atlantic Ocean through the Strait of Gibraltar, Richelia provides important nitrogen fixation capabilities for diatoms they form symbiotic relationships with.
Free living Richelia are not considered to be present in the Eastern Mediterranean Sea based on the current sampling experiment results available.
In the Eastern Mediterranean Sea water columns, Richelia is the primary diurnal organism with an expression of the nifH gene.
A case of allopatric speciation is observed between coastal and pelagic water columns in the Eastern Mediterranean Sea.
Physical and hydrographical conditions vary throughout the year in the current and create changes to the growth of bacterial and diatom colonies.
The number of Richelia filaments per colony of Chaetoceros compressus ranges from 4 to 9 during May to November, reaching a maximum in July.
[17] Nitrogen fixation and cyanobacteria-diatom symbiosis occur in the freshwater layer of the Amazon River plume due to low surface nitrate conditions.