Thalassiosira pseudonana

Thalassiosira pseudonana is a species of marine centric Bulka diatoms. It was chosen as the first eukaryotic marine phytoplankton for whole genome sequencing.^[1] T. pseudonana was selected for this study because it is a model for diatom physiology studies, belongs to a genus widely distributed throughout the world's oceans, and has a relatively small genome at 34 mega base pairs. Scientists are researching on diatom light absorption, using the marine diatom of Thalassiosira. The diatom requires a high enough concentration of CO₂ in order to utilize C₄ metabolism (Clement et al. 2015).^[2]

Thalassiosira pseudonana
A Thalassiosira pseudonana oogonium (egg cell) beginning to expand through the cell wall. Artificial coloring denotes chlorophyll (blue) and DNA (red).
Scientific classification
Domain:	Eukaryota
Clade:	Diaphoretickes
Clade:	Sar
Clade:	Stramenopiles
Phylum:	Gyrista
Subphylum:	Ochrophytina
Class:	Bacillariophyceae
Order:	Thalassiosirales
Family:	Thalassiosiraceae
Genus:	Thalassiosira
Species:	T. pseudonana
Binomial name
Thalassiosira pseudonana Hasle & Heimdal, 1970

The clone of T. pseudonana that was sequenced is CCMP 1335 and is available from the National Center for Marine Algae and Microbiota at Bigelow Laboratory for Ocean Sciences. This clone was originally collected in 1958 from Moriches Bay (Long Island, New York) and has been maintained continuously in culture.

Morphology

Left: Scanning electron microscope image of the T. pseudonana. Right: False coloring of the biosilica structure of T. pseudonana to highlight its valve region, fultoportulae and girdle band region.^[3]

Thalassiosira pseudonana has a radial symmetry. Its biosilica cell wall is divided into two halves, which are joined together by girdle bands, giving them a cylindrical shape or making them appear as a Petri dish.^[4] The diameter of their valves ranges from 2 to 9 μm.^[5][6] The valve is made up of silica ribs that radiate from the center with many 18 nm diameter nanopores between them.^[7] The face of the valve has 0-1 central fultoportula and a marginal ring of fultoportulae (6-12).^[8] The external openings of the central fultoportula appear as rimmed holes, whereas those of the marginal fultoportulae appear as short rimmed tubes, which are sometimes obliquely sectioned at the opening. On the internal face of the valve, two satellite pores surround the central fultoportula, while the marginal fultoportulae are surrounded by three satellite pores.^[8] The rimoportula is a rimmed pore located on the valve face, with a size similar to the fultoportula, and positioned between two fultoportulae. The pervalvar axis of T. pseudonana can be either shorter than or equal to the valve diameter.^[8] Their cell walls have been reported to mostly have low degree of silicification; however, their rims and ribs are highly silicified.^[5][6][8] This probably enables them to have high strength while being light and using silica economically.^[7][9]

Biomineralization

The distinct nano- to micro-scale structure of T. pseudonana follows a specific mechanism of formation. It begins with the formation of a thin base layer that outlines the valve.^[7] Next, there is radiation of the silica ribs and the buildup of the rims, while portulae develop. The formation of the radial structure starts from a central location and spreads out in x and y axes planes.^[7] During maturation, the valve surface becomes increasingly silicified, and the rim continues to develop, while the central portion of the valve becomes more rigid. Initially, the valve nanopores have irregular shapes, but they become circular during maturation.^[7] The nanoscale structures of T. pseudonana are genetically mediated. The silicification process involves three categories of molecules: silaffins, which are highly post-translationally modified phosphoproteins; long chain polyamines (LCPAs); and silacidins, which are acidic proteins.^[4] During valve synthesis, mRNA levels for silaffin 3 increase and lead to the formation of the base layer.^[7] The presence of lower concentrations of silaffin 3 or the light form of silaffin 1 and 2 leads to the generation of spherical silica structures, indicating possible mechanisms involved in the formation of spherical silica in the ridges.^[7] Over 150 genes have been identified as playing a role in the silicon biomineralization of T. pseudonana. A set of 75 genes were upregulated only during silicon limitation, while 84 genes were upregulated by both silicon and iron limitations, indicating a linkage between their iron and silicon pathways.^[4][10] T. pseudonana also possesses chitin-based scaffolds that are important in the formation of their biosilica structure.^[11]

Genomics and Model Organism Status

• Thalassiosira pseudonana* was the first diatom species to have its genome fully sequenced, with a genome size of approximately 34 megabase pairs containing around 11,000 genes. The landmark genome project, published by Armbrust et al. (2004) in *Science*, revealed unique gene families involved in silica metabolism and photosynthesis, establishing *T. pseudonana* as a key model organism in marine biology and biotechnology.[1] This species is extensively used to study cell cycle regulation, carbon concentrating mechanisms (CCMs), and responses to environmental changes. For instance, Smith et al. (2010) explored the molecular control of cell division in *T. pseudonana*.[2] Additionally, Johnson et al. (2015) characterized genes regulating CCMs, which enhance photosynthetic efficiency under varying CO₂ concentrations.[3]

Silica Shell Formation Mechanism

The characteristic silica shell, or frustule, of *Thalassiosira pseudonana* is composed of intricately patterned nanostructures formed through tightly regulated biological processes. Central to this process are silicon transporter proteins (SITs), which mediate the uptake and intracellular transport of silicic acid, the precursor of biogenic silica. Shrestha et al. (2008) demonstrated the conservation and functional importance of SIT gene families in *T. pseudonana* for frustule biosynthesis.[4] The unique nanostructured silica shells have attracted interest for applications in drug delivery systems and optical materials. Lee et al. (2017) reviewed the use of diatom biosilica as a biocompatible platform for targeted drug delivery.[5] Furthermore, Chen et al. (2020) investigated the optical properties of diatom frustules, highlighting their potential in photonics.[6]

Ecological Role and Environmental Impact

As a dominant phytoplankton species, *Thalassiosira pseudonana* plays a crucial role in global carbon cycling by contributing significantly to marine primary production and the biological carbon pump. Wang et al. (2019) quantified its contribution to carbon fixation and sequestration in ocean ecosystems, emphasizing its importance in mitigating atmospheric CO₂ levels.[7] However, rising ocean temperatures due to climate change affect its growth and photosynthetic performance. Liu et al. (2022) reported that temperature fluctuations influence *T. pseudonana*’s growth rate and photosynthetic efficiency, which may alter its distribution and ecological function.[8]

Pseudo-nitzschia australis and Domoic Acid Production

• Pseudo-nitzschia australis* is a diatom species known for producing domoic acid, a potent neurotoxin responsible for amnesic shellfish poisoning in humans and marine wildlife. This toxin accumulation during harmful algal blooms (HABs) poses significant threats to fisheries and public health. Trainer et al. (2013) provided a comprehensive review of *Pseudo-nitzschia* species and their ecological and toxicological impacts.[9] Recent genomic and transcriptomic studies by Brunson et al. (2018) have elucidated the gene clusters involved in domoic acid biosynthesis, advancing understanding of toxin regulation.[10] Environmental factors such as nutrient availability, water temperature, and hydrodynamics strongly influence bloom dynamics. McKibben et al. (2017) demonstrated correlations between these factors and *P. australis* bloom occurrences, informing monitoring and management strategies.[11]

Ecological Role and Carbon Cycling

• Thalassiosira pseudonana* is a key marine diatom contributing significantly to global carbon cycling. As a dominant phytoplankton species, it plays an essential role in primary production and the biological carbon pump, facilitating the sequestration of atmospheric CO₂ into ocean depths. Wang et al. (2019) quantified the species' contribution to carbon fixation and highlighted its importance in mitigating climate change effects through carbon sequestration.^[12]

Furthermore, environmental changes such as rising ocean temperatures impact the growth and photosynthetic efficiency of *T. pseudonana*. Liu et al. (2022) demonstrated that temperature fluctuations alter its physiological performance, potentially affecting its distribution and ecological function in marine ecosystems.^[13]

Symbiosis

Thalassiosira pseudonana and the heterotrophic alphaproteobacterium Ruegeria pomeroyi form a chemical symbiosis in coculture. The bacteria provide vitamin B12 to the diatoms, which in exchange provide organic nutrients to the bacteria. In the presence of the diatom, the bacteria start producing a transporter for dihydroxypropanesulfonate (DHPS), a nutrient produced by the diatom for the bacteria.^[14] A metabolic survey of the association between the bacterium Dinoroseobacter shibae and T. pseudonana showed that the bacterium has minimal impact on the growth of T. pseudonana, but it causes metabolic changes by upregulating the intracellular amino acids and amino acid derivatives of the diatom.^[15] It has been demonstrated that under conditions of environmental instability and extreme warming, biofilm formation can accelerate the evolutionary responses of T. pseudonana.^[16]