Lauderia annulata, copyright Kristobal22.

Belongs within: Mediophyceae.
Contains: Stephanodiscus, Thalassiosira.

United by silicon tubes
Published 12 March 2024

Diatoms are among the most instantly recognised of all aquatic micro-organisms. Their architectural silica tests are unique in the microbial biosphere. They are also among the most abundant organisms in many waters and play a pivotal role in the global distribution of aquatic silica. But, as is the consistent refrain on this site, making sense of the lower levels of diatom diversity has long presented a challenge. Take, for instance, the case of the family Thalassiosiraceae.

Scanning electron micrograph of Thalassiosira cf. gracilis, copyright FWC Fish and Wildlife Research Institute. Note the tubular fultoportulae around the outer margin.

The Thalassiosiraceae and their relatives in the order Thalassiosirales are among those diatoms with a radially centric morphology: that is, features of the valve face are arranged radially around a central point. However, molecular phylogenetic analyses do not associate the Thalassiosiraceae with other radially centric diatoms but instead place them closer to bi- or multipolar centric diatoms (in which more than one focal point is present on the valve), with this Thalassiosiraceae + polar group being in turn associated with the bilateral pennate diatoms (Kaczmarska et al. 2005). The radial centric morphology is thought to have been the original diatom condition, but their phylogenetic position suggests that it may have been a secondary reversion for the Thalassiosirales.

Structure of fultoportulae, from Kaczmarska et al. (2005).

Thalassiosirales as an order are united by the presence of fultoportulae, structures in which the basal layer of the silica shell is perforated by a strutted tube surrounded by satellite pores. In some species, the fultoportula appears externally as a simple pore; in others, it may extend as an elaborate tube. Both molecular and morphological data support fultoportulae as unique to a Thalassiosirales clade, as is the production of extruded threads of β-chitin from the fultoportulae (Alverson et al. 2007). These chitin threads are used to maintain buoyancy in the water column, and they are also used in many species to connect neighbouring cells into colonies. Such colonies may form elongate filaments, loose gelatinous aggregates, or well-defined sheets (Theriot & Serieyssol 1994).

Live chain of Skeletonema costatum, from the NOAA Photo Library.

The bulk of the Thalassiosirales have typically been divided between the families Thalassiosiraceae and Stephanodiscaceae, with a major distinguishing feature being their environment: Thalassiosiraceae are predominantly marine whereas Stephanodiscaceae are freshwater. ‘Thalassiosiraceae’ were thought to be paraphyletic with the broader Thalassiosirales clade originating in the marine environment in the late Cretaceous. The Stephanodiscaceae were then thought to have originated from a single invasion of fresh water in the mid-Miocene (Alverson et al. 2007). However, molecular phylogenetic analysis has not supported this scenario. Instead, the ‘Stephanodiscaceae’ appear to have had multiple origins within the ‘Thalassiosiraceae’. Transitioning between salinities is not an easy process for micro-organisms, involving as it does major changes in osmotic pressure. We don’t yet fully understand how the diatoms are managing to do it, but it appears possible that they may be making the change more readily than most.

Systematics of Thalassiosiraceae
    |  `--SkeletonemaC-SC06
    |       |--S. costatum Greville 1866C03
    |       |--S. menziesiiW87
    |       `--S. pseudocostatumOI05
    `--+--Porosira glacialisC-SC06
            |--L. annulataRA05
            |--L. borealisC-SC06
            `--L. glacialisB26
Thalassiosiraceae incertae sedis:
    |--D. confervaceaDG01
    `--D. cystiferaB26

*Type species of generic name indicated


Alverson, A. J., R. K. Jansen & E. C. Theriot. 2007. Bridging the Rubicon: phylogenetic analysis reveals repeated colonizations of marine and fresh waters by thalassiosiroid diatoms. Molecular Phylogenetics and Evolution 45: 193–210.

[B26] Bigelow, H. B. 1926. Plankton of the offshore waters of the Gulf of Maine. Bulletin of the Bureau of Fisheries 40 (2): 1–509.

[C-SC06] Cavalier-Smith, T., & E. E.-Y. Chao. 2006. Phylogeny and megasystematics of phagotrophic heterokonts (kingdom Chromista). Journal of Molecular Evolution 62: 388–420.

[C03] Chan, B. K. K. 2003. Studies on Tetraclita squamosa and Tetraclita japonica (Cirripedia: Thoracica) II: larval morphology and development. Journal of Crustacean Biology 23 (3): 522–547.

[DG01] Daugbjerg, N., & L. Guillou. 2001. Phylogenetic analyses of Bolidophyceae (Heterokontophyta) using rbcL gene sequences support their sister group relationship to diatoms. Phycologia 40 (2): 153–161.

[EB93] Edwards, D., J. G. Baldauf, P. R. Brown, K. J. Dorning, M. Feist, L. T. Gallagher, N. Grambast-Fessard, M. B. Hart, A. J. Powell & R. Riding. 1993. ‘Algae’. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 15–40. Chapman & Hall: London.

Kaczmarska, I., M. Beaton, A. C. Benoit & L. K. Medlin. 2005. Molecular phylogeny of selected members of the order Thalassiosirales (Bacillariophyta) and evolution of the fultoportula. Journal of Phycology 42: 121–138.

[OI05] Okamoto, N., & I. Inouye. 2005. The katablepharids are a distant sister group of the Cryptophyta: a proposal for Katablepharidophyta divisio nova/Kathablepharida phylum novum based on SSU rDNA and beta-tubulin phylogeny. Protist 156: 163–179.

[RA05] Rath, J., & S. P. Adhikary. 2005. A check list of algae from Chilika Lake, Orissa. Bulletin of the Botanical Survey of India 47: 101–114.

Theriot, E., & K. Serieyssol. 1994. Phylogenetic systematics as a guide to understanding features and potential morphological characters of the centric diatom family Thalassiosiraceae. Diatom Research 9 (2): 429–450.

[W87] Withers, N. 1987. Dinoflagellate sterols. In: Taylor, F. J. R. (ed.) The Biology of Dinoflagellates pp. 316–359. Blackwell Scientific.

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