Pleurosigma pacificum: A new species of Naviculoid Diatom from the western Pacific Ocean.

The Reverend William Smith established the genus Pleurosigma in 1852 to include Naviculoid Diatoms with a sigmoid (S) shape. Since then, the definition of the genus has been revised several times, now referring to a group of Diatoms with two or four ribbonlike plastids, (slightly) sigmoid valves, transverse and two obliquely intersecting striae and loculate areolae with external opening slits and internal poroids, which has been shown to be a genetically distinct group by molecular studies. The genus currently contains over 700 species, with the vast majority found living on sediments in brackish or marine environments. A much smaller group of these Diatoms is pelagic, living within the plankton of the ocean currents, typically having much straighter tests than their marine relatives. To date, only five species of pelagic Pleurosigma have been described (Pleurosigma antarcticum, Pleurosigma atlanticum, Pleurosigma indicum, Pleurosigma simonsenii, Pleurosigma directum), but metabarcoding (a technique of plant and animal identification based on DNA-based identification and rapid DNA sequencing) has suggested that this is a far larger and more diverse group.

In a paper published in the journal PhytoKeys on 2 June 2023, Fei-Chao Du of the Laboratory of Marine Organism Taxonomy and Phylogeny at the Institute of Oceanology of the Chinese Academy of Sciences, and the University of Chinese Academy of Sciences, Yu-Hang Li, also of the Laboratory of Marine Organism Taxonomy and Phylogeny at the Institute of Oceanology of the Chinese Academy of Sciences, and Kui-Dong Xu, again of the Laboratory of Marine Organism Taxonomy and Phylogeny at the Institute of Oceanology of the Chinese Academy of Sciences, and the University of Chinese Academy of Sciences, and of the Laboratory for Marine Biology and Biotechnology at the Pilot National Laboratory for Marine Science and Technology, describe a new species of Pleurosigma from the western Pacific Ocean.

The new species is described on the basis of samples collected from the upper 200 m of the ocean at a point (7°0.26'N, 141°59.63'E) roughly 1000 km to the north of Papua New Guinea. The new species is named Pleurosigma pacificum in reference to this point of discovery.

Location of the point (7°0.26'N, 141°59.63'E) where the samples from which Pleurosigma pacificum is described were collected. Google Maps.

The valves of Pleurosigma pacificum are lance-shaped rather than sigmoid, 45.0–51.5 µm long and 13.0–15.6 µm wide. The raphe (join between the valves) is straight and threadlike, curving slightly at the tips. The body is approximately round in profile, The valves have 21-22 straight and parallel transverse striae and 21-23 oblique striae, which intersect the transverse striae at an angle of 32° to 35°.

(A)–(F) Light microscope photographs of Pleurosigma pacificum. (A)–(D) Cleaned frustules showing parallel transverse striae under light microscope. (E) Valve centre showing a roundish central area (arrowhead). (F) Funnel-shaped terminal area in apical position. (G)–(L) Scanning electron microscope photographs of Pleurosigma pacificum. (G) Internal view of the whole valve. (H) Internal valve central area showing straight, slight expanded raphe fissures (arrowhead) and nodule bordered by two arched bars. (I) Internal raphe fissures terminate in helictoglossae at the apices (arrowhead). (J) External view of the whole valve showing the terminal fissures curving into opposite directions. (K) External central area of valve showing straight, simple raphe fissures (arrowhead). (L) Terminal fissures (arrowhead). Scale bars: 10 μm (A)–(F); 5 μm (G), (J); 1 μm (H), (I), (K), (L). Du et al. (2023).

A molecular analysis found that Pleurosigma pacificum grouped with other members of the genus Pleurosigma for which genetic data was available, but that it was the earliest branching member of the group. As such, Pleurosigma pacificum could potentially be considered outside the genus, particularly in light of its straight, rather than sigmoid, valves. However, Du et al. point out that other straight-valved species have been assigned to the genus, and that they are genetically deep within the group and do not form a monophyletic group, rather, the trait has arrisen in numerous different lineages within the genus, suggesting that it is of little phylogenetic significance. Since Pleurosigma pacificum has all of the other traits associated with the genus, Du et al. feel confident in assigning it to Pleurosigma.

Maximum likelihood (ML) and Bayesian inference (BI) phylogenetic trees based on the concatenated SSU rDNA and rbcL sequences. The values on each node indicate ML bootstrap and Bayesian posterior probabilities (%), respectively. Only bootstrap values over 50% are shown on the tree. Black dot indicate ML/BI = 100/1.00. The asterisk indicates the topological incongruence between ML and BI trees. Du et al. (2023).

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