Fragilariforma meireana: A new species of araphid Diatom from Belgium.

Diatoms are single celled algae related to Kelp and Water Moulds. They are encased in silica shells with two valves. During reproduction the cells divide in two, each of which retains one valve of the shell, growing a new opposing valve, which is slightly smaller and fits flush within the older valve. This means that the Diatoms grow smaller with each new generation, until they reach a minimum size, when they undergo a phase of sexual reproduction, giving rise to a new generation of full-sized cells.

In a paper published in the journal PhytoKeys on 9 June 2025, Bart Van de Vijver of the Research Department at Meise Botanic Garden, and the Department of Biology at the University of Antwerp, and David Williams of the Department of Life Sciences at the Natural History Museum, describe a new species of Diatom from a pond on the outskirts of the city of Damme, in the province of West Flanders, Belgium.

The new species was discovered in a pond on the outskirts of Damme while carrying out a biological survey of the region. Diatoms were collected by taking water samples and then centrifuging. A number of different Diatoms were recovered, including Eunotia bilunaris, Aulacoseira italica, Fragilaria campyla, Nitzschia archibaldii, Gomphonema utae, and an unknown species of Fragilariforma, which Van de Vijver and Williams name Fragilariforma meireana, in honour of Patrick Meire of the Department of Biology at the University of Antwerp, who formerly led the ECOBE research group and instigated the Damme biodiversity survey.

Fragilariforma meireana. Light microscope images taken from the holotype material (BR-4867, Damme, Belgium) (A)–(N) Valves in valve face view in decreasing length. The white arrows indicate the rimoportulae. The black arrows indicate anomalies in the striation pattern. (O)–(Q) Three frustules showing internal cells. Scale bars are 10 μm. Van de Vijver & Williams (2025).

Members of the genus Fragilariforma are 'araphid', which is to say elongate and symmetrical without a raphe system (slits on the midline from which mucus is secreted to aid motility). Valves of Fragilariforma meireana are 35–100 μm long and 4–5 μm wide, with no central area and parallel striae throughout the entire valve length, becoming very weakly radiate at the apices. 

Fragilariforma meireana, scanning electron microscope (SEM) images taken from the holotype material (BR-4867, Damme, Belgium). (A) SEM external view of an entire valve. (B) and (C) SEM external detail of two valve apices showing the rimoportulae (white arrows) and the apical pore field. Note the rudimentary spines and the irregular striation pattern on (C). (D) SEM internal detail of a valve apex showing the rimoportula (white arrow). (E) SEM internal view of an entire valve. The rimoportulae are indicated with white arrows. (F) SEM view of the broad valvocopula. Scale bars: are 10 μm in (A), (E) and (F), and 1 μm in (B)–(D). Van de Vijver & Williams (2025).

Some specimens of Fragilariforma meireana have an internal cell, this being large and robust, and filling most of the space between the valves. This is surprising, as such internal structures are unusual in bilaterally symmetrical Diatoms (it is more common in centric Diatoms, i.e. Diatoms with radial symmetry), however it has been seen in some other species, including Eunotia soleirolii, Meridion circulare, and Fragilariforma soleirolii. Internal cells such as these are interpreted to represent a resting phase, enabling the Diatoms to survive periods of adverse conditions, such as droughts or cold winters, although why the Damme specimens were forming them.

Fragilariforma meireana, SEM micrographs taken from the holotype material (BR-4867, Damme, Belgium). (A) SEM view of a complete frustule with the internal cell inserted between the valves. (B) SEM view of the resting spore covered by the valvocopula. At the apices, the valvocopula is well visible. Occasionally, very shallow ridge present on the plain side visible as a thin line. (C) SEM external detail of the valve apex with the valve, internal cell and one detached copula. (D) SEM external side view of the internal cell. Note the ridges on the top. (E) SEM external valve face view of the internal cell surrounded by the valvocopula. Scale bars are 10 μm in (A) and (B), and 1 μm in (C)–(E). Van de Vijver & Williams (2025).

The conditions in the pond from which Fragilariforma meireana was collected were not recorded, although most of the other species found there are known to prefer meso- to eutrophic conditions with moderate to higher electrolyte contents, and neutral to alkaline water. The exception to this is Eunotia bilunaris, which prefers oligotrophic, oligosaprobic conditions, although this species is known to be relatively environmentally tolerant.

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Using microfossils and pollen to determine the origin of a set of stocks in the collection of the Rijksmuseum, Amsterdam.

In 2019 a man donated a set of stocks to the Rijksmuseum in Amsterdam, which he claimed his  grandfather had acquired in the  1970s from a farm in Zeeland Province.  The museum put the stocks on display in 2021, as part of an exhibition about the Dutch colony in Brazil between 1630 and 1654. In 2023, the stocks were loaned to the United Nations in New York, where they formed part of an exhibition about slavery in Dutch colonies. However, during the New York exhibition, questions were raised as to whether the stocks had ever actually been to South America. A radiocarbon analysis suggested that the tree from which the stocks had been made was probably felled in about 1800, with the stocks probably being made in the first quarter of the nineteenth century. Furthermore, DNA testing established that the wood came from a lineage of Oak trees, which today is found growing in Europe between northern Spain in in the south and southern Scandinavia in the north. 

In a paper published in the journal Review of Palaeobotany and Palynology on 10 August 2024, Hans Piena of the Vrije Universiteit Amsterdam, the Dutch Open Air Museum, and the Royal Netherlands Historical Society, Bas van Geel of the University of Amsterdam, Tom Hakbijl of the Naturalis Biodiversity Center, Arie Kalis of Goethe University, Pim van der Knaap and Jacqueline van Leeuwen, also of the University of Amsterdam, and Kees Nooren of Utrecht University, describe the results of a study which looked at palynological evidence (preserved pollen and spores) to develop a forensic picture of the environment in which the stocks had been used.

Oak stocks in the collection of the Rijksmuseum in Amsterdam (Inv.nr. NG.2019–502). Measurements: 265 × 37.5 × 23 cm. Piena et al. (2024).

Sediment was extracted from holes and cracks in the stocks, and analysed for palynomorphs (pollen, algae and spores), and the surface of the stocks were analysed for surface wear and other traces of use.

The stocks comprise two oak beams resting on a pair of cross-braces. Each beam has nine semicircular openings, which correspond to similar openings on the other beam, with the pair making a hole. on the other beam. At one end of the beams is an iron hinge, at the other an obliquely worn padlock eye. The insides of the holes have been smoothed to a shiny surface, suggesting that the stocks were used frequently. The upper surface of the beams are also smooth, having apparently been rubbed frequently with sand; cracks and small holes on this surface were filled with sediment. There are also a large number of chopping and cutting marks on the upper surface, in places forming a chequerboard pattern. The underside shows no signs of similar wear or sanding, and is largely free of sediment. 

The two cross-braces yielded traces of attacks by Fungi and Woodworm, which are not seen on the beams, and are interpreted by Piena et al. as feet upon which the stocks sat. They were previously interpreted as side braces in the two slavery exhibitions, which would have led to the stocks being horizontal, holding prisoners seated on the floor. Under Piena et al.'s interpretation, the holes would have been vertical, forcing anyone  trapped in the stocks to stand.

The number '22' has been written on the upper surface of the stocks in blue chalk. The fact that this has not been rubbed of strongly suggests that it was written after the stocks ceased being used, possibly as part of an inventory process or auction sale. Before the twentieth century, chalk could be produced in few colours other than white, but blue is one of those colours, making it possible the number was written in the nineteenth century. 

The contents of a sediment-filled hole on the upper surface of the stocks were analysed for siliceous microfossils. This produced about 500 000 Diatoms per gram of sediment, with fifty different species identified. These were largely indicative of a mesotrophic freshwater environment, such as a slow-flowing stream or ditch.  This included the epiphytic Diatom Cocconeis placentula, which grows on the surface of aquatic Plants or Macroalgae. No marine or brackish water species were found. Phytoliths (opaline silica crystals which build up in the leaves of Plants as dissolved monosillicic acid taken up with groundwater and precipitated out as water is lost from the leaves via transpiration) associated with Grasses were found, but none associated with Palms. Also present in great numbers were the stomatocysts of Chrysophytes (Golden-brown Algae), which are also typical of freshwater environments, with about 460 000 stomatocysts per gram of sediment. Also present were samples of the freshwater Alga Spirogyra and pollen of the freshwater Plant Myriophyllum alterniflorum. All of these species are indicative of freshwater ecosystems, but otherwise fairly cosmopolitan, and can be found in tropical and temperate environments.

Samples of pollen and spores were extracted from eight different cracks and holes on the surface of the stocks, with five samples being extracted from one large hole. A total of 7106 individual specimens were found, including 142 taxa of spores and pollen, from 18 trees, 21 shrubs, 91 herbs, 5 Ferns, and 7 types of Fungi, Algae, and Moss. This high proportion of herb pollen is likely to be indicative of an open agricultural environment, probably with low nutrient levels. This contained only a small proportion of Chenopodiaceaetype and Plantago maritima-type pollen, making a salt marsh environment unlikely. Many of the pollen types are cosmopolitan in distribution, but others, including Armeria, Cistaceae, Erica arborea, Lavandula, Nerium oleander, Olea, Pinus pinaster, Quercus ilex, Urtica pilulifera, Eryngium tenue, Corrigiola telephiifolia, and Vitis vinifera, are typical of the Mediterranean region, while others, including Erica australis, Erica umbellata, Cistus albidus, Cistus populifolius, Cistus ladanifer, and Halimium halimifolium, are found only on the western Iberian Peninsula and in western North Africa. Notably, the pollen of Cedrus and Chamaerops, which are more-or-less ubiquitous in recent North African sediments, were absent. 

Many pollen samples from cereal species showed signs of heat-deformation, something which has previously been observed in samples from medieval cesspits, and which is thought to be associated with bread or porridge-making.

The stocks also show signs of Insect damage in places, and yielded some Insect samples. The Insect damage comes in two forms. The first of these is wide flat tunnelling in the cambium layer, which would have underlain the bark, and which is caused by the larvae of Buprestid or Cerambycid Beetles. The second form of damage is a series of small boreholes likely to have been caused by Anobium punctatum, a small boring Beetle commonly associated with wooden beams and objects in Europe, which favours humid indoor environments. The Insect remains found belong to Ptinus fur or Ptinus clavipes, Adistemia watsoni, Latridius minutus, and two unidentified members of the Corticariinae. Adistemia watsoni is a species native to South America, but which had spread to Europe by the nineteenth century. The Spider Beetles, Ptinus fur and Ptinus clavipes, are typically found in indoor settings, feeding on Human food or other Pland and Animal remains. A small Mite belonging to the family Macrochelidae was also found. This assemblage has been found in other archaeological settings, and is considered typical of an indoor environment with mouldy food, litter and/or hay, and possibly excrement. 

The underside of one of the cross-braces, which is worn and shows traces of Fungal and Woodworm attack. Piena et al. (2024).

Artefacts will typically begin to assemble traces and small microparticles of biological origin from the moment when they are made, enabling researchers to build up a life history of  objects of interest. Examination of the assemblage associated with the Rijksmuseum stocks showed no evidence of these ever having visited South America, despite earlier interpretations.

A prior DNA analysis of the wood suggests that the stocks were made from a tree which probably grew somewhere between northern Spain and southern Scandinavia, and it seems unlikely that it was transported for any great distance before being worked. This is because only wood with a high market value is typically shipped for any great distance, which effectively means wood with a straight grain and few knots. The wood from which the stocks are made has an irregular grain pattern and is somewhat knotty, which would generally only be used if the desired task required wood to be sourced locally. The stocks also show signs of having been worked using techniques typical for green wood usage, which again makes it highly unlikely that the wood was transported any distance before being used.

The style of the stocks, with holes on top forcing the prisoner(s) to stand is unusual, but is known to have been used widely on the Iberian Peninsula during the early nineteenth century, leading Piena et al. to conclude that the stocks are likely to be of Iberian origin.

The samples collected can be expected to have started to accumulate as soon as the stocks were  made, and to have continued to accumulate throughout their existence. Samples were taken from locations all over the stocks, and in one in one location, a hole, a succession of samples were taken from different depths. Despite this, other than samples from the lower side being dusty and samples from the upper side being sandy, there was remarkably little difference in the samples, suggesting that the stocks spent much of their lives in a single environment. 

The pollen samples collected from the stocks suggest that this environment was in western Iberia, while the DNA collected from the wood suggests that it grew no  further south than northern Iberia, which is at least a neighbouring region. 

The Insects collected from the stocks suggest a humid, indoor environment, while the presence of cereal pollen, and good preservation of the Diatoms and pollen suggest that the sediment in which they were preserved did was not exposed to moisture for long periods.

Based upon this evidence, Piena et al. consider two separate scenarios, which could account for the accumulation of microfossils seen on the stocks. 

It is possible that the entire assemblage is post-use, representing a time when the stocks had been abandoned in a slow-moving stream or ditch, in an open agricultural environment with poor soils and intensive Rye cultivation, an environment typical of the Iberian Peninsula in the nineteenth century. This scenario is supported by the good preservation of pollen and Diatoms, the type of sediment found, and the low number of Fungal spores, but contradicted by the presence of sediment only on the upper side of the stocks, not on the underside or in the cavities between the beams and cross-braces. 

Alternatively, the large amount of pollen from wild Grasses and Cereals, particularly Rye, makes it possible that the stocks were used in an indoor environment where Grass and Rye straw were used as litter. This is supported by the types of spores present, which are all from species associated with decaying Plant matter or Animal dung (including Human). This is consistent with the idea that the stocks were used in a humid, indoor environment. The presence of Fungal and Woodworm damage to the underside of the stocks would be consistent with them sitting on a moist floor covered with a litter of straw and hay, possibly in a stable or dungeon (with the latter being more likely, given the presumed purpose of the stocks). Thus the deformed Cereal pollen could have come from bread or porridge fed to prisoners, or their faeces if they were forced to produce this while trapped in the stocks, and the sand in the cracks on the upper surface would be consistent with frequent scrubbing of this surface. 

Historically, most stocks around the world were positioned on their sides, with prisoners thus able to lie down. The Rijksmuseum stocks, however, are of a far rarer design, which held the prisoner in an upright position, forcing them to stand upright. Such stocks were often ued in conjunction with a neck brace on a wall, so that prisoners could be held upright and restrained. This was once common in Spanish prisons, providing a likely origin for the stocks. Notably, such stocks were particularly common during the Peninsula War of 1807-1814 and the Spanish War of Independence of 1814-1823, both wars which were noted for numerous attocities. In other settings, however, stocks of this type were combined with a latrine bench with holes, with prisoners being held in a sitting position.

Por liberal?, 1814–1823, by Francisco de Goya y Lucientes, showing a  woman trapped in a horizontal type of stocks. Piena et al. (2024).

When obtained, little was known about the set of stocks present in the collection of the Rijksmuseum. They were identified as having (recently) come from Zeeland, and were considered appropriate for an exhibition about the Dutch colony in Brazil between 1630 and 1658. Subsequent investigations have suggested that they were most likely made in Europe around 1800, and never taken to South America. Subsequent investigations by Piena et al. have narrowed the likely point of origin to the western Iberian Peninsula, and suggest that they were used in a moist indoor environment, most likely a dungeon.

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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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Halamphora minima: A new species of Diatom from the mudflats of Hampyeong Bay, Korea.

Diatoms are single celled algae related to Kelp and Water Moulds. They are encased in silica shells with two valves. During reproduction the cells divide in two, each of which retains one valve of the shell, growing a new opposing valve, which is slightly smaller and fits flush within the older valve. This means that the Diatoms grow smaller with each new generation, until they reach a minimum size, when they undergo a phase of sexual reproduction, giving rise to a new generation of full-sized cells. The genus Halamphora was first described in 1895 as a subgenus of Amphora, but was elevated to a genus in its own right in 2009. It currently contains 154 species, with a global distribution, predominantly in marine and brackish water environments. Fifteen of these species have been recorded from Korea to date.

In a paper published in the journal Phytotaxa on 8 November 2022, Sung Min An of the Department of Microbial Resources at the National Marine Biodiversity Institute of Korea, Jihoon Kim of the Department of Biodiversity, also at National Marine Biodiversity Institute of Korea, and Nam Seon Kang, Kichul Cho, Jung Lee, and Fun Song Kim, also of the Department of Microbial Resources at the National Marine Biodiversity Institute of Korea, describe a new species of Halamphora from Hampyeong Bay, on the west coast of South Korea.

Samples were collected from an intertidal mudflat in Hampyeong Bay in July 2018, and cultured at the National Marine Biodiversity Institute of Korea. The recovered cells were then examined under light and scanning electron microscopes, with DNA being extracted to determine whether these Diatoms were a new species, and how they were related to other species within the group. The new species is named Halamphora minima, in reference to its small size in comparison to other members of the genus.

Map of sampling localities in Hampyeong Bay, the west coast of Korea. Type locality of Halamphora minima (35º03’41.94’’ N, 126º24’40.06’’ E). An et al. (2022).

The valves are only 5.9-7.4 μm in length and 2.4-3-3 μm in width, whereas most members of the genus exceed 15 μm in length, and the largest can exceed 80 μm. The tests of Halamphora minima have a deeply convex dorsal margin and a nearly straight ventral margin. The ventral side is expanded, and the valves on this size sealed by a series of striae. The raphe (a slit in the ventral edge of the shell) is straight on the and well developed. There is no axial longitudinal line on the dorsal edge. The surfaces of the shell are covered by a series of striae (lines) single on the ventral surface and double on the dorsal surface.

SEM micrographs of Halamphora minima. (11) External whole valve view, with central area (asterisk), and dorsal raphe ledge (arrow). (12) Internal whole valve view. (13) Detail of external valve apex,with dorsal raphe ledge (arrow). (14) Detail of internal valve apex showing tongue-like proximal helictoglossae (double asterisk), poorly developed distal helictoglossae (arrowhead), and internal longitudinal rib (arrow). (15) Detail of internal dorsal areolae occluded by hymens (arrows). (16) Dorsal girdle bands with two rows of poroids. (17) frustules showing the ventral girdle bands with dorsal raphe ledge (arrow). Scale bars: (11)-(14), (16), (17) 1 μm, (15) 0.5 μm. An et al. (2022).

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Aulacoseira wyomingensis: A new species of freshwater Diatom from a seep near Casper, Wyoming.

Diatoms are single celled algae related to Kelp and Water Moulds. They are encased in silica shells with two valves. During reproduction the cells divide in two, each of which retains one valve of the shell, growing a new opposing valve, which is slightly smaller and fits flush within the older valve. This means that the Diatoms grow smaller with each new generation, until they reach a minimum size, when they undergo a phase of sexual reproduction, giving rise to a new generation of full-sized cells. Members of the Family Aulacoseiraceae have elongate valves with many spines; these spines being able to  interdigitate with other members of the species, enabling them to form long chains. The family currently contains four genera, Aulacoseira, which is known from fossils dating back as far as the Cretaceous and still extant, with a global distribution, Eosira, which is known only from the Eocene of North America, Miosira, which is know from the Miocene of Europe, and Alveolphora, which is known from Miocene and Pliocene deposits across the Northern Hemisphere.

In a paper published in the journal Taxonomy on 8 June 2022, Jeremy Greifenstein, Rachel Shea, and John Patrick Kociolek of the Department of Ecology and Evolutionary Biology at the Museum of Natural History of the University of Colorado Boulder describe a new species of Aulacoseira from a small seep near Casper, Wyoming.

The new species is named Aulacoseira wyomingensis, where 'wyomingensis' means 'from Wyoming'. The new species is described from a series of specimens extracted from a sample collected on 22 August 2021. These Diatoms are cylindrical in shape, and while they can adhere together, chains of longer than two Diatoms have been observed.

Aulacoseira wyomingensis. Scanning electron microscopy. External girdle views of entire frustules. Valves have striae that are disorganised. Column has small ridges. Cingulum is composed of numerous ligulate elements. Spines are small in length and shield-like. Valve on the right in (C) appears to be incompletely formed. Scale bars are 5 µm. Greifenstein et al. (2022).

The valves of Aulacoseira wyomingensis are 7-14 μm in diameter, with faces covered by large areolae (openings) up to 1 μm in diameter. The sides are covered by striations, these having smaller areolae. The central part is covered by ridges. 

Aulacoseira wyomingensis. Scanning electron microscopy. Internal views. (A) Valve view showing areolae and interior of valve. Scale bar is 2.5 µm. (B) Side view showing part of valve interior and exterior. Scale bar is 2.5 µm. (C) Valve view of interior showing ringleiste. Scale bar is 2.5 µm. (D) High magnification view of single areola showing fine hymenate occlusion over opening. Scale bar is 0.3 µm. (E) Side view showing part of the valve interior and exterior. Scale bar is 2.5 µm. Greifenstein et al. (2022).

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Mayamaea vietnamica: A new species of soil-dwelling Diatom from Vietnam.

The genus Mayamaea was proposed by Horst Lange-Bertalot in 1997. Initially, 13 taxa formerly attributed to Navicula sensu lato were included in the genus. Mayamaea includes small-celled species that are elliptical in shape. The main characteristic of the genus is the arrangement of the pore occlusions, which are hymenes that cover the areolae from the outside, as well as the presence of a pronounced sternum found in the vast majority of species. The location of the hymenes is an important characteristic that differentiates Mayamaea from the genera Eolimna and Sellaphora, otherwise these genera have much in common including the structure of the raphe, central pores and distal raphe ends. Therefore, accurate identification by morphological characteristics is possible only when studying the material using a scanning electron microscope. Currently, the genus has about 30 taxa. Species of the genus Mayamaea are distributed worldwide, though most species are known from Europe. The species Mayamaea agrestis was described from Southeast Asia (Indonesia).

Species of Mayamaea prefer humid terrestrial habitats, as well as freshwater ecosystems (oligotrophic and mesotrophic), including in the littoral zone of these environments. In discussing the ecology of the species, Carlos Barragán, Luc Ector and Carlos Wetzel suggest that soil habitats are typical for Mayamaea lacunolaciniata, Mayamaea muraliformis, Mayamaea terrestris, and Mayamaea petersenii. However, some other species have also been found in the soil. For example, Mayamaea atomus was found in the soil of paddy fields in Central Japan. A large number of Diatom species (147 taxa) were identified from cultivated soils in Podkarpacie Province, Poland, and the most numerous was Mayamaea atomus and 5 other species. Others species of Mayamaea were found as well, including Mayamaea agrestis, Mayamaea excelsa, Mayamaea permitis, Mayamaea cf econdita, and Mayamaea sp2. Jasper Foets, Carlos Wetzer, Adriaan Teuling, and Laurent Pfister found Mayamaea atomus as a dominant in the soil of the Attert River basin (Luxembourg). The authors revealed that Mayamaea agrestis was typically on agricultural grasslands, Mayamaea permitis was one of the most abundant species in the territory, and Mayamaea fossalis was found in undisturbed grassland soils.

In a paper published in the journal Algae on 21 December 2020, Elena Kezlya and Anton Glushchenko of the К.А. Timiryazev Institute of Plant Physiology, John Patrick Kociolek of the Museum of Natural History and Department of Ecology and Evolutionary Biology at the University of Colorado, Yevhen Maltsev and Nikita Martynenko, also of the К.А. Timiryazev Institute of Plant Physiology, Sergei Genkal of the Papanin Institute for Biology of Inland Waters, and Maxim Kulikovskiy, again of the К.А. Timiryazev Institute of Plant Physiology, describe a new species from the genus Mayamaea isolated from a soil sample from the territory of Cát Tiên National Park in Vietnam.

Cát Tiên National Park is located in southern Vietnam, 150 km northeast of Ho Chi Minh City. The park was established in 1978 and consists of three parts with a total area of 73 878 hectares (738.78 km²). The region belongs to the bioclimatic type of monsoon tropical climate with summer rains. Relative humidity, as a rule, always exceeds 70%, with an average annual temperature of about 26°C. From December to March there is almost no rainfall. Southwest monsoons last eight months, from April to November, when most of the precipitation is observed. The wet season peaks in August-September. At this time of the year, up to 400-450 mm of precipitation falls per month, which leads to flooding of a significant part of the park. The main part of the territory is occupied by forests, which are of the monsoon, semi-deciduous type. These forests are characterized by high biological diversity and high productivity, second only to moist tropical forests in this respect.

 

Geographical position of the studied area. Kezlya et al. (2020).

The sample used was collected on 9 June 2018 from the Cát Tiên National Park, Đồng Nai Province, Vietnam and designated No. 06460. It was comprised examof wet soil, which had a pH of 6.5. Samples in general were taken as follows: first, the surface of the test site was examined in order to detect macrogrowth of algae, then a combined sample was taken from an area of 10-30 m² using the metal scoop or shovel. The composite sample consists of 5-10 individual samples. For an individual sample, the topsoil was removed from an area of 5 to 20 cm². After selection, the instruments were cleaned and sterilised with ethanol. Samples were placed in plastic zip bags, labeled. Immediately after the selection, the absolute humidity was determined in the laboratory room by the 'hot drying' method, then air dried and packaged.

The new species is named Mayamaea vietnamica, in reference to the country where it was discovered, and the specimens were placed in the Collection of Maxim Kulikovskiy at the Herbarium of the К.А. Timiryazev Institute of Plant Physiology.

Under light microscope the cells are solitary, rectangular in girdle view. One H-shaped plastid, with one arm lying against each side of the girdle, connected by a narrow central isthmus. Valves small, from almost linear to elliptical and oval (29 specimens) with pronounced sternum and broadly rounded ends. Length 9.1-10.5 μm, breadth 3.9-4.8 μm. Raphe narrow, linear. Central area more or less expressed, rounded to asymmetrical, rarely transversally elongated, and bordered on each margin by 3 shortened striae and/or 3 isolated areolae. Axial area tapers from the central area, becoming narrower towards the ends. The striae are radiate throughout the valve, 19-22 in 10 μm. Areolae coarse, clearly visible in light microscope.

 

Mayamaea vietnamica Strain VP 1, slide No. 06460. Light microscopy, differential interference contrast. (A) & (C)-(Z) Valves face. (A)-(H) Live cells with chloroplast structure. (B) Сell in girdle view. (I)-(Z) Size diminution series. (K) Holotype. Scale bar represents: 10 μm. Kezlya et al. (2020).

Under scanning electron microscope in external view the raphe is narrow, linear. Proximal raphe ends straight, drop-shaped. Distal raphe fissures bending strongly to the same side of valve mantle. Striae composed in 1-3 round areolae, not extending to valve margin. Areolae round, covered by individual hymenes.

 

Mayamaea vietnamica. Strain VP 1, slide No. 06460. Scanning electron microscopy, external views. (A) & (B) Whole valve. Areola occluded by hymen (arrow). (C) Central area. (D) Valve end. Scale bars represent: (A) & (B) 1 μm; (C) & (D) 0.5 μm. Kezlya et al. (2020).

Under scanning electron microscope in internal view the raphe is narrow, linear. Proximal valve ends slightly deflected to one side. Distal raphe ends terminating in small helictoglossae.

 

Mayamaea vietnamica. Strain VP 1, slide No. 06460. Scanning electron microscopy, internal views. (A) Whole valve. (B) Central area. (C) & (D) Valves ends. Scale bars represent: (A) 1 μm; (B)-(D) 0.5 μm. Kezlya et al. (2020).

Phylogenetic analysis of molecular sequence data shows that Mayamaea vietnamica is part of a branch with other species of Mayamaea. The branch with Mayamaea species is well-supported and independent from other branches. Closely related to the monophyletic group of Mayamaea species is a branch with strains of species from the genera Sellaphora and Eolimna. The branch containing these genera also has high statistical support. Six strains of Mayamaea permitis are divided in two branches in Kezlya et al.'s molecular tree. This suggests the presence of at least two cryptic species from specimens whose morphology suggest they could be assigned to the taxon Mayamaea permitis sensu lato. Mayamaea fossalis is close to two strains of Mayamaea permitis, however a difference between these species is supported by high statistical support. This result shows that these two species are independent taxa based on Kezlya et al.'s molecular investigation.

 

Bayesian tree of Mayamaea vietnamica (indicated in bold) constructed from a concatenated alignment of 107 partial rbcL and partial 18S rDNA sequences of 1380 characters. Values above the horizontal lines are bootstrap support from RAxML analyses (those below 50 are not shown); values below the horizontal lines are Bayesian posterior probabilities (those below 90 are not shown). All sequences have strain numbers (if available) and GenBank numbers. Species from the centric diatoms were used as an outgroup. Asterisks represent 100% statistical support. Kezlya et al. (2020).

Mayamaea vietnamica possesses all of the typical characters of the genus Mayamaea. These features include hymenes that covers the areolae from the outside, uniseriate striae, large areolae, small size, as well as the presence of a pronounced sternum. Our new species is morphologically similar to some known species from the genus Mayamaea, but it differs from them by both quantitative and qualitative features. The results of Kezlya et al.'s molecular investigation show the new species is part of an independent branch the includes three taxa. Mayamaea vietnamica is more closely related to Mayamaea terrestris than either is to Mayamaea atomus. Additionally, Mayamaea permitis and Mayamaea fossalis were included in the molecular investigation. The lineage of Mayamaea taxa is part of a broader monophyletic group that includes species of the genera Sellaphora, Eolimna and Rossia. However, the results of Kezlya et al.'s molecular investigation show that there is high statistical support for recognising Mayamaea as a genus distinct from these other genera.

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