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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Multicellular Eukaryotic fossils from the Mesoproterozoic Chuanlinggou Formation of North China.

All life found on Earth today is made up of cells, with the vast majority of organisms still being unicellular; it is generally presumed that the earliest forms of life would have been single-celled Prokaryotes (organisms with cells which lack internal divisions and organelles).  Multicellularity has arisen numerous times within both Prokaryotic and Eukaryotic groups, although complex multicellularity, with cells differentiated into specialist forms and organised communication between cells, has only arisen six or seven times, and only in Eukaryotes.

The earliest widely accepted multicellular Eukaryotic fossils, filaments and spherical groups of cells, appear around the Mesoproterozoic-Neoproterozoic boundary, while filamentous Prokaryotes are known from the Archaean. Early multicellular Eukaryotes include Bangiomorpha pubescens, a putative Red Alga from 1050 million-year-old deposits in the Canadian Arctic, Eosolena loculosa, a Eukaryote of uncertain affinities from 1030 million-year-old deposits in Siberia, Arctacellularia tetragonala, another species of uncertain affinities from 1000 million-year-old deposits in the Democratic Republic of Congo, Proterocladus antiquus, a possible Green Alga from 950 million-year-old deposits in North China, Archaeochaeta guncho, another species of uncertain affinities from 950 million-year-old deposits in northwestern Canada, and Ourasphaira giraldae, a possible Fungi from 890 million year old deposits in the Canadian Arctic. 

Some putative multicellular Eukaryotes have also been recorded from earlier in the Mesoproterozoic, including Eosolena minuta, from 1500 million-year-old deposits in northern Siberia, or the carbonacious impressions of the Gaoyuzhuang Formation in North China, which can reach tens of centimetres across, or the possible Eukaryotic microfossils from the 1600 million-year-old Tirohan Dolomite of central India. The oldest examples of the coilled microfossil Grypania are currently dated to about 2100 million years before the present (i.e. Late Palaeoproterozoic) although it is debated whether this is a Eukaryote or a Cyanobacterium. Of similar age are the pyritic macrostructures of the Francevillian Biota of Gabon, though there is some debate as to whether there are of biological origins at all.

In 1989, micropalaeontologist Yan Yuzhong published a description of a filamentous Eukaryotic fossil from the 1630 million-year-old Chuanlinggou Formation of North China in the Bulletin of the Tianjin Institute of Geology and Mineral Resources. At this time journals were only available in print, and the Bulletin, which was printed in Chinese, had almost no circulation outside of China. Furthermore, the quality of the images in Yan's paper were rather poor, leading to the publication being largely overlooked.

In a paper published in the journal Science Advances on 24 January 2024, Lanyun Miao and Zongjun Yin of the State Key Laboratory of Palaeobiology and Stratigraphy at the Nanjing Institute of Geology and Palaeontology, Andrew Knoll of the Department of Organismic and Evolutionary Biology at Harvard University, Yuangao Qu of the Institute of Deep-sea Science and Engineering of the Chinese Academy of Sciences, and Maoyan Zhu, again of the State Key Laboratory of Palaeobiology and Stratigraphy at the Nanjing Institute of Geology and Palaeontology, and of the College of Earth and Planetary Sciences of the University of the Chinese Academy of Sciences, re-examine the Chuanlinggou Formation fossils, and discuss the implications of these for the origin of multicellular Eukaryotic life.

Samples of grey shale were collected from the Wengjiazhuang Section of the Chuanlinggou Formation in Kuancheng County of Hebei Province, which has been dated to 1634.8 million years before the present (±6.9 million years) using uranium/lead ratios in zircons. Zircons are minerals formed by the crystallisation of cooling igneous (or in this case, impact) melts. When they form, they often contain trace amounts of uranium, which decays into (amongst other things) lead at a known rate. Since lead will not have been present in the original crystal, it is possible to calculate the age of a zircon crystal from the ratio between these elements. Microfossils were then extracted from these shales by acid maceration.

This technique recovered flattened greyish or pale brown filamentous fossils, which Miao et al. describe as Qingshania magnifica, the name used by Yan Yuzhong for his material. These are not the only filamentous fossils derived from the Chuanlinggou Formation shales, but are significantly larger than other forms, supporting the idea that they are Eukaryotic in origin, while the other forms are Prokaryotic, probably Cyanobacteria. The 278 individual specimens Miao et al. identified ranged from 20 to 194 μm in diameter, with a maximum length of 860 μm. The filaments were straight or curved, and made up of smooth-walled cells, more than 20 of which were present in the longest specimens. These cells are generally cylindrical in shape, with a cell length of 15 to 190 μm. Terminal cells, where preserved, are hemispherical. None of the specimens had any form of external sheath or holdfast.

Transmitted-light photomicrographs of Qingshania magnifica from the Chuanlinggou Formation. (A) to (D) and (K) Filaments with cells of varying length and width. (E) Four-celled filament with hemispherical terminal cell. (F) and (G) Filament with notably decreasing cell width toward one end. Note that (F) and (G) represent the same specimen; (F) lost the narrowest part of the filament as shown in (G). (H to J) Filaments displaying more uniformity of cell dimensions. (L) Two-celled filament with ovoid terminal cell. All specimens were handpicked from organic residues of acid maceration and photographed in wet mounts, except for (K), which was photographed from a permanent strew mount. Solid and empty gray triangles in (A), (C), and (K) indicate the longest and the shortest cells, respectively, within single filaments. tb, transverse band (interpreted as cross wall); tr, transverse ring (interpreted as partially preserved cross wall). Scale bars 50 μm (A) to (E), (I), (J), and (L) and 100 μm (F) to (H) and (K). Miao et al. (2024).

The specimens show considerable variation, with the largest being ten times as wide as the smallest, individual cells being cylindrical, barrel-shaped, or cup-shaped, and filaments being of even width or tapering towards one end. Despite this variation, Miao et al. treat them all as a single species, suggesting that the variations reflect s different growth or developmental stages within the population.

Micrographs of Qingshania magnifica from the Chuanlinggou Formation. (A) Transmitted-light photomicrograph of a five-celled filament with constant width and dark narrow transverse bands. (B) Scanning electron microscope image of (A) showing surface features and the preservation as a complete compression. Note the obliquely compressed cross wall of the right terminal cell showing smooth surface and no other particular features. (C) to (E) Magnifications of (B), showing smooth wall surface and the well-defined contact between adjoining cells manifested by a very shallow groove (marked by cyan arrowheads) along transverse bands. (C) and (E) represent dashed boxes in (A) and (B); (D) corresponds to the dashed box in (C). Scale bars, 50 μm (A) and (B), 10 μm (C), and 2 μm (D) and (E). Miao et al. (2024).

Some of the filaments have small, round-to-ovoid structures within some of their cells. These structures are faint, but always contained entirely within the cell, making it unlikely that they are separate structures superimposed upon the filaments. Inclusions within cells, from both Proterozoic and Phanerozoic settings, have variously been interpreted as endocysts, collapsed cytoplasm, or organelles. The fossils are interpreted as being compressed cell walls, which makes it likely that structures withing them would be endocysts. A variety of Eukaryotic and Prokaryotic groups produce endospores in response to worsening conditions (such as the end of a growing season), but these tend to have protective envelopes thicker than the outer cell wall, which is not the case with these structures. However, the structures are found only in larger cells, and are only slightly smaller than the smallest cells, which suggests that they may be some form of asexual reproductive spore; similar spores are produced by some extant filamentous Algae, such as Urospora wormskioldii.

Transmitted-light photomicrographs of Qingshania magnifica with a small round or ovoid inclusion from the Chuanlinggou Formation. (A), (C), and (D) Filaments with constant width. (B) and (E) Magnifications of dashed boxes in (A) and (C), respectively, showing details of round inclusions. (F) Filament of notably varying width. Note that the middle cell of the filament is cyathiform in shape. (G and H) Magnifications of dashed box in (F) and (D), respectively. All specimens were handpicked from organic residues of acid maceration and photographed in wet mounts. Scale bar, 50 μm (A), (C), (D), and (F). Miao et al. (2024).

Microscale Raman and Fourier transform infrared spectroscopic investigations of the composition of the filaments suggested that the cell walls of Qingshania magnifica were composed largely of aromatic compounds, with a lower proportion of aliphatic compounds, with the aliphatic compounds forming long chains with little branching. This is not sufficient to make any  assessment of the taxonomic status of Qingshania magnifica on its own, but is quite distinct from the composition of Cyanobacterial cells found in the same deposits.

The original specimens of Qingshania magnifica described by Yan in 1989 were identified from thin sections of yellowish-green shales, and had a maximum width of about 250 μm and were up to 6000 μm in length. Yan identified these as Green Algae, placing them in the modern family Ulotrichaceae. Miao et al.'s specimens are slightly smaller, but preserve more detail, allowing for a more detailed reconstruction.  They interpret Qingshania magnifica as a simple multicellular organism with large cells and a degree of morphological variation, with a life cycle that involved spores produced within cells, which then produced thin filaments, which grew into thicker filaments, which were capable of producing more spores.

A wide range of both Prokaryotic and Eukaryotic organisms produce filaments of cells today. Among Prokaryotes, these include at least eleven phyla of Bacteria and one of Archaeans. The most sophisticated filamentous Prokaryotes are Cyanobacteria, which produce a range of forms including straight, tapering, and branching filaments. However, no known Cyanobacterium, or other Prokaryote, living or fossil, closely resembles Qingshania magnifica. Filamentous Eukaryotes include Algae such as Archaeplastids (the group that includes both Red and Green Algae) and Ochrophytes (the group that includes Brown Algae, Golden Algae, and Diatoms), as well as filamentous Fungi and Oomycetes (Water Molds). The cells of Qingshania magnifica are completely surrounded by cell walls, which suggests that each cell acquires its own nutrition, by either photosynthesis or osmotrophy (absorbing nutrients from the environment). This is also quite different from the hyphal structure seen in Fungi and Oomycotes, even the septate forms. making it unlikely that Qingshania magnifica could be assigned to either of these groups. Furthermore, molecular clock estimates suggest that Fungi did not appear till about 1000 million years ago, and Oomycotes probably around the dawn of the Cambrian.

Based upon this analysis, Miao et al. conclude that Qingshania magnifica is most likely to have been a Eukaryotic Algae. This is consistent with molecular clock analyses, which suggest plastids (chloroplasts) were first acquired by unicellular Algae during the Palaeoproterozoic. The morphology of Qingshania magnifica is also consistent with younger fossils interpretted as Green Algae, as well as several modern members of that group. However, Miao et al. do no conclude there is sufficient evidence to confidently place Qingshania magnifica within the Green Algae, as originally proposed by Yan, instead concluding that it could be a Green Algae, a Red Algae, a stem group Archaeplastid, or even a member of an entirely extinct Eukaryotic group. Whichever of these is true, Qingshania magnifica provides strong support for a Late Palaeoproterozoic appearance of the crown group Eukaryotes, rather than a Late Mesoproterozoic one, which has sometimes been proposed. 

Overview of early evolution of the Eukarya along with fossil records. (A) Simplified Eukaryotic tree with divergence time estimates of major branches by molecular clock study. LECA, Last Eukaryotic Common Ancestor. Dashed grey lines represent hypothetical stem-group Eukaryotes, which are extinct. Abbreviation: Pha., Phanerozoic. (B) Representative fossil records of early Eukaryotes. The oldest unambiguous Eukaryotic fossils are unicellular forms, e.g., Tappania plana and Shuiyousphaeridium macroreticulatum from the approximately 1650 million-year-old Ruyang Group; Dictyosphaera macroreticulata, Germinosphaera alveolata, and Valeria lophostriata from the Changzhougou Formation and lowermost Chuanlinggou Formation of North China. The Qingshania magnifica represents the current oldest convincing multicellular Eukaryote from the approximately 1635 million-year-old upper Chuanlinggou Formation of North China. The oldest Red Alga is Bangiomorpha pubescens from the approximately 1050 million-year-old Hunting Formation, Canada. The oldest Green Alga is Proterocladus antiquus from the approximately 950 million-year-old Nanfen Formation of North China. The oldest putative Fungus is Ourasphaira giraldae from the approximately 890 million-year-old Grassy Bay Formation of Canada. The oldest Amoebozoans are vase-shaped microfossils, e.g., Cycliocyrillium torquata from the approximately 750 to 730 million-year-old Kwagunt Formation, part of the Chuar Group of Arizona. Scale bars, 500 μm for the oldest Green Alga and 50 μm for all other specimens. Miao et al. (2024).

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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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