Friday 26 February 2021

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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