Diversity and distribution of the genus Tetmemorus (Desmidiaceae, Zygnematophyceae) in Brazil

: In the present study, a taxonomic review was conducted on representatives of the genus Tetmemorus (Desmidiaceae, Zygnematophyceae) documented within Brazilian territory. This review involved compiling data from the literature and analyzing samples collected throughout the Bahia State, updating our knowledge about this genus in Brazil. For each identified taxon, we provided information such as description, distribution across biomes and states, watersheds, ecological aspects (including habitat and community types), a list of examined (and excluded) materials, and taxonomic comments. Additionally, a taxonomic key for all species reported in Brazil was provided. Through this comprehensive review, we identified a total of eight Tetmemorus taxa occurring in Brazilian territory, comprising five species ( T. brebissonii , T. furcatus , T. granulatus , T. laevis , T. planctonicus ) and three non-typical varieties ( T. brebissonii var. minor , T. laevis var. borgei , T. laevis var. minutus ).


INTRODUCTION
Desmids, belonging to the Zygnematophyceae class, form a diverse group of Streptophyta algae, comprising approximately 60 genera found exclusively in continental aquatic environments.This class has two subclasses: Spirogloeophycidae, with one representative Spirogloea muscicola (De Bary) Melkonian, a sister group of all desmids, and Zygnematophycidae, containing the other genera (Cheng et al. 2019), with four orders (Desmidiales, Serritaeniales, Spirogyrales, and Zygnematales) (Hess et al. 2022).A striking and exclusive feature of the Zygnematophyceae is conjugation-type sexual reproduction, in which two adult cells unite and release their protoplasmic contents, which act as flagellated amoeboid gametes that fuse to form a diploid zygospore.Asexual reproduction occurs by bipartition (Brook 1981, Hall & McCourt 2017).
In Brazil, 45 genera of desmids have been recorded (Bicudo & Menezes 2017), including Tetmemorus Ralfs ex Ralfs.This genus is characterized by solitary, cylindrical, fusiform cells, shallow median constriction, and deep apical incision; a smooth cell wall with scattered pores or ornamented with elongated scrobicles arranged in transverse or longitudinal series; and axial chloroplasts, with one or more pyrenoids.
Molecular data have shown that species such as Tetmemorus brebissonii and T. laevis form a well-supported clade embedded in a clade with Euastrum taxa (Hall et al. 2008), or in a clade with Euastrum, Cosmarium, and Actinotaenium cucurbita (Brebisson) Teiling taxa (Gontcharov & Melkonian 2011).However, to date, the small number of sequences from Tetmemorus and Euastrum does not allow for a better assessment of the phylogenetic relationships within the Euastrum assemblage (Gontcharov & Melkonian 2011).According to these authors, some taxa of this assemblage (mainly those of the Euastrum2 clade) share characteristics with species of the genus Tetmemorus, such as elongated cells, smooth walls, and apical incision in the semicells.According to Guiry & Guiry (2021), there are 17 known species of the genus, eight of which are taxonomically valid, in addition to 40 infraspecific taxa.
The present study aimed to carry out a taxonomic review of representatives of the genus Tetmemorus Ralfs ex Ralfs (Desmidiaceae) occurring in Brazil.

MATERIALS AND METHODS
Samples of continental microalgae collected in different aquatic environments in Bahia State, most of them from rivers and lakes on the north coast and Chapada Diamantina, were analyzed at the Phycology Laboratory, State University of Feira de Santana.These samples are stored in the Herbarium of the State University of Feira de Santana (HUEFS).
All materials were analyzed using an Olympus BX43 binocular microscope.At the Instituto Gonçalo-Moniz (Fiocruz-Bahia) a part of the material was submitted to complementary analyses by scanning electron microscopy (SEM), according to the protocols of Ramos et al. (2017) and Moura et al. (2021), to detail the cell wall characteristics of Tetmemorus species.
The morphological features of Tetmemorus representatives were analyzed, pointing out those of greater taxonomic importance.In addition to analyzing the material registered in HUEFS, all taxa reported in Brazil were reviewed based on the characteristics described and illustrated in the literature, as well as compared with their original protologues.
Information was provided for each taxon, such as descriptions, illustrations, updated geographic distribution in Brazil, list of analyzed material, ecological aspects such as type of habitat and life form, biome, watersheds, and taxonomic comments.In addition, for some taxa, there is a List of Material Excluded, which comprises records with insufficient information for reanalysis, as generally there are only citations or the illustrations are lacking.
Illustrations have been provided in drawings and photographs using a light microscope and SEM to demonstrate the variability of the material analyzed in all taxonomic views.The drawings were made using the Inkscape software, while the optical microscopy photographs were taken with a MicroPublisher camera (QImaging MP5.0-RTV-CLR-10-C).SEM images were taken at the Instituto Gonçalo Moniz (Fiocruz-Bahia) with a camera adapted to the JEOL 6390 LV Microscope.

Tetmemorus Ralfs ex Ralfs, 1848
Cells free-living, solitary, straight, ranging from cylindrical to fusiform, longer than wide, with shallow median constriction and open sinus; apices rounded to slightly truncated, with narrow, deep vertical midline incision, usually closed, and rarely open.The cell wall may be smooth, punctate, or ornamented with scrobicles.Chloroplast single, axial, stellate, with 1-9 pyrenoids arranged in a median series.
In Brazil, the genus Tetmemorus is represented by eight taxa, distributed in five species and three non-typical varieties, which can be identified using the key below.
Material Note: Tetmemorus brebissonii has a wide distribution, being recorded on most continents, making it one of the best-known species of the genus.The presence of a cell wall decorated with scrobicles organized in a longitudinal series, in addition to its cylindrical cell outline with margins parallel and widely rounded apices, allows T. brebissonii to be separated from the other species of the genus.
Based on populations of T. brebissonii var.intermedius Flensburg in Europe, Van Westen (2015) considered that cell morphometry values (4.5 to 6 times longer than wide, 100-123 μm long, 19.5-23 μm wide, 16.5-20.5μm isthmus) and ecological preferences [pH = 3.8-4.3 (-4.7);EC = 20-80 μS cm -1 ] were distinct from those reported for T. brebissonii and elevated this variety to the category of species, as Tetmemorus flensburgii Van Westen.To date, it has only been registered in Europe and North America.
Considering only the cell dimensions, it is possible that some of the Brazilian records, identified as T. brebissonii (Martins & Bicudo 1987, Menezes et al. 2011, Oliveira et al. 2014) and T. brebissonii var.minus (Förster 1964, Bicudo 1969), are representatives of Tetmemorus flensburgii.However, according to Růžička (1981), T. brebissonii var.brebissonii shows a wide variation in cell metric limits [(100-) 150-220 (-270-320) compr.× (19-) 25-40 (-48) μm wide], which naturally depends on the environmental conditions to which the species is subjected.Therefore, for the time being, we consider that the aforementioned records from Brazil, with metric limits close to T. flensburgii, remain as representatives of T. brebissonii var.brebissonii until integrative studies are carried out and ecological preferences are clarified.
Another species worth noting is Tetmemorus penioides A.W.Bennett, described for Europe (Lancashire, Furness Fells, England).Although this taxon is considered taxonomically valid in the Algaebase (Guiry & Guiry 2021), it had already been referred to by Krieger (1937) as a synonym of Tetmemorus brebissonii.We fully agree with this synonymy because all morphometric features of T. penioides are within the circumscription of Tetmemorus brebissonii.
Regarding materials from Brazil, Bicudo & Ventrice (1968) reported the occurrence of T. laevis var.tropicus Willi Krieger, a taxon described in 1937 by Krieger for Indonesia.According to the first authors, the Brazilian specimens studied had elongated cells (5-5.5 times longer than wide) and sharply tapered poles.However, despite the wall not being represented by the longitudinal series of pores, the illustrated specimens (Bicudo & Ventrice 1968: figs. 39-40), as well as the metric limits of the cells, were identical to T. brebissonii, also reported for the same place.Thus, we consider it more appropriate that this record of T. laevis var.tropicus to Brejo da Lapa be treated as a synonym of T. brebissonii.Note: According to Bicudo et al. (2014), the specimens identified as T. brebissoni var.minor by Bicudo (1969) for the State of São Paulo, refer to T. laevis, due to the presence of a finely punctuated wall.Among the most common morphological variations reported for var.minor, are the base of the semicells, which may present a small protrusion in the isthmus region, in addition to the margins of the semicells, which are straight to slightly concave.

Material excluded:
Tetmemorus furcatus G.J.P. Note: This species is apparently endemic to Brazil and so far, known only from its type locality.It has been reported to occur in metaphyton and periphyton of filamentous algae, on the bank of a stream formed by small waterfalls in the region of Águas Claras in Vale do Capão, Chapada Diamantina, Bahia, northeastern Brazil.
Tetmemorus furcatus is easily recognized by its fusiform cell outline with a noteworthy open apical incision, V-shaped.However, it should be compared with Tetmemorus fissus West & G.S.West, another species of the genus that presents an open apical incision.This latter species, described for Huíla, Angola, differs in that it has an elliptical cell outline, semicells with convex lateral margins, an open sinus in the median region of the cell, in addition to the much smaller dimensions (42 μm long, 18 μm wide).
It is possible that this pattern with an open apical incision is typical of the tropical region, since the only two species of the genus with this characteristic were recorded only in Brazil and Angola.
Material Note: Tetmemorus granulatus is one of the best-known species of the genus, with several records on different continents.This species is notable for its markedly fusiform cells with rounded, narrow apices.Despite this, certain morphotypes are commonly confused with T. laevis, especially those that are wider and with not so sharp apices.As these two species are commonly found in the same environments, and present a certain polymorphism, the differentiation of these taxa in some cases can be a little complicated, requiring a more detailed population analysis.However, Tetmemorus granulatus has cells that are strongly tapered towards the apices, generally longer than 150 μm, whereas those of T. laevis are slightly tapered towards the apices, which are largely rounded, and the cells are generally smaller than 150 μm.
Tetmemorus laevis var.tropicus is an example of how representatives of these two species can be confused.This variety was proposed by Krieger (1937), based on material from Sumatra and Java (Indonesia), to accommodate remarkably thinner specimens of T. laevis.We analyzed several samples from Bahia and this slender morphotype was quite common to be found, especially in samples from the northern coast of the state.However, such specimens are much closer morphologically to T. granulatus than to T. laevis, mainly due to their markedly fusiform contour with sharper apices.
From population studies, we have noticed a great morphological variation, mainly in the base and middle region of the semicells, which varied from little to very inflated.This variation was noticeable many times in the same cell (one semicell more inflated than the other), but in all studied specimens the fusiform pattern was maintained.A variable constriction near the apex was also observed in some specimens of T. granulatus, similar to that reported in T. granulatus var.constrictus Kurt Forster.However, we do not consider this feature a sufficient taxonomic character to support this variety, as it is quite variable at the population level.Thus, considering this broad phenotypic plasticity, T. laevis var.tropicus and T. granulatus var.constrictus were accounted here as heterotypic synonyms of T. granulatus.
Material Note: Tetmemorus laevis is distinguished from other species of the genus by presenting slightly fusiform cells with widely rounded apices in the frontal view.In the material from Bahia, we noticed a certain polymorphism in the cell outline, with slightly fusiform to almost oblong cells.Another variable feature is the apex, which can be either rounded (more common) or slightly truncated in the lateral view.This phenotypic plasticity was also observed by Bicudo et al. (2014) based on analyses of material collected in São Paulo.These latter authors also pointed out the variation in the intensity and density of cell wall punctuation.However, in specimens from Bahia, we noticed little variation in the cell wall pattern, in which punctuations were generally evenly spaced.According to Coesel & Meesters (2007), T. laevis can be found in both acidophilic and oligotrophic aquatic environments as well as in subaerial environments associated with Sphagnum and on the surface of moist soils.
Habitat: oligo-mesotrophic environments such as ponds, rivers, Sphagnum fields in mountainous regions.
It is notably one of the smallest species of the genus, showing the same cellular outline in both side and frontal views.As observed by Förster (1974), the studied populations showed morphological variations both at the apex of the cells (from rounded to truncated) and at the edges of the semicells (more or less convex).The cell wall is distinctive for being smooth and devoid of ornamentation or prominent pores, which also differs from other species.In addition, according to the authors, this species can be compared to T. laevis var.minutus, differing mainly by its smaller dimensions, rotational shape (front and side views), hyaline cell wall, and cell outline.Förster also highlighted the different ecological preferences of these two taxa: T. laevis var.minutus occurs more frequently in humid terrestrial environments, such as soils, rock surfaces, and Sphagnum fields, whereas T. planctonicus has only been recorded in plankton from aquatic environments.
From the present study, knowledge about the genus Tetmemorus from Brazil has been expanded.However, gaps remain in the distribution of this genus along that country, especially in the Central-West, North, and Northeast (Figures 4-5).Most records are for São Paulo and Bahia, which reflects the large numbers of samplings and of desmidiologists working in those states.Regarding the type of habitat, we noticed that representatives of this genus in Brazil are more commonly found in Sphagnum fields, shallow streams, and temporary ponds (Fig. 6), especially under acidic conditions, as pointed out by Coesel & Meesters (2007) for Europe.Furthermore, other desmids that generally occur with Tetmemorus include representatives of the genera Actinotaenium, Cylindrocystis, Netrium, and Penium.
As some species are possibly endemic to Brazil (T.furcatus and T. planctonicus), we recommend expanding the investigation of the diversity of this genus, mainly in mountainous regions.Finally, the use of taxonomic tools, such as SEM, physiology, molecular data, and geometric morphometry, is desirable to complement morphological analysis from light microscopy, as well as modelling to comprehend the relationship between the taxa of Tetmemorus and its distribution patterns.

Figure 4 .
Figure 4. Geographic distribution of Tetmemorus brebissonii, T. furcatus and T. granulatus in the Brazilian watersheds and biomes.

Figure 5 .
Figure 5. Geographic distribution of Tetmemorus laevis and T. planctonicus in the Brazilian watersheds and biomes.

Figure 6 .
Figure 6.Types of habitats where Tetmemorus taxa commonly found in Brazil.a) stream.b) Sphagnum field.c-d) temporary ponds.e) Population of Tetmemorus laevis with some algae associated.