Planothidium minuanum M.G.Junqueira, L.O.Crossetti & C.E.Wetzel, 2024

Planothidium minuanum M.G.Junqueira, L.O.Crossetti & C.E.Wetzel sp. nov. ( Figs 2–41 View FIGURES 2–41 LM, 42–56 SEM)

Light microscopy (LM) ( Figs 2–41 View FIGURES 2–41 ):― Valves elliptic but with a rather with a quadratic aspect, asymmetric, with broadly protracted, subrostrate apices. Valve dimensions: length 10–17.0 µm, width 5.5–8 µm. Rapheless valve ( Figs 2–25 View FIGURES 2–41 ): axial area narrow, linear to lanceolate, expanded in the central area, in opposition to the cavum. Central area with a unilateral large, horseshoe-shaped, hyaline area with a visible cavum; on the opposite side, striae slightly shortened in the axial area and slightly spaced. Striae radiate, more strongly radiate towards the apices, 12–14 in 10 µm measured at the central part of the valve, opposite to the cavum. Raphe valve ( Figs 26–41 View FIGURES 2–41 ): axial area narrow, linear. Central area irregular, rectangular to slightly rounded, bordered by 1 or 2 shortened striae on each side. Raphe straight, with expanded drop-like proximal raphe endings. Terminal raphe fissures deflected to the same side. Striae strongly radiate throughout the entire valve, 12–14 in 10 µm.

Scanning electron microscopy (SEM) ( Figs 42–56 View FIGURES 42–47 View FIGURES 48–52 View FIGURES 53–56 ):― Rapheless valve ( Figs 42−52 View FIGURES 42–47 View FIGURES 48–52 ): striae composed externally of two (three) to one, irregularly-rounded shaped areolae ( Figs 42−47 View FIGURES 42–47 ), the ones closer to the mantle being slightly smaller than the inner areolae (i.e. towards the axial area). Striae near the axial area often composed of one row of areolae. Near the valve mantle, striae occasionally composed of two to three areolae. Striae mostly discontinuing on the valve mantle. Areolae on the valve mantle usually composed of a cluster of four to five equally sized areolae. Irregular shallow depressions present in the central area and slit-like depressions in the axial area. In internal view ( Figs 48−52 View FIGURES 48–52 ) cavum aperture wide with the borders usually linked to the neighbouring striae. Striae internally sunken between raised virgae. Areolae internally covered by individual hymenes ( Fig. 52 View FIGURES 48–52 ). Raphe valve ( Figs 53−56 View FIGURES 53–56 ): striae broader than the virgae, composed of three to four rows of rounded areolae, considerably smaller than the rapheless valve. Straight raphe, with proximal external raphe terminating in rimmed expanded pores. Terminal raphe fissures unilaterally bent, continuing shortly onto the valve mantle. Internally central nodule raised, with proximal raphe endings slightly deflected to opposite sides ( Fig. 56 View FIGURES 53–56 ), terminating inconspicuously. Terminal raphe endings terminating on faint helictoglossae ( Fig. 56 View FIGURES 53–56 ) Striae internally sunken between raised virgae; areolae internally covered by individual hymenes.

Type:— BRAZIL. Rio Grande do Sul, Balneário Pinhal, Cerquinha coastal lake, 30°15’92.6”S, 050°17’08.0”W, coll. date 17 Feb. 2022, leg. Luciane O. Crossetti (holotype slide ICN-212270, Fig. 6 View FIGURES 2–41 represent the holotype).

PhycoBanck Registration:— http://phycobank.org/104936

Etymology:― The “ Minuanos ” were one of the native nations of Uruguay, Argentina and Brazil (especially in the state of Rio Grande do Sul). It is also a cold southwesterly wind that blows in the southern Brazilian state of Rio Grande do Sul and in Uruguay.

Ecology & associated diatom flora:— Planothidium minuanum was present in several of the 24 studies coastal lakes. Hermany et al. (2013) identified the species as Planothidium rostratum (their figs 5x - aa) and it could eventually be identified as P. minuanum .According to Hermany et al. (2013) it was highlighted as a periphytic taxa from freshwater or tolerant to very restricted concentrations of salinity, marking the rise of the water column as a result of the Holocene marine transgression which, as recorded by Lorscheitter & Dillenburg (1988) in the Tramandaí Lagoon (1,800 years BP). The main associated species mentioned by Hermany et al. (2013) were Placoneis opportuna ( Hustedt 1950: 436) Chudaev & Gololobova (2016: 53) , Aulacoseira ambigua (Grunow 1880: 88) Simonsen (1979: 56) , and Platessa hustedtii ( Krasske 1923: 193) Lange-Bertalot (2004: 445) in Krammer & Bertalot (2004). In our case, the most common species that co-occurred were Achnanthidium minutissimum ( Kützing 1833: 578) Czarnecki (1994: 157) , Aulacoseira ambigua , Cocconeis euglypta Ehrenberg (1854: 34) , Pseudostaurosiropsis connecticutensis E. A. Morales (2001: 117), and Nitzschia angustatula Lange-Bertalot in Lange-Bertalot & Krammer (1987: 18). Planothidium minuanum was registered in 12 (light-blue highlighted in Fig. 1 View FIGURE 1 ) out of 24 coastal lakes but was considered as a rare species in terms of abundance, with the mean 0.56% relative abundance across the diatom communities. The relative abundance of P. minuanum arrange 0.36% in Emboabinha lake and 1.66% in Fortaleza lake. The species’ optimum, were close to the average value of environmental variables considering the total set of lakes ( Figure 57 View FIGURE 57 ). The ecological preference of P. minuanum was for higher electrical conductivity and transparency values, low total suspended solids, mesotrophic waters, and neutral pH ( Figure 57 View FIGURE 57 ). Thus, they were found growing attached as epiphyte in freshwater environments and occasionally found in slightly brackish waters.

Taxonomic comments:— Planothidium minuanum is characterized by the presence of a cavum,which distinguishes it from several species that have a similar valve shape, but differ due to the presence of a sinus. This distinction prevents that Planothidium minuanum from being confused with Planothidium dubium (Grunow in Cleve & Grunow 1880: 23) as described by Round & Bukhtiyarova (1996: 352), and Planothidium reichardtii Lange-Bertalot & Werum (in Werum & Lange-Bertalot 2004: 172). Both have a similar elliptic-lanceolate valve outline with truncated, rostrate apices and an asymmetrical central area in the raphe-bearing valve, but both taxa lack the distinct cavum structure excluding, hence, all conspecificity. The African species Planothidum comperei discussed in N’Guessan et al. (2014) sufficiently differs not to be confused with P. minuanum as well. Planothidium incuriatum split off in 2013 from P. biporomum (M.H. Hohn & Hellerman 1963: 273) Lange-Bertalot (1999: 275) is a similar species, also occurring in southern Brazil, but it is larger and has elongated, lanceolate to elliptic-lanceolate valves. Planothidium rostratum , as illustrated in Wetzel et al. (2019, figs 109–119), has striae composed externally of three to four rows of small, rounded areolae on the rapheless valve, with the areolae on the valve mantle composed usually of a cluster of four to five areolea. Irregular shallow slit-like depressions lines are also present in the central and axial area, while the raphe valve also shows the striae composed of three to four rows of rounded areolae. This is not the case on Planothidium minuanum where the sunken areolae with irregular shape forms a uni- to biseriate striae on the rapheless valves ( Figures 42–47 View FIGURES 42–47 ). Planothidium minuanum also forms a cluster of areolae (up to 6) on the valve mantle (see Figure 46 View FIGURES 42–47 ). Furthermore, Planothidium minuanum differ to Planothidium rostratum in the shape of the apices: the new diatom species has protracted and subrostrate apices, while Planotdhium rostratum has shortly rostrate and longer rostrate apices. Among all species recently described, the most similar in ultrastructural elements is P. xinguense described from Brazilian Amazon (Xingu basin). However, P. xinguense has clearly protracted and rostrate to subcapitate ends. Other rostrate, cavum-bearing taxa such as P. africanum and P. rostratoholarcticum Lange-Bertalot & Bak in Bak & Lange-Bertalot (2015: 354) differ in having multiple slit-like grooves in their axial and central area, so far not observed in P. minuanum . Moreover, Planothidium minuanum show a valve with elliptical, but with quadratic aspect shape, and protracted and subrostrate apices, while P. rostratoholarcticum has a valve with elliptical shape, and weakly elongated, or broadly protracted or subrostrate apcies.