Fragilaria huebeneri, Krahn & Schwarz & Wetzel & Cohuo-Durán & Daut & Macario-González & Pérez & Wang & Schwalb, 2021

Krahn, Kim J., Schwarz, Anja, Wetzel, Carlos E., Cohuo-Durán, Sergio, Daut, Gerhard, Macario-González, Laura, Pérez, Liseth, Wang, Junbo & Schwalb, Antje, 2021, Three new needle-shaped Fragilaria species from Central America and the Tibetan Plateau, Phytotaxa 479 (1), pp. 1-22 : 16

Fragilaria huebeneri

As well as F. huebeneri , F. asiatica was described from the Tibetan Plateau. However, valves are smaller (31‒102 µm) compared to F. huebeneri (99–121 µm) and have a fusiform to linear outline. In contrast to F. huebeneri , F. asiatica does not possess any spines and up to two rimoportulae can be present per valve. Furthermore, striae of F. asiatica are opposite instead of alternate.

Fragilaria asterionelloides Tuji & D.M.Williams (2017: 46) can be distinguished from F. huebeneri by the formation of star-shaped colonies and smaller size (60–70 µm). Apices of F. asterionelloides are more capitate and valves have a higher stria density of 22–23 per 10 µm. In SEM, bifurcate spines can be observed at the valve center of F. asterionelloides , whilst F. huebeneri rather develops spatula-shaped spines.

Regarding colony formation, Fragilaria huebeneri can be confused with F. crotonensis ssp. crotonensis Kitton (1869: 110) . Both taxa form long ribbon-like colonies linked together by spatula-shaped spines. Otherwise, species have clear differentiating characteristics. Fragilaria huebeneri has a higher stria density (17–21 striae per 10 µm) than F. crotonensis ssp. crotonensis (15–18 striae per 10 µm), has a higher length-to-width ratio (39–52 to 18–38 and more, respectively) and shows no proximal constriction.

Fragilaria crotonensis ssp. lacus-vulcani forms only few-celled colonies and has generally lower stria density (14–18 striae per 10 µm) than F. huebeneri . The central area of F. crotonensis ssp. lacus-vulcani is distinct in LM, while it is often difficult to see in F. huebeneri .

Although found in a different geographical and climatic region, F. huebeneri is morphologically close to F. neotropica . The latter is mainly distinguished by shorter valves (52–72 µm), much higher stria density (28–32 in 10 µm), and capitate apices. Almeida et al. (2016) also described teratological forms in populations of F. neotropica from Brazilian reservoirs. Comparable to F. huebeneri , the valves show a central deformation. The authors suggest that deformation might be caused by metal contamination in the reservoirs.

Different types of environmental stressors have long been discussed as modifying valve morphology, including increased metal concentrations and trace elements (e.g. McFarland et al. 1997, Falasco et al. 2009, Cantonati et al. 2014, Lavoie et al. 2017). For example, the central deformation of valves of F. tenera (as Synedra tenera W. Smith ) in Lake Orta, Italy have been ascribed to pollution by copper (Cu) ( Ruggiu et al. 1998). In saline lakes on the Tibetan Plateau, extremely high levels of arsenic (As) were found. The accumulation is probably caused by natural loading and evaporation ( Xiong et al. 2020). Soils on the Tibetan Plateau possess high natural As concentrations and in general, heavy metals mostly originate from weathering processes ( Li et al. 2009, Sheng et al. 2012). Surface soil samples collected from the Lhasa–Shigatse–Nam Co region were found to be polluted with As, Cu, nickel (Ni), and mercury (Hg) to different degrees and may pose a potential ecological risk ( Xie et al. 2014). Therefore, we propose that the deformation of the valves of F. huebeneri might be related to increased concentration of these elements in the lake water of Nam Co. However, further water analyses are needed to confirm this hypothesis.

A differentiation between F. saxoplanctonica and F. huebeneri can be made by the acutely rounded apices and the higher stria density (23–28 in 10 µm) of F. saxoplanctonica . Additionally, a central area and marginal spines are absent.

Fragilaria huebeneri resembles F. tenera var. tenera and the other two varieties. All taxa have a relatively similar stria density. However, F. tenera var. tenera forms loose aggregates at most, while F. tenera var. lemanensis forms stellate colonies, and no colony formation has been observed in F. tenera var. nanana . Valve lengths of F. tenera var. lemanensis and F. tenera var. nanana are smaller (70–80 µm and 29–85 µm, respectively) compared to F. huebeneri (99–121), whereas the length of F. tenera var. tenera is quite similar (60–120 µm). In contrast to F. huebeneri , F. tenera var. tenera has a more sublinear valve outline and apices of F. tenera var. lemanensis and F. tenera var. nanana are more capitate.

Fragilaria tenuissima is also difficult to distinguish from F. huebeneri in LM. Valve size, width, length-to-width ratio, and stria density largely overlap. Fragilaria tenuissima , however, does not form long colonies and apices are only weakly subcapitate. In SEM, two rimoportulae per valve are present in F. tenuissima instead of one in F. huebeneri , and spines are small or reduced. Moreover, F. tenuissima exhibits four to seven areolae per stria, whilst F. huebeneri has three to five areolae per stria.