Kobayasiella micropunctata Lange-Bertalot 1999

Kobayasiella micropunctata (Germain) Lange-Bertalot (Figs 3 U-AM; 9 A-F)

Iconographia Diatomologica 6: 267 (Lange-Bertalot 1999). — Navicula subtilissima var. micropunctata Germain, Flore des diatomées – Diatomophycées – eaux douces et saumâtres du Massif armoricain et des contrées voisines d’Europe occidentale: 234 (Germain 1981). — Navicula micropunctata (Germain) Kobayasi & Nagumo, The Botanical Magazine, Tokyo 101 (1063): 247 (Kobayasi & Nagumo 1988).

ECOLOGY AND DISTRIBUTION. — Kobayasiella mircopunctata was the most commonly found Kobayasiella species in our study, present in 33 lakes. However, its mean relative abundance was only 0.8%, with a maximum of 6.2% in Lake 16-F. No clear distributional pattern was distinguished according to water chemistry conditions. However, a medium alkalinity level seems to be consistent for this species. Mg and Na concentrations were linked to the distribution with respective optima at 0.87 mg /L and 0.34 mg /L. Kobayasi & Nagumo (1988) observed this species from Imandra Lappmark (Russia) and Japan in waters with circumneutral pH and low conductivity (131 µS/cm). In North America, K. micropunctata has been reported from Montana (United States) in waters with pH 8.46 and low conductivity (19 µS/cm), indicative of fluctuating pH (Bahls 2012a). This species at present has a global distribution in northern and subalpine environments.

The frustules are rectangular and narrow in girdle view. The valves exhibit a linear-lanceolate shape with more or less narrow subcapitate to capitate protruding ends. Based on a sample size of 22, the valve dimensions range from a length of 17 to 24.5 µm and a width of 3.5 to 6 µm. The stria density varies from 40 to 44 in 10 µm. The axial area is lanceolate and narrow. The central area is elliptic and occupies more than half the width of the valve, with seven to eight long and short striae between the proximal raphe ends (Fig. 9C). Externally, the valve face is flat. The raphe is linear, with a kink-like irregularity halfway between the mid-valve and apex (Fig. 9A, B). Externally, the central raphe fissures are widely spaced and linearly expanded, with enlarged rounded ends (Fig. 9C). The terminal raphe fissures are hooked to the secondary side of the valve opening and exhibit a small external funnel-like surface depression (Fig. 9D). Internally, the raphe is located on a thickened sternum. The terminal fissures end on small elongated helictoglossae, isolated from the apex mantle (Fig. 9E). The striae are strongly radiate at the mid-valve and become strongly convergent at the apices. The mantle striae are interrupted by a thick hyaline ridge at the valve face/mantle junction. The striae on the mantle apex are reduced and continuous around the apices. Distinct Voigt faults are present on both the primary and secondary sides, close to the apex. The striae are covered with four rows of small multiseriate pores. Internally, the multiseriate pores are positioned between thickened virgae. The cingulum is composed of finely silicified open wide copulae bands, and the copulae feature two rows of areolae covered with fine poroid occlusions. At the base of the copula, there is a fringe of fine pores (Fig. 9F).

Our taxon, with a linear-lanceolate outline and subcapitate to capitate protruding ends with hooked terminal raphe fissures, matches the species K. micropunctata (synonym: Navicula subtilissima var. micropunctata (Germain) Kobayasi & Nagumo). In the original description, Germain (1981) reported his illustrated valve (in TEM) to be 2 µm wide, but based on the magnification it is actually 3.8 µm wide, which is in line with the specimens of Kobayasi & Nagumo (1988) and those of this study. Kobayasi & Nagumo (1988) emended the species with detailed SEM and TEM images. In their specimens the broad central area is documented along with the absence of virgae projections and hooked terminal raphe fissures. Kobayasi & Nagumo (1988) did not examine the type material of Germain, but looked at materials from Imandra Lappmark (Russia) and Japan. Species with similar valve outlines but with a larger size include K. pseudostauron, K. tursujuqensis sp. nov. and K. subtilissima. In LM, this taxon is distinguished by size, subcapitate apices and a broad central area.