Kobayasiella parasubtilissima Lange-Bertalot 1999

Kobayasiella parasubtilissima (Kobayasi & Nagumo) Lange-Bertalot (Figs 2 A-P; 4 A-H)

Iconographia Diatomologica 6: 268 (Lange-Bertalot 1999). — Navicula parasubtilissima Kobayasi & Nagumo, The Botanical Magazine, Tokyo 101 (1063): 245 (Kobayasi & Nagumo 1988).

ECOLOGY AND DISTRIBUTION. — Kobayasiella parasubtilissima was found in ten lakes. A rare species, with a mean relative abundance of 0.2% and a maximum of 5.5% in Lake 16-H. Although rare, this species was more abundant in acidic conditions (Appendix 3) with a pH optimum of 5.7 and a relatively high DOC optimum of 6.9 mg /L (Appendix 2). Camburn & Charles (2000), Fallu et al. (2000), and Siver & Hamilton (2011) all reported this taxon from low alkalinity, low pH waters across eastern North America. Siver et al. (2005) and Bahls (2012b) also observed this taxon with a mixed population of more than one species from western North America. Kobayasiella parasubtilissima has also been reported from Europe, suggesting that this is a globally distributed species.

The frustules exhibit a rectangular and narrow shape in girdle view. The valves are narrow, ranging from linear to linear-lanceolate, with capitate ends. Based on a sample size of 21, the valve dimensions vary, with a length ranging from 27 to 33.5 µm and a width ranging from 3.5 to 5 µm. The length-to-width ratio is 6.5 to 7.0. The stria density ranges from 45 to 50 in 10 µm. The axial area is linear to lanceolate and narrow. The central area is weakly linear-elliptic, exhibiting 7-8 marginal striae of varying lengths. The valve face is flat. The raphe is linear, with a distinct kink-like irregularity occurring halfway between the mid-valve and apex (Fig. 4A). The central raphe fissures are widely spaced, linearly expanded, and rounded. The terminal raphe fissures are weakly curved on the valve face, opening with an external linear to funnel-like groove (Fig. 4A, G). Internally, the raphe is situated on a thickened sternum, appearing straight without a kink-like regularity. The proximal raphe fissures are T-shaped depressions, while the terminal fissures end on elongated helictoglossae, isolated from the apex mantle (Fig. 4F, E, H). The striae exhibit a radial pattern at the mid-valve and become strongly convergent towards the apices (Fig. 4B). From the mid-valve to the Voigt fault, the individual striae become straight to bent, while from the Voigt fault to the apex, they change from bent to straight. The striae possess 4-6 (sometimes 9) rows of pores. The striae are interrupted by a hyaline valve face/ mantle margin along the valve sides and extend uninterrupted around the apices. Distinct Voigt faults can be observed on both the primary and secondary sides of the valve, located at 2/3 of the distance between the mid-valve and apex. The striae are covered with 4-6 (sometimes more) rows of small multiseriate pores. Internally, the multiseriate pores are positioned between thickened virgae. Small silica projections from the virgae may or may not be present. The cingulum features 3-4 open copulae bands, with the copulae exhibiting two linear rows of pores at the pars interior. Additionally, a velum-like cover with 6-8 openings covers the pores. Occasionally, a fringe can be observed at the base of a copula.

Specimens from Tursujuq National Park match specimens from the original publication of Kobayasi & Nagumo (1988) with the characteristic linear narrow valves with capitate ends and a length to width ratio between 6.5 and 7.0. Populations in Tursujuq National Park were at the larger end of the described size range, with slightly wider valves (up to 5 µm) and a higher stria density (up to 50 in 10 µm) when including irregular smaller striae along the valve margin.