A reassessment of Geminella (Chlorophyta) based upon photosynthetic pigments, DNA sequence analysis and electron microscopy

A cultured microalgal strain (UTEX 2540) originally identified as Heterotrichella gracillas Reisigl (Xanthophyceae) was re-examined using various techniques. Morphological evidence, particularly the absence of dimorphic cells (one blunt, the other tapered to an acute point), indicate that strain UTEX 2540 has been misidentified. Heterotrichella gracillas is considered to be a member of the chlorophyll a and c-containing class Xanthophyceae (Chromista). However, HPLC analyses of photosynthetic pigments indicated the presence of chlorophylls a and b, ß-carotene, lutein and violaxanthin while ultrastructural data revealed the presence of starch stored inside the plastid. These data, as well as small subunit (18S rRNA) gene sequence analysis, indicate that this alga belongs in the Chlorophyta, not the Xanthophyceae (Chromista). Further DNA sequence analyses suggest that UTEX 2540 is most closely related to Geminella terricola Petersen and certain Microspora species that are currently classified in the Ulotrichales. However, unlike other Geminella species, UTEX 2540 exists as single cells or forms poorly organized (2-8 celled) ephemeral pseudofilaments. A conspicuous extracellular mucilaginous sheath characterizes other Geminella species but this feature is lacking in UTEX 2540. Furthermore, our analyses convincingly demonstrate that Geminella and at least some isolates of Microspora do not belong in the Ulotrichales. These results suggest that (1) the generic concept for Geminella must be broadened to include unicellular species that lack an apparent mucilaginous envelope, (2) Geminella does not belong in the Ulotrichales, and, instead, (3) its closest relatives among other green algae are almost certainly found within the Trebouxiophyceae