Fine structure of the vegetative frustule, auxospore, and initial cell of Licmophora gracilis (Bacillariophyta) from Potter Cove (25 de Mayo/King George I., Antarctica)

This study presents the first comprehensive ultrastructural analysis of the vegetative frustules, auxospores and initial cells of Licmophora gracilis from Potter Cove, 25 de Mayo/K. George I, Antarctica. Using light and scanning electron microscopy, we characterized key morphological features, including valve microarchitecture, cingulum organization and reproductive structures. Licmophora gracilis dominated glacial ice samples discharged into coastal marine environments, exhibiting high relative abundances (≥50%) alongside L. antarctica and L. belgicae. Notably, auxospores were frequently observed (2–3% abundance), suggesting active sexual reproduction synchronized with Antarctic summer conditions. Mucilaginous colonies form short branches bearing cuneate frustules with dense, asymmetrically distributed lenticular plastids. The valve face is centrally flat, gradually curving into the mantle without a distinct boundary. The cingulum comprises five bands (valvocopula and four pleurae), with the valvocopula exhibiting a unique abvalvar ligula – a taxonomically diagnostic trait. Auxospores appeared in paired arrangements, supporting cis-type anisogamy, and developed a perizonium with transverse (open bands, ventral sutural split) and longitudinal siliceous elements, alongside cap-like incunabular remnants. Initial cells differed from vegetative cells in reduced girdle bands and heterovalvy, with epivalves showing arched morphology. Our findings resolve discrepancies in previous descriptions of the vegetative frustule, such as narrower stria density ranges and variable rimoportula count, while revealing novel morphological features, including the valvocopula ligula and auxospore wall architecture. This study underscores the ecological adaptability of L. gracilis to glacial ice dispersion and reinforces its role as a valuable bioindicator for Antarctic coastal ecosystems.