Low Temperature SEM findings on the ultrastructure of Trebouxia lynnae (Trebouxiophyceae, lichenized microalgae)

The lichenized green microalga Trebouxia lynnae Barreno has been recently described and is considered a model organism for studying lichen chlorobionts. Its cellular ultrastructure has already been studied in detail by light, electron, and confocal microscopy, and its nuclear, chloroplast and mitochondrial genomes have been sequenced and annotated. Here, we investigated in detail the ultrastructure of in vitro grown cultures of T. lynnae observed by Low Temperature Scanning Electron Microscopy (LTSEM) applying a protocol with minimum intervention over the biological samples. This methodology allowed for the discovery of ultrastructural features previously unseen in Trebouxiophyceae microalgae. In addition, original Transmission Electron Microscopy (TEM) images of T. lynnae were reinterpreted based on the new information provided by LTSEM. The nucleolar vacuole, dictyosomes, and endoplasmic reticulum were investigated and reported for the first time in T. lynnae and most likely in other Trebouxia lineages.