ACUF (Algal Collection University Federico II): custodian of microalgae diversity and a valuable resource for research. The case of Stichococcus bacillaris (Trebouxiophyceae, Chlorophyta) strains isolated from Italian low pH environments 50 years ago

Living culture collections play an essential role in preserving and studying microbial biodiversity. The ACUF collection represents one of the long-standing repositories of extremophilic microalgae in Europe, and the strains analysed here, isolated nearly 50 years ago from acidic springs, volcanic substrates and geothermal habitats, provide a clear example of the scientific value of such preservation. The possibility of obtaining fresh DNA from historical isolates allows questions to be addressed that were not accessible at the time of isolation. In this study, we revisited a group of strains originally identified as Stichococcus bacillaris and show how modern phylogenetic tools can refine their taxonomic placement. The combined use of rbcL and the nuclear rRNA cistron provided a consistent picture across markers. Both datasets clearly separated one strain from the remaining isolates, and both placed this strain within Deuterostichococcus, specifically to D. epilithicus. The remaining nine ACUF strains consistently grouped with Tetratostichococcus jenerensis. The presence of T. jenerensis-like strains in multiple Italian localities expands the known ecological and geographic range of this lineage. The congruence between plastid and nuclear markers supports the robustness of the revised phylogenetic placements. At the same time, the genealogical structure revealed by the TCS analysis suggests that subtle diversification may have occurred among strains collected from distinct geothermal and volcanic habitats. The distinct position in both phylogenies suggests that the original assignment to S. bacillaris did not reflect its true relationships, likely due to the limited diagnostic value of morphology alone in these coccoid green algae.