A polyphasic taxonomy of the pseudocryptic diatom species complex <i>Pinnularia acidicola</i> , including description of new species

Diatoms are one of the most species-rich and environmentally significant protist groups on Earth, playing important roles in the biogeochemical cycles and food webs of many (semi)aquatic ecosystems. However, much of their species-level diversity lies beyond the resolution of traditional microscopy-based methods that are routinely, and sometimes exclusively, utilized in diatom taxonomy. One such cryptic species complex is Pinnularia acidicola, which was recently shown to consist of at least three different species-level lineages inhabiting remote southern hemisphere islands. Here, we (1) compare the P. acidicola complex to its closest relatives from P. subcapitata and P. sinistra using both published and newly established strains, (2) integrate all currently available evidence – including from light and scanning electron microscopy, conventional and geometric morphometrics, molecular phylogenetics, eDNA-based biogeography and ecology – and (3) formally resolve the three species. The P. acidicola name is reserved for the type population found on Ile de la Possession and Marion Island, while the population found on Réunion is described as P. acidireunionensis. The population sampled on Ile Amsterdam is newly recognized as the already described P. vixconspicua, and epitypes including DNA sequences are designated. The three species are genetically distinct, exhibit non-overlapping geographic distributions and are morphologically differentiated by subtle, yet significant morphometric differences in valve dimensions, fascia and striation. Despite this, all three species appear to share an ecological preference for acidic soils, ponds or streams. Altogether, our comprehensive analysis of the P. acidicola species complex highlights the importance of combining both morphological and genetic approaches in uncovering diatom species diversity.