Micro- and ultrastructural features for the distinction of Phymatolithon lusitanicum from Phymatolithon calcareum

Calcifying red algae foster unique and rich biological communities and are important component of the global C cycle. Rhodolith beds are globally distributed biodiversity hotspots that are engineered by free-living calcifying red algae, and maërl beds are a type of rhodolith bed typically characterized by free-living, twig-like coralline algae with a branched growth form. Phymatolithon calcareum, along with the more recently described Phymatolithon lusitanicum is considered a major component of maërl beds in Europe. Here, we explore the morphology of the vegetative thallus of P. calcareum and P. lusitanicum. Our aim is to identify statistically valuable morphological features that can be used to differentiate these two species of algae that are macroscopically very similar, frequently sterile, and share the same habitat. Morphological features have historically failed to aid in distinguishing P. lusitanicum from P. calcareum. Our observations of seasonal growth patterns, the arrangement of perithallial cells in filaments, and the size of cell fusions provide noteworthy advances in our ability to use morphological features to identify the different species. Additionally, ultrastructural characteristics appear to be a reliable distinguishing feature between the two Phymatolithon species.