New discoveries reveal unexpectedly wide taxonomic diversity and call for a new classification of the green algal family Prasiolaceae (Trebouxiophyceae)

Algal lineages with small size and low morphological complexity are typically among the most difficult to resolve taxonomically. They are often characterized by high levels of cryptic diversity and environment-driven plasticity that complicate species recognition. Their full taxonomic diversity is even more difficult to unravel when their evolution has also resulted in great ecological diversity, encompassing many different habitats including unusual ones. An example of this case is the Prasiolaceae, a family of filamentous, leafy and packet-like green algae occurring in marine, freshwater and terrestrial environments. They are well known for their preference for habitats subject to high loads of organically derived nutrients, mainly in the form of seabird guano found in upper littoral habitats. In recent years, new surveys and new molecular phylogenetic studies have produced the surprising discovery of many new lineages, revealing an unexpected high taxonomic diversity. DNA sequence data obtained from cultured strains called for the description of several new genera and species. Prasiola, type genus of the family, was subjected to a major rearrangement. The genus was shown to be polyphyletic, which required its splitting into four genera: the real Prasiola (in which the type species Prasiola crispa belongs), Eaprasiola, Mariprasiola and Vittaprasiola. The marine species of this grouping, classified in the genus Mariprasiola, represent a remarkable case: they exhibit clear morphological and life history differences, although sequences of the most common markers used for species delimitation are identical. Conversely, in the genus Vittaprasiola DNA sequence divergence demonstrates the existence of cryptic entities that require separation at species level. Further studies of this group should expand the body of molecular markers available and, in combination with fieldbased studies making use of environmental DNA, might reveal an even higher phylogenetic and taxonomic diversity.