Diversity of the genus Ulva (Ulvophyceae, Chlorophyta) on the Conero Riviera: an assessment based on DNA sequence data

Ulva L. is one of the most ubiquitous, widespread, and species-rich genera of green macroalgae. Species of Ulva are found in coastal areas of temperate and tropical regions and, in eutrophic conditions, may produce extensive blooms known as green tides. Some species also occur in coastal lagoons and in freshwater bodies. Until the early 2000s the genus was subdivided in two different genera, Ulva (thallus consisting of 2-layered blades, forms referred to as sea lettuce) and Enteromorpha (tubular hollow thallus formed by 1 layer of cells). These, however, were merged into the single genus Ulva based on molecular evidence. The study of the taxonomy of Ulva has a long history dating back the description of the genus by Linnaeus in 1753. Ulva species are notoriously difficult to identify based only on morphological characters. Colour, habit of the thallus, presence/absence of microscopic teeth on the blade margins, size and arrangements of cells in surface view, and number of pyrenoids in the chloroplasts are the main characters that have been used in morphology-based identifications. It is well known, however, that the morphology of many species is highly variable and affected by phenotypic plasticity related to environmental conditions. Therefore, in recent years DNA barcoding based on the use of the plastid rbcL and tufA genes has become the preferred approach to achieve reliable species identifications. Classification and taxonomy of the genus have greatly advanced in recent years thanks to the availability of sequences obtained from type specimens. Ulva is widespread on the Conero Riviera, where specimens with sea lettuce morphology are especially abundant in late spring-early summer. In the past literature these have been referred to Ulva rigida. We reassessed the taxonomy of Ulva in this area by sequencing the tufA gene in 37 specimens collected from 6 sites (Piscinetta del Passetto, Spiaggia del Passetto, Scalaccia, Due Sorelle, Sassi Neri, Spiaggia del Frate). Most of the specimens (31) with sea lettuce morphology were assigned to Ulva lacinulata. However, 3 specimens from Scalaccia were identified as Ulva rigida, highlighting a case of cryptic diversity. Species assignment for these two species were unambiguous, due to complete identity with sequences from type specimens. Sequences of specimens with tubular morphology were recovered in tufA clades corresponding to Ulva compressa (2 sequences) and Ulva intestinalis (1 sequence). Based on the results, no introduced species of Ulva were identified. The results highlight the critical importance of DNA sequence data for identification of Ulva species and the need for continued monitoring of the diversity of Ulva in the study area in future years.