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Species assigned to Porphyra sensu lato (Bangiales) make up one of the largest groups of red algae in the world. While the Bangiales are monophyletic, Porphyra is not, but the number of genera remains unresolved and a consensus needs to be reached on how to proceed. Here, it is proposed that a global taxonomy for the group would enable resolution and consensus. Using examples from our work on Porphyra in both the northern and southern hemispheres, including the North Atlantic, Mediterranean and Pacific, notably Chile, we have addressed the following questions: i) how many species are there, ii) how many species are there in different parts of the world, and iii) how do these species relate to those in different parts of the world? A cumulative total of species described in the North Atlantic and Mediterranean since 1800 indicates that there are still species to be added to the flora for these regions. Determination of intraspecific diversity or diversity within species complexes was examined using the mitochondrial cox1. The results split the species into two distinct groups, revealed differences between the British and Faroese floras and indicated a new species in the Faroes. Detection of species diversity in different geographical areas of the world and their relationship with the North Atlantic and southeast Pacific (Chile) was studied using the plastid Rubisco spacer and partial rDNA SSU data. While the Rubisco spacer was limited for global comparisons, it was useful for species identification adding five more species to the Atlantic flora. The partial rDNA SSU data, while its shortness made it unsuitable for generic interpretation, revealed the spread of taxa from different geographical areas throughout the tree. We have concluded that to achieve a global taxonomy standard molecular markers should be used and we propose that the rbcL and complete rDNA SSU would be suitable. We have also concluded that while a few regions of the world have been well studied, before a consensus on generic boundaries can be achieved detailed taxonomic studies are needed in many other parts of the world including circumpolar Arctic, mid and southwest Atlantic and southeast Asia.

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