Resurrecting the red algal genus Grania within the order Acrochaetiales (Florideophyceae, Rhodophyta)

An isolate of the filamentous red alga Acrochaetium efflorescens from the NE Atlantic was followed in culture through a triphasic sexual life history. Morphology of each phase matched previous descriptions for this alga. However, inclusion of A. efflorescens in the genus Acrochaetium is problematic. In current taxonomic treatments this genus includes acrochaetioid algae with a stellate plastid and a large central pyrenoid. Acrochaetium efflorescens, on the other hand, has consistently been reported to possess spiral to ribbon-shaped plastids. To determine the phylogenetic affinities of A. efflorescens, we sequenced large subunit ribosomal DNA and resolved full support (Bayesian posterior probabilities; maximum likelihood, neighbour joining, and parsimony bootstrap percentages) for inclusion of A. efflorescens within the order Acrochaetiales. However, it does not associate closely with any of the existing genera in this order, showing only a weak affinity to the genus Rhodochorton. The genus Grania, originally established by Kylin, is currently available for this species. Given its distinct anatomy (ribbon-shaped plastids in combination with seriate carposporangia), unique molecular signature, and lack of congruence with other current generic concepts within the Acrochaetiales, we advocate resurrecting the genus Grania for G. efflorescens.