Rexia erecta gen. et sp nov and Capsosira lowei sp nov., two newly described cyanobacterial taxa from the Great Smoky Mountains National Park (USA)

Two newly discovered taxa of Cyanobacteria from the Great Smoky Mountain National Park (USA) are presented. The first is the newly described species Capsosira lowei (Capsosiraceae), differing from the only other previously described species C. brebissonii Kutz. ex Born. et Flah. in regard to cell size and filament morphology. In addition, C. brebissonii is described as an aquatic or subaerophytic taxon, while our isolate was obtained as a phycobiont from the lichen Hydrothyria venosa J. L. Russell. Capsosira is currently placed in the Capsosiraceae of the Stigonematales due to its ability to have division in two planes. However, molecular evidence gathered in this study indicates closest affinity with Aulosira and Nostoc commune Vaucher, both in the Nostocaceae, Nostocales. Rexia erecta was isolated from concurrently collected aerophytic, epilithic sites. The hormogonia production, near absence of heterocysts and division in two planes are all typical of the Stigonematales, but it fits none of the currently circumscribed families in that order. This genus in other ways appears morphologically similar to members of the Scytonemataceae and Microchaetaceae. Molecular evidence (nearly complete 16S rRNA sequence data and 16S-23S internal transcribed spacer ITS region) places Rexia in the Microchaetaceae. These taxa are both problematic as they indicate that cell division in two planes has likely arisen more than once in the Nostocales, and thus the Stigonematales as currently circumscribed is not a monophyletic group. The Nostocales and Stigonematales are, in our opinion, in need of revision at the family and order level of classification.