Marine necklace-chain Fragilariaceae (Bacillariophyceae) from Guam, including descriptions of Koernerella and Perideraion, genera nova

Light microscopy (LM) and scanning electron microscopy (SEM) studies of necklace-chain forming colonial diatoms from benthic habitats in Guam revealed five species. Bleakeleya notata (Grunow in Van Heurck) Round is widespread and well-known. Asterionella notata var. recticostata Korner has been reported only twice before; we describe the plastids for the first time. On phylogenetic grounds we assign A. notata var. recticostata to a new genus, Koernerella, and emend the description of Bleakeleya. Three new species belonging in a new genus, Perideraion, are described as P. montgomeryi, P. decipiens and P. elongatum. Cultures from single-cell isolates yielded four gene sequences for three of these taxa. The genera are easy to identify in the LM and SEM. Perideraion differs from Bleakeleya and Koernerella in having two large H-shaped plastids, as reported for Asterionellopsis and Asteroplanus, and is further distinguished from all these genera by the following character set: a distinct rim around the basal field of circular pores, an apical rimoportula laterally on the valve mantle, pores in the valves that are unlike those in the basal pore field, and absence of apical slits. Although the basal pole attachment and colony morphology seem synapomorphic, molecular evidence suggests that the Bleakeleya-Koernerella-Perideraion clade is separate from the Asterionellopsis-Asteroplanus clade and these five genera do not form one monophyletic group. It remains to be seen whether they are part of a somewhat larger group at the base of the araphid grade or if the genera we studied are truly separate.