Analysis of the taxonomic position of Planktoniella muriformis (Thalassiosiraceae): a proposal of Thalassiosira muriformis comb. nov.

Planktoniella is a genus of marine diatoms currently comprising nine species, including the type species Planktoniella sol. The taxonomic history of Planktoniella muriformis has been controversial; it was originally described as Coenobiodiscus muriformis and later transferred to Planktoniella. Morphological and molecular studies indicate that this species is phylogenetically distant from the type species. In this study, we clarify the taxonomic position of P. muriformis using an integrative approach combining morphological and molecular evidence. Colonies consisted of approximately 135–200 cells embedded in an organic matrix, with valves exhibiting eccentric areolation, a marginal ring of fultoportulae, each bearing four satellite pores, and a single rimoportula. Phylogenetic analyses based on LSU rDNA sequences recovered Planktoniella as a well-supported monophyletic clade, while the two P. muriformis sequences clustered within a subclade of Thalassiosira, as sister lineage to T. angulata. Genetic distances showed that P. muriformis diverged by 10.2 % from P. sol and by 4.9 % from T. angulata, confirming its closer affinity to the latter. Morphological and molecular evidence support the transfer of P. muriformis to the genus Thalassiosira, resulting in the new combination Thalassiosira muriformis . However, this change underscores the need for a comprehensive taxonomic revision of Thalassiosira , which remains polyphyletic.