Recent studies on biodiversity and eco-physiological characteristics of the genus Skeletonema (Bacillariophyceae)

The cosmopolitan diatom Skeletonema costatum is widely considered to be one of the most important phytoplankton species because as a primary producer, it contributes to the productivity of global marine food chains and occasionally forms heavy blooms. New taxonomic methods using electron microscopy for fine morphological observations, and gene analysis of ribosomal DNA (rDNA) in 2005 and 2007 showed that “S. costatum” actually includes eight species: S. ardens, S. costatum sensu stricto (s.s.), S. dohrnii, S. grethae, S. grevillei, S. japonicum, S. marinoi and S. pseudocostatum. With the addition of these species to the three existing species, S. menzelii, S. subsalsum and S. tropicum, the genus Skeletonema comprises 11 species. The results of identification using both morphology and gene analysis agree well and reinforce the validity and applicability of the new classification methods. Building on this progress and my own findings over the last five years, I herein comment on the methods of identification and classification of the 11 species of the genus Skeletonema, introduce the eco-physiological characteristics and biogeography of the new Skeletonema species, and present results of population studies of S. marinoi using genetic methods. A noteworthy outcome of the new taxonomic methods was the resolution of 21 strains in the S. marinoi-dohrnii complex. The global distribution of S. costatum sensu lato (s.l.), with opportunistic features, is explained by the discovery that this species was comprised of eight related species expected to have different eco-physiological characteristics. New research has revealed that the global distribution pattern of S. japonicum could be explained by its temperature–growth characteristics. The high diversity of the genus Skeletonema is evident in Dokai Bay, Japan, where seven or eight species were reported, and four species were counted in a single sample. Interesting results from ongoing research suggest that the genetic structure of the S. marinoi population in Mariager Fjord, Denmark has been stable for over one hundred years. Hopefully, the taxonomy will continue to develop using other gene markers, and these taxonomic methods will be applied to different areas.