New insights into the morphology and phylogeny of Heterosigma akashiwo (Raphidophyceae), with the description of Heterosigma minor sp. nov.

Abstract:

Heterosigma akashiwo is one of the smallest raphidophytes and is known for its ability to form dense, fish-killing blooms. In many parts of the world, H. akashiwo is a permanent resident of the algal community, and blooms have caused extensive financial losses for the aquaculture industry. Different aspects of its biology have been the focus of numerous investigations over the last two decades but, perhaps because of its small size and fragile nature, few morphological studies have been carried out. We conducted phylogenetic and morphological studies of 24 strains of Heterosigma from the coastal waters of Europe, North America, Australia and New Zealand. Complete 18S, ITS region and the D1–D2 region of 28S rDNA were sequenced and compared with all available raphidophyte sequences, revealing a novel species of Heterosigma, Heterosigma minor sp. nov. All strains of H. akashiwo were genetically very similar across the rDNA region (> 99.8% similarity) and morphologically very similar in light microscopy and scanning electron microscopy. Heterosigma minor shared only 98.2% similarity with H. akashiwo, and there were several morphological differences: it was smaller, rounder and had fewer mucocysts than H. akashiwo. The secondary structure of ITS2 was reconstructed, revealing three compensatory base changes between the two species, further indicating that H. minor is a sister to H. akashiwo. The phylogeny of Raphidophyceae was reconstructed to include all currently described raphidophytes.

Key Words::

• Ichthyotoxic
• ITS2 Secondary structure
• Morphology
• Phylogeny
• Raphidophyceae
• Taxonomy

ACKNOWLEDGEMENTS

The authors are grateful to the excellent staff at UNCW for valuable help with SEM preparation and analysis: D. Mark Gay, Bob York and Professor Emeritus Richard Dillaman. We are also grateful to Linda Medlin for invaluable help with the reconstruction of ITS2. We thank the culture collections where we have obtained our material, especially the curator of SCCAP culture collection, Gert Hansen, for allowing us the use of his LM picture of strain SCCAP K-1549 (Fig. 3). We thank Jahn Throndsen for permission to use his line drawing (Fig. 2). The Norwegian Research Council is thanked for funding project 196702/E40 Toxic Algae. The Basque Government is acknowledged for funding project IT-699-13, which allows for maintenance of the culture collection at Microalgae Culture Collection of the Department of Plant Biology and Ecology of the University of the Basque Country. We would also like to thank two anonymous referees for the constructive comments regarding this manuscript.

SUPPLEMENTARY DATA

Supplementary data associated with this article can be found online at http://dx.doi.org/10.2216/15-115.1.s1