Diversity and taxonomic identification of Shionodiscus spp. in the Australian sector of the Subantarctic Zone

Abstract

Diatoms are siliceous phytoplankton that play a major role in global carbon fixation, as well as forming the basis of most marine food webs. In the Southern Ocean, diatoms are responsible for significant volumes of carbon export and sequestration in the deep sea. Diatoms are unicellular and microscopic, thus diatom taxonomy has naturally progressed as imaging and molecular technologies have developed. Despite recent advancements, some aspects of diatom taxonomy remain unclear. The genus Shionodiscus was separated from Thalassiosira in 2006, and possesses, as a group, internal extensions of the marginal strutted processes as well as a labiate process on the valve face, usually distant from the margin. The features distinguishing Shionodiscus from Thalassiosira, and Shionodiscus spp. from each other, are difficult to examine under light microscopy, and most taxonomists will group similar Shionodiscus species together when encountered, rather than discriminating species. As a result, the full extent of Shionodiscus diversity is rarely examined in individual studies. In this study, sediment trap material captured in the Australian sector of the Subantarctic Zone was examined using both light and scanning electron microscopy, and compared to known measurements on Shionodiscus species and varieties. This study had two aims; to catalogue for the first time the diversity of Shionodiscus spp. within the Australian sector, and to create a formalized set of criteria for grouping Shionodiscus species, when only light microscopy analysis is possible. By documenting Shionodiscus diversity in the Australian sector and establishing a standard identification protocol, researchers will be better able to determine the ecological significance of the genus in diatom assemblages.

Keywords:

• diatom
• Shionodiscus
• taxonomy
• Southern Ocean
• sediment traps

Acknowledgements

Deep thanks to Dr Andrés Rigual-Hernández who provided samples for photographic plates. We would like to extend warmest thanks to the Macquarie University Microscopy unit, particularly N. Vella, and also to Andrew J. Alverson who provided helpful advice on this study. We express great appreciation to two anonymous reviewers and Dr Eileen Cox for thoughtful feedback provided during the review process.

ORCID

Jessica V. Wilks http://orcid.org/0000-0001-6217-4952

Additional information

Funding

This study was funded by Macquarie University under an MQRES scholarship to JW, and Australian Antarctic Science (AAS) grant 4078 (LA, TT, SB, ARH). The SAZ Project, from which the material was obtained, was funded by the Australian Antarctic Division, ASAC grants, 1156 and 2256 (TT), as well as the National Science Foundation (NSF) Grant (R. Francois, T. Trull, S. Honjo and S. Manganini), the Belgian Science and Policy Office (F. Dehairs), CSIRO Marine Laboratories, and the Australian Integrated Marine Observing System (IMOS).