Moorthi Stefanie D.

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Stefanie D.

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  • Article
    Mixotrophy : a widespread and important ecological strategy for planktonic and sea-ice nanoflagellates in the Ross Sea, Antarctica
    (Inter-Research, 2009-03-04) Moorthi, Stefanie D. ; Caron, David A. ; Gast, Rebecca J. ; Sanders, Robert W.
    Mixotrophic nanoflagellates (MNF) were quantified in plankton and sea ice of the Ross Sea, Antarctica, during austral spring. Tracer experiments using fluorescently labeled bacteria (FLB) were conducted to enumerate MNF and determine their contribution to total chloroplastidic and total bacterivorous nanoflagellates. Absolute abundances of MNF were typically <200 ml–1 in plankton assemblages south of the Polar Front, but they comprised 8 to 42% and 3 to 25% of bacterivorous nanoflagellates in the water column and ice cores, respectively. Moreover, they represented up to 10% of all chloroplastidic nanoflagellates in the water column when the prymnesiophyte Phaeocystis antarctica was blooming (up to 23% if P. antarctica, which did not ingest FLB, was excluded from calculations). In ice cores, MNF comprised 5 to 10% of chloroplastidic nanoflagellates. The highest proportions of MNF were found in some surface water samples and in plankton assemblages beneath ice, suggesting a potentially large effect as bacterial grazers in those locations. This study is the first to report abundances and distributions of mixotrophic flagellates in the Southern Ocean. The presence of MNF in every ice and water sample examined suggests that mixotrophy is an important alternative dietary strategy in this region.
  • Preprint
    Defining DNA-based operational taxonomic units for microbial-eukaryote ecology
    ( 2009-06-19) Caron, David A. ; Countway, Peter D. ; Savai, Pratik ; Gast, Rebecca J. ; Schnetzer, Astrid ; Moorthi, Stefanie D. ; Dennett, Mark R. ; Moran, Dawn M. ; Jones, Adriane C.
    DNA sequence information has been increasingly used in ecological research on microbial eukaryotes. Sequence-based approaches have included studies of the total diversity of selected ecosystems, the autecology of ecologically relevant species, and the identification and enumeration of species of interest to human health. It is still uncommon, however, to delineate protistan species based on their genetic signatures. The reluctance to assign species-level designations based on DNA sequences is partly a consequence of the limited amount of sequence information presently available for many free-living microbial eukaryotes, and partly the problematic nature and debate surrounding the microbial species concept. Despite the difficulties inherent in assigning species names to DNA sequences, there is a growing need to attach meaning to the burgeoning amount of sequence information entering the literature, and a growing desire to apply this information in ecological studies. We describe a computer-based tool that assigns DNA sequences from environmental databases to operational taxonomic units at approximate species-level distinctions. The approach provides a practical method for ecological studies of microbial eukaryotes (primarily protists) by enabling semiautomated analysis of large numbers of samples spanning great taxonomic breadth. Derivation of the algorithm was based on an analysis of complete small subunit ribosomal RNA (18S) gene sequences and partial gene sequences obtained from GenBank for morphologically described protistan species. The program was tested using environmental 18S data sets from two oceanic ecosystems. A total of 388 operational taxonomic units were observed among 2,207 sequences obtained from samples collected in the western North Atlantic and eastern North Pacific.