Mixotrophy : a widespread and important ecological strategy for planktonic and sea-ice nanoflagellates in the Ross Sea, Antarctica

Abstract

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.