Growth and survival of Neoceratium hexacanthum and Neoceratium candelabrum under simulated nutrient-depleted conditions

The dinoflagellate Neoceratium is commonly observed in oceanic waters, depleted in major inorganic nutrients such as nitrogen and phosphorus. Using culture isolates, we investigated whether two Neoceratium species (N. hexacanthum and N. candelabrum) can grow phototrophically at low nutrient concentrations found in surface waters of oligotrophic subtropical gyres (OSGs). No phototrophic growth (indicated by changes in cell numbers, the presence of dividing cells or cellular protein increase) was observed when N. hexacanthum and N. candelabrum were grown in low nutrient seawater. In separate experiments, to determine survival time under oligotrophic nutrient conditions, 68% of N. hexacanthum cells were able to re-establish growth after spending 1–10 days in North Atlantic gyre seawater; 40% recovered after 11–20 days and only 3% recovered after 21–30 days. The longest period any single cell survived, and then went on to divide, was 26 days. These findings demonstrate that Neoceratium cells could remain viable for >3 weeks in surface waters of OSGs, but to sustain their growth nutrients must be obtained periodically from an alternative source: via ephemeral upwelling of nutrient-rich waters, phagotrophy and/or movement to and from the nutricline.