Investigating reasons for the growth and survival of the dinoflagellate genus Neoceratium in oligotrophic subtropical gyres

Abstract

The dinoflagellate Neoceratium is frequently observed in oligotrophic subtropical gyres (OSGs) where major inorganic nutrients such as nitrogen and phosphorus are depleted in the surface waters. In Chapter 2, continuous plankton recorder (CPR) data demonstrate that Neoceratium inhabit surface waters of the North Atlantic subtropical gyre (NAG) throughout the year. The presence of Neoceratium in surface waters contrasts with the fact that nutrients are severely depleted in the surface 100 to 130 m. In Chapter 3, it is shown that Neoceratium cannot grow when exposed to low nutrient concentrations comparable to those found in surface waters of OSGs. However, cells are able to survive and re-establish growth after a maximum of >3 weeks of “nutrient starvation”, with signs of stress becoming noticeable after 10 days. In Chapter 4, nutrient ratios in large areas of surface waters of OSGs in the Atlantic are shown to be favourable to mixotrophy (86 % of sites in the North and South Atlantic OSGs). It is estimated that 0.01 to 0.44 ciliates per day would need to be ingested for Neoceratium to survive in these waters. In Chapter 5, growth was only observed when cells were exposed to a range of irradiances (6 to 60 ?mol quanta m-2 s-1), above 22 ?mol quanta m-2 s-1; an irradiance that is typically only found 15 to 22 metres above the nutricline at midday, implying that VM would be required to access the nutricline. A mechanism for VM in OSGs is suggested whereby vertical movement is triggered by phosphate-limitation of cells. In Chapter 6, the thesis concludes with a conceptual model to explain how a number of the above findings likely interact to enable Neoceratium to successfully survive and grow in OSGs.

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