All plankton experience a dynamic fluid environment due to the effects of wind, waves, tides, and currents. Large-scale physical processes are known to affect plankton spatial distribution, vertical mixing, and nutrient supply. At the small scale of the individual cell, fluid motion generated on larger scales is experienced as a laminar shear and is known to affect cell physiology and behavior. Growing field and laboratory evidence suggests that dinoflagellate population growth is influenced by shear. Dinoflagellates are important members of the plankton community and one of the most shear-sensitive organisms known. The objective of this project is to provide a mechanistic understanding of how shear affects population growth, and to identify environmental and physiological factors that cause shear sensitivity to vary.
The effect of shear on population growth and cell cycle dynamics will be studied in the red tide dinoflagellates Lingulodinium polyedrum (=Gonyaulax polyedra) and Gymnodinium sanguineum (=G. nelsonei, G. splendens). Although the effect of shear on population growth of both species has previously been studied, new preliminary data suggest that the inhibition of population growth by shear is variable within a species. In addition, these data suggest a different mechanism for the effect of shear on cell cycle progression than previously proposed. Laboratory experiments will subject cells to Couette flow with quantified levels of fluid shear approximating levels found in oceanic turbulence. Three hypotheses will be tested: (1) Shear sensitivity is affected by growth conditions; (2) Population growth is inhibited by shear levels which are plausible for small-scale turbulence in the physically-forced upper ocean; and (3) Exposure to shear interferes with the normal progression through the cell cycle. Changes in population growth will be measured using daily cell counts and the cell cycle will be resolved using flow cytometry and cell volume measurements.
The factors which affect shear sensitivity in laboratory cultures may be relevant to field conditions, such as during the progression of a dinoflagellate bloom. Therefore, the results provide a foundation for future field studies of the effect of turbulence on in situ dinoflagellate populations. A mechanistic understanding of the effect of shear on dinoflagellate growth inhibition strengthens the argument that turbulence is important in regulating population growth in the field. Turbulence is a poorly understood environmental factor affecting phytoplankton physiology. Ultimately, turbulence may be as important as light and nutrients in the regulation of dinoflagellate populations, including red tides and toxic blooms.
