Swimming speed of three species of Alexandrium (Dinophyceae) as determined by digital in-line holography

Digital in-line holographic (DIH) microscopy Was Used to track motility in several related species of the marine dinoflagellate Alexandrium in response to temperature after acclimation at selected temperatures. Numerical reconstruction of DIH holograms yielded high-contrast three-dimensional images of the trajectories Of many Motile Cells swimming simultaneously throughtout the sample volume. Swimming speed and trajectory were determined for clonal isolates of A. ostenfeldii, A. minutum and A. tamarense within the temperature range front 8 to 24 degrees C. The strains of these species revealed difference in temperature optima for growth and tolerance that were a function of both acclimation responses and genetic factors reflecting the origin of the isolates. The fastest swimming, speeds were recorded at 24 degrees C for cells of A. minutum. Acclimated strains of all three species swam significantly slower at lower temperatures, although fastest swimming speeds did not always occur at temperature optima for growth. Aged cells from stationary phase cultures swain more slowly than cells in exponential growth phase. Doublets from a rapidly dividing culture swam faster than singlets from the same culture, confirming the propulsive advantage of paired cells. Holographic microscopy is a powerful tool for the acquisition of detailed observations of swimming behaviour of microalgal cells in the form of three-dimensional trajectories over the appropriate temporal (sub-second) and spatial (micrometer) scales.