The reversible effect of flow on the morphology of Ceratocorys horrida (Peridiniales, Dinophyta)

Most cells experience an active and variable fluid environment, in which hy drodynamic forces can affect aspects of cell physiology including gene regu lation, growth, nutrient uptake, and viability, The present study describes a rapid yet reversible change in cell morphology of the marine dinoflagell ate Ceratocorys horrida Stein, due to fluid motion, Cells cultured under st ill conditions possess six large spines, each almost one cell diameter in l ength, When gently agitated on an orbital shaker under conditions simulatin g fluid motion at the sea surface due to Light wind or surface chop, as det ermined from digital particle imaging velocimetry, population growth was in hibited and a short-spined cell type appeared that possessed a 49% mean dec rease in spine length and a 53% mean decrease in cell volume, The reduction in cell size appeared to result primarily from a 39% mean decrease in vacu ole size, Short-spined cells were first observed after 1 h of agitation at 20 degrees C; after 8 to 12 d of continuous agitation, long-spined cells we re no longer present, The morphological change was completely reversible; i n previously agitated populations devoid of long-spined cells, cells began to revert to the long-spined morphology within 1 d after return to still co nditions, During morphological reversal, spines on isolated cells grew up t o 10 mu m.d(-1). In 30 d the population morphology had returned to original proportions, even though the overall population growth was zero during thi s time, The reversal did not occur as a result of cell division, because si ngle-cell studies confirmed that the change occurred in the absence of cell division and much faster than the 16-d doubling: time, The threshold level of agitation causing morphology change in C. horrida was too low to inhibi t population growth in the shear-sensitive dinoflagellate Lingulodinium pol yedrum. At the highest level of agitation tested, there was negative popula tion growth in C, horrida cultures, indicating that fluid motion caused cel l mortality, Small, spineless cells constituted a small percentage of the p opulation under all conditions, Although their abundance did not change, si ngle-cell studies and morphological characteristics suggest that the spinel ess cells can rapidly transform to and from other cell types, The sinking r ate of individual long-spined cells in still conditions was significantly l ess than that of short-spined cells, even though the former are larger and have a higher cell density, These measurements demonstrate that the long sp ines of C. horrida reduce cell sinking, Shorter spines and reduced swimming would allow cells to sink away from turbulent surface conditions more rapi dly, The ecological importance of the morphological change may be to avoid conditions that inhibit population growth and potentially cause cell damage .