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BIOCONVECTION AND GRAVITAXIS IN THE CALCIFYING ALGA PLEUROCHRYSIS CARTERAE.  V. Chheda, D. Montufar-Solis, and P.J. Duke.  Dept. of Orthodontics, Dental Branch, UT Houston Health Science Center, Houston, TX. 

     Bioconvection is the formation of convection patterns within a culture of swimming microorganisms due to their collective behavior.  Swimming strains of the calcifying alga Pleurochrysis carterae exhibit bioconvection when cultured in aquaria, flasks, or Rose Chambers, affirming the presumed negatively gravitaxic behavior of the cell.  These unicells are being grown in our lab for preliminary studies relating to an International Space Station experiment that will study the relationship of mineralization and morphogenesis to gravitaxis.  In the present study, we used Wintrack 2000 to determine the influence of bioconvection on the direction of cell movement by analysis of videos acquired at different magnifications and using different cell concentrations.  Cells were grown in a completely filled Rose chamber, eliminating the variable of an oxygen gradient. 

Results showed that an evenly distributed number of cells swimming up concentrate at the top of the container and then abruptly fall, forming a convection current visualized as a thin but dense column of downflowing cells.  The greatest influence on movement of individual cells is seen in or near these streams.  Cells detaching from the stream move upwards at an angle at least in part influenced by the convective fluid flows.  This angle determines if the cell will reenter the stream, or move into the region between downstreams, where they swim directly up.  Therefore, cells in a population do not swim independently of one another, but this effect is diminished at lower densities.  The videotaping plan for the experiment aboard Space Station includes taping with white light and infrared, allowing us to address the question of bioconvection in the absence of gravitaxis and phototaxis, which have not been previously studied.

     (Support:  NASA grant NAG2-1261 and UTHSC Summer Research Program)